Frequently Asked Questions –Battery Chargers.

If you have a technical question relating to Battery Chargers for Lead acid or SLA batteries, please review the following list to see if the topic you need is covered below.


Safety and liability issues - please read this first.

  1. Should I switch it off when the green Ready light comes on, to save electricity and to avoid over charging my battery?

 

  1. Your competitor will sell me a 12 Volt, 10 Amp battery charger at a lower cost?

 

  1. What’s the difference between a DC Power supply and a battery charger?

 

  1. I want a charger that switches to standby when the current drops below a certain level. Do you make a charger that does that?

 

  1. I can’t get any output from my charger when I measure it using a DC Voltmeter?

 

  1. In your spec you claim something called “mains zero crossing voltage monitoring”. What is that and why would I want that?

 

  1. What is the override timer and what’s it’s purpose, why is this feature included in all your battery chargers?

 

  1. What is parasitic loading and why is it important to check this spec in some battery applications?

 

  1. My battery isn’t getting fully charged, I’m not getting the run time on load that I was expecting. Why is that?

 

  1. My battery is getting over charged, it’s not switching to standby soon enough, or the green Led doesn’t stay on?

 

  1. Can I run a DC load from the battery while it is charging?

 

  1. How should I connect one battery charger to charge more than one battery?

 

  1. How does one check and adjust the Voltage settings of my battery charger?

 

  1. How does one check and adjust the Current Limit setting of my battery charger?

 

  1. What is the Battery Type DIP Switch for, how should it be set?

 

  1. How long will my battery support my load, how can I calculate the expected runtime?

 

  1. How long will it take for a certain current rating charger to fully recharge a given size battery?

 

  1. What’s the difference between a Sealed Lead Acid battery, a Gel Cell Battery, an Absorbed Electrolyte Battery, a Valve Regulated Battery, and why does it matter?

 

  1. Important safety and liability issues – please read this first.


 


This Battery Chargers FAQ page is provided by Dektron Ltd. Copyright 2003-2005
THIS PAGE MAY NOT BE COPIED AND REPOSTED IN MODIFIED FORM.

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If you have any suggestions for improvement to this page, or any questions on the topic that are not covered here, you are welcome to contact the author by email at phil@dektron.com


ANSWERS:

  1. “Should I switch it off when the green Ready light comes on, to save electricity and to avoid over charging my battery?”


Absolutely not! We recommend that, where possible, you leave the charger plugged in and switched on, with the batteries connected, until you next need the battery for use. There are several reasons for this. At the end of the charge cycle, when the green ready light is on, the charger is trickle charging the battery in constant voltage float/standby mode, nominally at 2.3 Volts per cell. This is the same charge method used for batteries in standby applications such as alarm panels or emergency lighting, where the battery is intended to be charged 24 hours a day, every day. At this voltage, the battery will not be gassing so loss of electrolyte is minimal. The charge current drops exponentially to a very low level, sufficient to maintain the battery in a fully charged state and to compensate for any self discharge. Over time this low rate of charge will tend to equalise charge imbalance between the cells, which can extend the battery life. By leaving the charger switched on, you will prevent any risk of damage to the battery from sulphation (which can be caused by allowing the battery to stand in the discharged state). The energy consumed in standby mode is minimal, typically about 10 Watts for a medium size charger, so one unit (One KWHr) of electricity is used every 100 hours, which costs about one and a half Pence per day. The only exception to this recommendation is in cases where the battery manufacturer specifically states that the battery is not suitable for constant voltage float operation, or when running from an intermittent AC supply such as a generator.

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  1. “Your competitor will sell me a 12 Volt, 10 Amp battery charger for a lower price than your unit. Why should I pay more than I have to?”


Well, the lower cost charger may be fine for some applications. But, if you are using a battery in a demanding application where performance and battery lifetime are important, you might find that saving money on the battery charger is not cost effective in the long term. If the battery is overcharged or undercharged then your product will not perform as well as it could, and the battery will not give the lifetime, in terms of cycles of discharge and standby time, and so will need to be replaced more frequently than you, or your customers, were expecting. Batteries can fail within the warranty period, and the battery manufacturer may decline warranty claims for replacement batteries where incorrect charging has contributed to the problem. This is why it’s advisable to test your system carefully using the exact battery, charger and load in a simulation of actual use. Also, the system designer should ensure that the cyclic and float voltage settings of the charger are within the ranges specified by the battery manufacturer. Our chargers are designed to offer the best battery performance and lifetime with features such as three stage charging, precise voltage regulation, proportional timing, overrun timer, low start voltage, and low parasitic loading. Some of our regular customers started using our product only after they had experienced a problem. Don’t find this out the hard way - there is much more to the specification of a battery charger than the Voltage, Current rating, and price.

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  1. “What’s the difference between a DC Power supply and a battery charger?”


A battery charger is a type of DC Power supply (PSU) which is specifically designed for charging batteries. While any DC Power supply can be used to charge batteries, there are serious potential pitfalls to using a generic power supply as a battery charger. For example, a DC Power supply may include regulation circuits, which may be damaged if a battery is connected to the output, before the AC Power supply is switched on. The regulation circuit in a power supply is not designed to reduce parasitic load and so may draw power from the battery if left connected when AC power is switched off. These two issues can be addressed by adding a blocking diode, but then the volt drop of the diode (which is temperature dependant) needs to be allowed for. Generic power supplies do not provide multiple stage charging with different voltage limits, or temperature compensation of the charge voltage, or reverse battery connection protection. In general, it’s better to use a battery charger that was designed for the job, rather than a general purpose DC power supply, for battery charging. If using our chargers, there is no need to fit any external blocking diode or contactor to prevent current flow from the battery back into the charger, when the AC supply is off, as may be required with some generic power supplies.

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  1. “I want a charger that switches to standby when the current drops below a certain level. Do you make a charger that does that?”


No, we don’t offer that type of charge termination. We use an alternative technique called proportional timing, which does the same thing, but does it better. We have done extensive testing on different types and sizes of batteries to reach this conclusion. Many competitors multi stage chargers use a current comparator to determine when to switch from bulk charge (constant voltage at the cyclic voltage limit)to float/standby mode (constant voltage at the float voltage limit). This method, although widely used, has some drawbacks. The problem is that the current at end of charge varies with a number of parameters external to the charger, such as the temperature, the age of the battery, and the size of the battery. In a constant voltage charge system, the charge current falls off exponentially as the battery EMF increases and the charger voltage is held constant. At the end of the charge, where the determination of switch to float has to be made, the slope of the current against time graph is quite flat, so a small change in the current setting can make a wide difference to the charge time. When a battery approaches the end of it’s life it tends to draw a higher self discharge current due to sludge accumulation increasing electrical leakage between the plates, so if a current comparator is used the charger may never switch down to the float/standby voltage, resulting in overcharge, gas emission, and premature battery replacement. Our chargers use proportional timing where the switch to float is timed optimally, eliminating the need for sensing low currents, and eliminating adjustments to the charge termination controller to match the Amp-hour size of the specific battery.

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  1. “I can’t get any output from my charger when I measure it using a DC Voltmeter so do I need to send it back for repair?”


Probably not. Our chargers feature short circuit and reverse polarity shutdown, so they don’t produce any output voltage unless they are actually connected to a battery. The charger waits to “sense” the battery voltage on the output before it starts producing voltage, so you cannot test for DC output with a volt meter or test lamp, when there is no battery connected to the charger output. Try switching the AC supply to the charger off and on, the Led indicators should show the power on test sequence (Green-Yellow-Red, each for about a half second) each time the AC power is applied. If there is no Led test indication, check that AC input power is getting to the charger, and the AC Power input fuse is intact. If the Led power-on indication is OK, try connecting the charger to a known good battery (of the correct voltage, but almost any size will do for testing), the yellow charge indicator should come on, and the battery voltage should rise to around 2.4 Volts/Cell. If this happens, the charger is producing output. If the yellow charge Led does not come on, when the battery and AC power are connected, check carefully that your connections from the charger to the battery are sound and that the battery is wired the correct way around (Red lead from charger to the battery Positive). If the battery is very excessively discharged (to less than 1 or 2 Volts DC in total) then the charger may not start because it can’t detect that the battery is there. If this happens, try removing the DC load to allow the battery voltage to recover, or connect another battery in parallel momentarily to provide starting bias. Note that batteries discharged to zero voltage are liable to be damaged by sulphation if allowed to remain in a discharged state for more than a few hours.

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  1. “In your spec you claim something called “mains zero crossing voltage monitoring”. What is that and why would I want that?”


SCR controlled chargers have un-smoothed output, so the DC output to the battery is in the form of a pulse of current each half cycle of the AC supply. During the time when the AC input is crossing zero, in between pulses of output, there is no current flowing in the cables from the charger to the battery. We take advantage of that, by using a sample-and-hold circuit to measure the battery voltage at mains zero crossing, so that the charger can monitor the battery voltage without errors that would otherwise be caused by volt drop on the DC cables. When in Constant Voltage mode, the charger will maintain a constant voltage at the battery terminals, by increasing the voltage at the charger end of the cable if needed to compensate for volt drop in the cable. In some applications, especially when using long DC cables, this feature can improve performance and eliminate the requirement to run separate voltage sensing leads. This feature does not apply to switch mode or other smoothed output chargers.

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  1. “What is the override timer and what’s it’s purpose, why is this feature included in all your battery chargers?”


The override (sometimes called overrun) timer is a software timer, which starts at each beginning of each charge, and runs until the green “Ready” light comes on. There is a fixed maximum time allowed for completion of each charge cycle, the default setting is 18 hours, but this setting can be modified if required by changing the software. If the override timer times out before the “Ready” led comes on, the unit enters “fault mode” and shuts down, producing no further output. The fault mode is indicated by a continuous rapid flashing of the Green “Ready” Led. The fault mode can be cleared by either switching the AC supply off and on, or by disconnecting from the battery. Note that, providing the charge cycle completes normally, the charger will normally remain in float/standby mode with the green Led on, and 2.3V/Cell constant voltage output, indefinitely because the override timer is stopped when the green “Ready” Led comes on. The override timer is intended to prevent continuous charging (and possibly overcharging) under fault conditions, such as a shorted cell in the battery, or a charger fault causing low output current, or a voltage sensing failure. For very unusual applications, if a charger is used on a disproportionately large battery (such as sometimes used in a float/standby application) where the charger may normally take over 18 hours to reach the end of the charge cycle, we can supply a modified control chip with the override timer disabled (-NT option). Normally, even in float/standby applications, the charger current rating should be selected so that it is large enough to fully recharge the battery in less than 18 hours, so the override timer will never terminate the charge under normal conditions.

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  1. “What is parasitic loading and why is it important to check this spec in some battery applications?”


Parasitic loading means the DC current that flows into the charger from the battery when there is no AC power supply to the charger. In some competitors units the control circuits in the charger are powered from the DC output circuit, so that the charger may “leak” several tens of milliamps (or sometimes more) back out of the battery, if it’s left connected when there is no AC power, or when it’s switched off. This can cause a problem in applications where the charger is normally, or may be, left wired to the battery, when the AC input power is switched off or the supply fails. A load of just 50mA will discharge the battery by 1.2 Ah every 20 Hours, and by 8.4 Ah in a week. If , over time, the battery becomes over-discharged, that can lead to sulphation, or excessively low voltage, so that when the AC power is restored, the battery will not recharge even though power is available. Ideally, the charger should be specified so that the parasitic loading is less than, or comparable to, the battery self discharge rate. Our chargers typically have a parasitic load spec of less than 300 micro Amps, or 0.3 mA, which is low enough to be insignificant in normal applications. No series isolation diode between charger and battery is needed when using chargers with a low parasitic load current.

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  1. “My battery isn’t getting fully charged, I’m not getting the run time on load that I was expecting. Why is that?”


This could be due to a number of things, because the battery, the load, and the charger have to work together as a system, so a problem in any one of them may result in sub-optimal performance. First, review answer to “How long will my battery support my load, how can I calculate the expected runtime?” below and check the expected runtime of the load current against the size of the battery. Measure the actual load current and verify that it is as expected. Check the “Cyclic Voltage Limit” and “Float/Standby Voltage Limit” settings of the charger are correct per the recommendations of the manufacturer of your battery. For details on how to check these voltage settings, see answer “How does one check and adjust the Voltage settings of my battery charger?” below. If the voltage settings are OK, try leaving the battery on charge for an extended period (for example, over the weekend) to make sure it’s as fully charged as possible. Also see answer to “How does one check and adjust the Current Limit setting of my battery charger?” to confirm that the current output of the charger is up to specification. In the cables from the charger to the battery, check that there are no excessively long cables, thin wiring, or badly connected terminals causing power loss in the cable run, verify the charge current flowing using an Amp meter connected in series with the battery terminal under the actual conditions of typical charging. If the charger voltage or current values are not correct, either adjust them or return the charger for repair. Consider having the battery capacity tested using a constant current test load, if you have access to one, typically a good battery will run a 1xC rate discharge for 30 minutes to 1.5V/Cell, for example, a 32Amp-hour battery, discharged at 32 Amps, should run for 30 minutes before the battery terminal voltage drops to below 9 Volts. The runtime of the battery drops over time, a good quality equipment battery will typically provide 200 cycles of discharge, to 100% depth of discharge, before needing replacement. These figures are typical, check the published spec from the battery manufacturer for the exact type of battery you are using.

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  1. “My battery is getting over charged, it’s not switching to standby soon enough, or the green Led doesn’t stay on?”


This can be due to a number of things. If the battery has a faulty cell, then it’s on charge voltage will not reach the charger set point to switch to Constant Voltage Mode, which results in overcharging of the remaining cells, until the overrun timer terminates charge after 18 hours. If there’s a fault in the charger which causes the voltage setting to drift upwards, or if the charger is not set for the correct battery type, that can cause overcharging. In any case, the appropriate test, is to measure the battery voltage when in the constant voltage charge stage, and confirm that the voltage is correct per the specification of the battery. To do this, switch the charger off and on to reset it, and then wait until the “charge” light starts to flash (or, on some units, until the “80%” led comes on. The charger is now in the constant voltage mode. Measure the battery voltage using an accurate digital volt meter, measuring at the battery terminals. If the voltage is too high (for example, more than about 14.7V on a 12V, absorbed electrolyte sealed battery, then the charger is faulty or needs adjustment. In some very unusual applications, if the AC power supply is unreliable (frequent supply interruptions) that may result in overcharging, because the proportional timer always holds the battery at the cyclic charge voltage limit for a minimum of one hour before switching back to float/standby. If the battery is supporting a load while charging, and the nature of the load is regular, high current demand pulses (greater than the charger current rating),that may reset the proportional timer and cause overcharging. In this case, the charger can be modified to eliminate the 1 hour minimum time offset, contact the factory if this modification is needed in your application.

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  1. “Can I run a DC load from the battery while it is charging?”


Yes, but there are a few points to watch for. Firstly, the load will be subjected to the on-charge voltage of the battery, which is of necessity somewhat higher than the battery’s normal on load voltage. For example, a 24 Volt battery system will normally be held at about 29 Volts DC for several hours during the Constant Voltage charge stage, so you should check that your DC load is specified to be OK at the higher voltage, including some allowance for voltage overshoot and charger adjustment tolerance. If it looks like there might be a problem, consider lowering the charger cyclic voltage adjustment setting (this will result in a longer recharge time but will reduce the stress on the load). Or consider using a voltage regulator, or voltage reducer, between the battery and the load. Secondly, any load current drawn from the battery while charging, will reduce the effective charge current and so extend the recharge time. It’s best to keep the average level of DC load current to not more than about 20% of the charger current rating, for this reason. Thirdly, if the charger is an un-smoothed SCR type, it will cause superimposed AC ripple on the battery DC output, which can upset sensitive electronic loads, for example causing a background hum noise on radios. This can be reduced by keeping the charger cables and the load cables separate if possible – run the charger cables (both Positive and Negative) directly to the battery terminals, separate from any other wiring. Alternately, a DC filter circuit can be added to the charger output.

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  1. “How should I connect one battery charger to charge more than one battery?”


Charging more than one battery, or battery pack, from a single charger, is something of a compromise and should be avoided if possible. It’s much better to use two smaller chargers, one for each battery. We also offer “bank” chargers which include several independent charging circuits. If the batteries are not equally discharged, that is if they support different loads, then it’s not possible to charge them optimally using one charger, because the timing of the stages of charging should be matched to the battery depth of discharge for optimal charging performance. But, this is often done, for example in a boat or RV/caravan application where there is a “starting” battery and a “house” battery, and it’s desired to charge both from a single battery charger. A common arrangement is to use a “diode splitter” to divide the charger output between the two batteries, while maintaining isolation between the batteries, so that, for example if the “house” battery gets discharged, the vehicle can still be started. Our chargers are designed to be connected directly to the battery, they will not operate correctly, if there is a diode splitter fitted between the charger and the battery, because the diode does not allow reverse current flow from the battery to the charger so the charger cannot measure the battery voltage accurately. To get around this, we suggest fitting a 1K Ohm, half watt, resistor across each of the diodes. This is a readily available component, and it will allow enough current to pass through the diode to allow the charger to operate normally. If more than one battery is connected, it’s advisable to try to make the lengths and thickness of the cable to each battery about the same so as to avoid unequal resistances. Even so, the charger will measure the battery voltage as halfway between the two actual voltages, if they are different, and so the charging will not be as optimal as it should be. This is a fundamental problem and the best solution is to fit a separate charger for each battery bank. Charging batteries of multiple cells, either in series or in parallel, to make a higher voltage or Amp-hour rating, is acceptable, providing the batteries are of the exact same type, capacity, and age, and are connected in series or parallel at all times so that there is no unequal load. A common error, is to charge two 12V batteries in series with a 24V charger, and then to “tap” a 12V supply from the centre connection, this always results in one battery overcharged and the other undercharged which shortens the life of both batteries, and so should be avoided. It’s much better to use two 12V chargers, if there is any load driven from the connection between the batteries.

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  1. “How does one check and adjust the Voltage settings of my battery charger?”


There are three preset pots on the PCB inside the charger, these are marked as V-LIM1, V-LIM2/STBY, and I-LIM. Some chargers also have a DIP switch for setting the battery type. In any case, to check and adjust the charger voltage limits, proceed as follows. First, connect the charger to a fully charged battery. The battery used for this test can be a small one, or it can be the battery normally used with the charger, but it must be in good condition, fully charged and of the correct number of cells (for example, 12 cells for a 24 Volt charger, or 6 cells for a 12 V charger, and so on. The test battery does not have to be exactly the same type as the actual battery used in the application. Connect a calibrated accurate digital volt meter or multi-meter in parallel with the battery terminals. The volt meter should be connected directly to the battery terminals if possible. Switch the charger on and observe the green-yellow-red Led indication (Power on self check) showing the circuit board appears to be working OK. Then the Charging (usually yellow) Led should come on, indicating that a battery is connected to the charger. After a few seconds, the charger should reach the voltage limit and enter the constant voltage stage of charge. This is indicated, either by the yellow charging Led starting to flash off and on about once per second, or by the “80% Charged” led coming on, if fitted. (Some non standard chargers do not flash the yellow charging Led to indicate when the voltage limit is reached, but those are very unusual). When the charger is in constant voltage mode, observe the volt meter reading. The reading should be correct per the “Cyclic charge voltage limit” for the type of battery being used. The default setting, which works OK with most batteries, is 14.5V (2.42 Volts per Cell). If the voltage is more than 0.1 Volt wrong, adjust the preset marked V-LIM 1 to get the correct voltage. Next, locate the test point link on the PCB. On PCB’s with a 3-pin header, the test point is the 2 pins nearest the rear of the unit. On PCB’s with a 2-pin header marked “test”, that is the test point. Bridge the test point pins momentarily using a small flat blade screw driver, and observe that the green “ready” Led comes on and stays on. When the green Led is on, allow the battery voltage to settle for a few seconds, then check the reading which should be 13.8V on a 12V battery, or 2.3 Volts per cell. If necessary, adjust using the preset pot marked as either “V-LIM 2” or “STBY” (Standby). Note that, if the charger is fitted with temperature compensation (usually there is a thermistor sticking out the side or rear in a pigtail bush if this is fitted), then the voltage setting should be adjusted to allow for the temp comp at the actual ambient temperature at time of adjustment, if it is significantly different to 20 degrees C. The temp comp adjustment is –0.004 Volts per cell per degree C difference from 20C. For example a 12V (6 Cell) battery, if adjusted at 30C ambient temperature, should be set to 0.24 Volts below the nominal setting, so the float voltage would be 13.56V instead of 13.8V.

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  1. “How does one check and adjust the Current Limit setting of my battery charger?”


The current limit setting is adjusted using the preset pot marked “I-LIM” (short for Current Limit). It is set when the charger is made and does not normally need to be re adjusted. The current limit is a little more difficult to check and adjust than the voltage limit, because the amp meter has to be connected in series, and a load is required to hold the battery voltage down. If you do need to check and adjust it, proceed as follows. Connect the charger, either to a recently discharged battery in good condition, or to any battery with a DC load in parallel that is draws more current than the charger’s current rating. For example, for adjusting a 10 Amp charger, a 12 Amp DC Load would be suitable. A good current load for small 12V chargers, is a car battery with the car headlamps switched on, or a battery with a resistive or lamp load connected across it. Connect an amp meter in series with the charger output. Switch the charger on, observe the current reading. It should correspond with the charger nominal current rating. If the current is too high, adjust the I-Lim preset to correct it. If the current is too low, and will not adjust to the correct value, confirm that the AC input voltage is within spec, and that the battery voltage when charging is around 2.1 Volts per cell (approximately 12.6V on a 12V battery). The charger must be in current limit when adjusting the I-Lim preset, or the adjustment will have no effect. Note that the amp meter must be connected in series with the charger output in such a way that it does not add any significant amount of resistance, for example if using a digital multi meter, the standard set of meter probes should not be used because they are relatively long and thin, and may give a falsely low current reading. A pair of substantial thick and short test leads with 4mm plugs to plug directly into the amp meter should be used instead. A DC reading clamp meter is ideal, if available. A moving pointer type of meter is best because it reads arithmetic mean value, digital meters may not give the correct reading when measuring un-smoothed DC current. Meters which read RMS values should be avoided because the arithmetic mean value corresponds to battery charging time, and this can be significantly lower than the RMS or equivalent heating effect current, if there is superimposed AC ripple present.

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  1. “What is the Battery Type DIP Switch for, how should it be set?”


On chargers that are fitted with a Battery Type DIP switch inside on the PC Board, the charger can be quickly configured for use with either gel cell, sealed lead acid, or liquid electrolyte battery types. The difference is the cyclic voltage limit setting (This is the first voltage limit, where the charger changes to constant voltage mode, which happens when the battery reaches about 80% level of charge). The DIP switch setting also has a small effect on the float/standby voltage. If in doubt, we suggest use of the default normal setting, as that will give satisfactory performance with most battery types, with a voltage limit of 14.5V (per 6 cells). The sealed lead acid or normal setting is appropriate for absorbed electrolyte or AGM batteries. The two switch levers are marked on the PCB next to the switch, as N for normal and G for gel. The default (factory) setting, unless otherwise specified, is the “Normal” or “SLA” (Sealed Lead Acid) setting, referred to as normal. To set this mode, the switch marked N should be on, and the switch marked G should be off. The gel cell setting lowers the cyclic limit voltage to 14.1V (per 6 cells) and to select this, the switch marked G is on, and the switch marked N is off. The Liquid electrolyte battery setting increases the cyclic voltage limit to 15.6V (per 6 cells) and to select this both switches should be off. Note that, if the liquid electrolyte setting is used, there will be significant gassing in the battery when approaching full charge, if the charging is done indoors with limited ventilation, it may be better to select the Normal/SLA setting instead, which will give reduced gas emission, but will take longer to fully charge the battery. The benefit of having the dip switch is that the setting can be changed in the field without having to use a volt meter and test battery, so it allows use of the one charger type with different sorts of lead acid battery technology. The DIP switch is only fitted on the larger units, on the smaller units that don’t have a switch, the same effect can be obtained by manually adjusting the voltage limit settings using a fully charged battery and volt meter, as described elsewhere. If special or custom voltage settings are required, to suit a specific application, that can usually be arranged providing the settings are specified when ordering.

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  1. How long will my battery support my load, how can I calculate the expected runtime?”


To a first approximation, to calculate how long the battery will run the load, just measure or calculate the current that the load will draw when running, and divide the battery Amp-Hour (Ah) capacity rating by the load current, to give runtime in hours. This will be the runtime to 100% depth of discharge (DOD) and should be de-rated by 20% to avoid over discharge. Note that the battery capacity is expressed in Amp Hours (Ah), this is not the same as any figure in Amps which is a unit of current flow. If a battery supplier offers you a “100 Amp Battery” you might want to avoid that supplier!. It’s important that the system designer calculates the maximum depth of discharge, because the battery will not give good lifetime or the expected performance if it’s too small to support the load. In a cyclic application, (meaning an application where the battery is charged and discharged on a regular basis) the battery depth of discharge should be limited to no more than about 80% of maximum, in order to get a cost effective battery cycle life. For reliability and good conservative engineering, it’s advisable to use a large battery with plenty of capacity, that way the depth of discharge will be low and the battery will last a long time. But there are often commercial pressures to keep the cost as low as possible, so the designer must balance these carefully, and it may be necessary to calculate the run-time accurately. There are, though, some complicating factors. The first is de-rating the battery capacity to allow for the rate of discharge. Battery de-rating is only needed when discharging at rates faster than the rate at which the Ah is specified in the battery data sheet. The battery capacity is rated in Ah by the battery manufacturer, and is usually available from the battery data sheet. But, the higher the discharge current, the less efficient the battery becomes, so the Ah available is highest at a low discharge current, and needs to be de-rated to a lower figure at higher levels of discharge. The battery manufacturer does not know what current your load will draw, so they specify the battery at a given discharge current, or over a specified time to discharge. If they use a long (20 hour) discharge, that yields the highest figure in Ah. Typically, equipment batteries (like SLA or Gel batteries) are specified in Ah over a 20 Hour discharge. Large cyclic or traction batteries are often specified in Ah over a 5 Hour discharge. In any case, there should be a graph available from the battery maker showing actual capacity against discharge current. Not all battery suppliers give this data in the same way, and some don’t give it at all, so it can be difficult to compare one battery against another. Batteries designed for automotive use sometimes have their capacity rated in “reserve minutes” meaning minutes at a constant load current, which can be converted into a figure in Ah. If in doubt, contact the battery manufacturer, and ask for clarification. For an extreme example, if a battery is discharged in a half an hour, it will usually provide only half of it’s rated 20 hour Ah rating. Thus, a good quality 50Ah battery, fully charged and in good condition, will supply a load that draws 50 Amps, for only 30 minutes. The same battery, would supply a load that draws 1 Amp, for 50 Hours, or a load that supplies 2 Amps, for 25 hours. So, it’s essential, if discharging at high rates, to consult the battery supplier’s data, to determine the actual battery discharge capacity at the load current you are using. The load current should be measured or calculated accurately, since the load current determines the size and cost of the battery and charger. In cases where the battery load will vary during the discharge (for example, when driving a motor, where current is proportional to torque) the calculations can get complicated, as the battery de-rating factor should be applied to each value of load current. Another issue is the end of discharge voltage, which again varies with discharge current. Batteries are specified in Ah to a given end point voltage, typically 1.5 Volts per cell, or 9 Volts per nominal 12V pack of 6 cells, and this may not be enough voltage to drive the load properly, in which case the discharge time must be de-rated. Another factor to consider is how conservative the battery supplier is with their specifications. The battery industry is very competitive, and there are some manufacturers who claim maximum or optimal figures for their battery, while others may give minimum or guaranteed figures. Brand new batteries direct from the factory, will typically have about a 10%reduced capacity for the first few cycles of discharge as the plates are not fully “formed” when the battery is new. Some makers allow for this in their figures for Ah capacity, others do not. It’ certainly advisable, in any case, for you to test your system (Charger, battery, and load) extensively as part of the design process, to collect your own data and base your claims to your own customers, on that.

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  1. “How long will it take for a certain current rating charger to fully recharge a given size battery?”


This can be calculated approximately as follows. The recharge time in hours equals the battery capacity in Ah, multiplied by the Depth of Discharge in %, multiplied by 0.8, multiplied by 1.5, divided by 100 times the charger current rating in Amps, plus one hour. For example, a 55Ah battery, discharged to 80%, on a 6-Amp charger, would take about 9.8 hours. A 110 Ah battery, discharged to 50%, on a 10 Amp charger, would take about7.6 Hours. The battery reaches 80% recharge relatively quickly, the last 20% of the charge is done in constant voltage mode where the current is dropping exponentially, so it is charging more slowly, this is the reason for the 1.5 factor and the plus one hour constant. Our chargers usually provide an indication when the 80% level and switch to CV mode has been reached (either an indicator Led marked 80%, or the Charge Led starts to flash) showing that the battery could be used at this point, with some loss of run time. At the end of the charge cycle, the Green Ready Led will show that the battery is ready for use. It’s recommended to leave the charger connected and switched on, if possible, even after the green Led shows, as the charger is still supplying a small current in standby mode, which tops off the charging process. The heating effect on the battery is proportional to the square of the charge current, while the recharge time is inversely proportional to the linear value of the charge current.

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  1. What’s the difference between a Sealed Lead Acid battery, a Gel Cell Battery, an Absorbed Electrolyte Battery, a Valve Regulated Battery, and why does it matter?”


These are many different types of Lead-Acid rechargeable battery, and there is some confusion. Quite often customers refer to a battery as a “Gel Cell”, when in fact it’s another type of SLA battery. There is not much difference in discharge performance, but there is often a difference in recharge voltage limit. Sealed Lead Acid is a generic term for all lead acid batteries which have fixed tops, so the electrolyte is supplied with the battery when it’s manufactured, and it’s not intended that the battery ever be opened or topped up in the field. These are also sometimes known as “maintainance free” batteries. Sealed Lead Acid (SLA) has become a popular generic term, and is widely used in the industry. It’s actually a rather misleading term, since all lead acid batteries must have vents to allow any excess gas pressure to escape from the battery casing, especially if cells become overcharged under fault conditions such as a shorted cell. Lead acid batteries should, in general, never be charged in a completely sealed cabinet or enclosure, for this reason. The terms “Valve regulated battery” or “Recombinant battery” which some makers (more correctly) use instead of SLA, but which do not seem to be very widely used. All types Valve regulated. or Recombinant batteries, normally release very little or no gas during charge and discharge, as they are designed to operate with a small positive gas pressure inside the battery casing. These SLA battery types can be further divided into “gel electrolyte” and “absorbed electrolyte” types. The Gel cells have the acid electrolyte in the form of a gel, the absorbed electrolyte type have the acid in liquid form, trapped in a glass fibre mat between the plates. Absorbed electrolyte batteries are also sometimes called AGM (Absorbed Glass Mat) batteries. A possible advantage of a gel electrolyte may be that if the battery plastic casing is damaged in transit or in an accident, the electrolyte is not in liquid form and can’t run out of the battery and cause further damage or corrosion. But a disadvantage may be that some gel batteries are more easily damaged by overcharging, because gas bubbles form in the gel and may push the electrolyte away from the plate surface, permanently reducing the capacity. In all cases, it’s advisable to check the battery manufacturer’s spec for the recommended constant voltage charging voltage range, and check that the charger is set within that range, to provide the best performance with the type of battery used in the application. Usually battery makers specify two settings for the charge voltage limit, a higher value for Cyclic (short term charging) and a lower value for Float (long term charging). Usually cyclic setting is around 2.45 Volts per cell (14.7V on a 12V battery), and the float setting is around 2.3 V/Cell (13.8v on a 12V battery). Our chargers use both of these settings (V-Lim1 and V-Lim 2 settings) to provide both fast recharge and long term maintainance charge.

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  1. Important safety and liability issues – please read this first”.

WARNING: SAFETY CONCERNS.

These FAQ notes are intended for use of suitably qualified persons only. This page is provided for free, in good faith, on an “as-is” basis, by and on behalf of Dektron Ltd. The answers given are BASED UPON EXPERIENCE AND ARE EFFECTIVE, to the best of our belief. However, the reader is responsible for verifying these points, by checking with the battery charger manufacturer, and the battery manufacturer, and for testing their system, to ensure that the systems they design and supply to their customers are safe and reliable. Nothing in this FAQ page should be relied upon to contradict information available elsewhere.
Battery chargers are safe and effective if used correctly and in accordance with the supplied instructions. However, repairs and adjustments should only be carried out by suitably trained technicians or engineers. If you are in any doubt, or if you are not suitably trained, you should not operate the unit with any covers or screws removed, or make any internal adjustments or modifications, or operate the unit in any way other than as set out in the instructions supplied with it. The safety aspects to consider when working on battery chargers include, but are not limited to, danger of electric shock from the AC input and primary circuits, danger of burns or fire from short circuits or poorly made connections in the high current DC output battery circuits, and danger of explosions due to spark ignition of hydrogen gas produced by the battery when charging.

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If you have any suggestions for improvement to this page, or any questions on the topic that are not covered here, you are welcome to contact the author by email on phil@dektron.com

Revision 1.06, September 2005. File name: chgr_faq.htm