Frequently Asked Questions
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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.