Frequently Asked Questions
–
If
you have a technical question relating to Battery Chargers for Lead
acid or
Safety
and liability issues -
please read this first.
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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.
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.
Back to top.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.