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Battery Balancer


The problem: the service life of an expensive battery bank can be substantially shortened due to state of charge unbalance


One battery with a slightly higher internal leakage current in a 24V or 48V bank of several series/parallel connected batteries will cause undercharge of that battery and parallel connected batteries, and overcharge of the series connected batteries. Moreover, when new cells or batteries are connected in series, they should all have the same initial state of charge. Small differences will be ironed out during absorption or equalize charging, but large differences will result in damage due to excessive gassing (caused by overcharging) of the batteries with the higher initial state of charge and sulphation (caused by undercharging) of the batteries with the lower initial state of charge.


The Solution: battery balancing


The Battery Balancer equalizes the state of charge of two series connected 12V batteries, or of several parallel strings of series connected batteries.

When the charge voltage of a 24V battery system increases to more than 27,3V, the Battery Balancer will turn on and compare the voltage over the two series connected batteries. The Battery Balancer will draw a current of up to 0,7A from the battery (or parallel connected batteries) with the highest voltage. The resulting charge current differential will ensure that all batteries will converge to the same state of charge.


If needed, several balancers can be paralleled.

A 48V battery bank can be balanced with three Battery Balancers.


LED indicators

Green: on (battery voltage > 27,3V)

Orange: lower battery leg active (deviation > 0,1V)

Orange: upper battery leg active (deviation > 0,1V)

Red: alarm (deviation > 0,2V). Remains on until the deviation has reduced to less than 0,14V, or until system voltage drops to less thVa.n 26,6V.


Alarm relay

Normally open. The alarm relay closes when the red LED switches on and opens when the red LED switches off.


Alarm reset

Two terminals are available to connect a push button. Interconnecting the two terminals resets the relay.

The reset condition will remain active until the alarm is over. Thereafter the relay will close again when a new alarm occurs.


Even more insight and control with the midpoint monitoring function of the BMV-702 Battery Monitor The BMV-702 measures


the midpoint of a string of cells or batteries. It displays the deviation from the ideal midpoint in volts or p. ercent. Separate deviation percentages can be set to trigger a visual/audible alarm and to close a potential free relay contact for remote alarm purposes.

Please see the manual of the BMV-702 for more information about battery balancing.


Learn more about batteries and battery charging

To learn more about batteries and charging batteries, please refer to our book ‘Energy Unlimited’ (available free of charge from Fangpusun and downloadable from


Battery Balancer connected to two series connected 12V batteries (24V system)


Three Battery Balancers connected to four series connected 12V batteries (48V system)

Battery Balancer

Input voltage range

Up to 18V per battery, 36V total

Turn on level

27,3V +/- 1%

Turn off level

26,6V +/- 1%

Current draw when off

0,7 mA

Midpoint deviation to start balancing

50 mV

ncing 50 mV Maximum balancing current

0,7A (when deviation > 100mV)

Alarm trigger level

200 mV

Alarm reset level

140 mV

Alarm relay

60V / 1A normally open

Alarm relay reset

Two terminals to connect a push button

Over temperature protection


Operating temperature

-30 t0 +50°C

Humidity (non-condensing)




Blue (RAL 5012)

Connection terminals

Screw terminals 6 mm² / AWG10

Protection category



0,4 kg

Dimensions (h x w x d)

100 x 113 x 47 mm



EN 60950


EN 61000-6-3, EN 55014-1


EN 61000-6-2, EN 61000-6-1, EN 55014-2

Automotive Directive

EN 50498



Battery Balancer connected to six series-parallel connected 12V batteries (24V system)



1) The Battery Balancer(s) must be installed on a well -ventilated vertical surface close to the batteries (but, due to possible corrosive gas ses, not above the batteries !)

2) 2) In case of series -parallel connection, the midpoint

interconnecting cables must besized to at least carry the current that arises when one battery becomes ope n-circuited.

- In case of 2 parallel strings: cross section 50% of the series interconnecting cables.

- In case of 3 parallel strings: cross section 33% of the series interconnecting cables, etc.

3)  If required: first wire the alarm contact and the alarm reset.

4)  Use at least 0,75 mm? to wire the negative, positive and midpoint connections (in this order).

5)  The balancer is operational.

When the voltage over a string of two batteries is les s than 26,6V the balancer switches to standby and all LEDs will be off.

When the voltage over a string of two batteries increases to more than 27,3V (during charging) the green LED will turn on, indicating that the balancer is on.

When on, a voltage deviation of more than 50 mV will start the balancing process and at 100 mV one of the two orange LED s will turn on. A deviation of more than 200 mV will trigger the alarm relay.


What to do in case of an alarm during charging

In case of a new battery bank the alarm is probably due to differences in initial state-of-charge. If the difference between the lowest and highest battery voltage reading is more than 0,9V: stop charging and charge the individual batteries or cells separately first, or reduce charge current substantially and allow the batteries to equalize over time.


If the problem persists after several charge-discharge cycles:

a) In case of series-parallel connection disconnect the midpoint parallel connection wiring and measure the individual midpoint vol tages during absorption charge to isolate batteries or cells which need additional charging, or:

b) Charge and then test all batteries or cells individually or:

c) Connect two or more battery balancers in parallel (on average one balancer will take care of up to three parallel 200 Ah strings).


In case of an older battery bank which has performed well in the past, the problem may be due to:

d)  Systematic undercharge: more frequent charging needed (VRLA batteries), or equalization charge needed (flooded deep cycle flat plate or OPzS batteries). Better and regular charging will solve the problem.

e) One or more faulty cells: replace all batteries.


Three Battery Balancers connected to 12 series-parallel connected 12V batteries (48V system)


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