I
f you’re in the market for a 24V battery then you have
probably discovered that it costs a lot more than two
12V batteries. The obvious solution is two 12V batteries
in series, but as our introduction explains, that’s a solution
that brings a new set of problems. Fortunately, an economi-
cal answer is within your reach – a ‘battery balancer’.
Our version can’t handle quite as much current as some
commercial designs because it lacks a large heatsink, but
you can easily parallel several of our balancers if you
need a higher current capacity and the cost would still
be quite reasonable.
It can be used with pretty much any battery chemistry,
as long as the battery voltages will stay within the range
of 5-16V.
Balancing is most critical with lithium-based rechargeable
batteries because they tolerate overcharging much less than
a similar lead-acid battery would.
This design incorporates a low-voltage cut-out which
prevents the batteries being discharged too far if it is un-
able to keep them balanced and its very low quiescent
current of under 0.02mA means it will have virtually no
effect on battery life.
It also incorporates an LED to show when it is monitoring
the battery voltages and two more LEDs to show when one
or the other is being discharged or shunted.
By default, the low-voltage cut-out is set up so that the
batteries are only balanced when they are being charged.
However, there are defi nitely situations where you might
also want the batteries to be balanced during discharge.
In that case, you just need to change a resistor or two
in order to adjust the cut-out threshold so it is near the
minimum battery voltage. In this case, the cut-out will still
act to protect the batteries but will allow balancing during
charging and discharge, right down to that lower threshold.
It’s a compact unit at just 31.5 × 34.5 × 13mm, so you
can tuck it away inside just about any device. And if the
300mA balancing current is not suffi cient for your purposes,
all you need to do is wire two or more units in parallel and
they will operate in concert to keep the batteries balanced.Balancing operation
There are two sections to the circuit; the bal-
ancer and the low-battery cut-out. The entire
circuit is shown in Fig.1, with the balancing
circuitry in the top half and the low-voltage
cut-out below.
Starting with the balancing section, schottky
diodes D1 and D2 are connected in series with
the two batteries so that no damage should occur if
they are wired up incorrectly. These diodes are then con-
nected to MOSFETs Q1a and Q2b at the right-hand side of
the circuit diagram, via a pair of 27Ω 3W resistors.
These MOSFETs are normally switched off and no current
can fl ow through them. If the voltage across one battery2× 12V
Battery
Balancer
By Nicholas Vinen
Two 12V batteries are often signifi cantly cheaper than one equivalent 24V
battery, but you need to be careful connecting batteries in series as their
voltages and state-of-charge may not be identical. The difference in voltage
can increase over time, leading to battery damage from over-charging and/
or under-charging. This compact, low-cost device keeps them balanced so
that they last a long time.
Look after your lithiums!
can tuck it away inside just about any device. And if the
300mA balancing current is not suffi cient for your purposes,
all you need to do is wire two or more units in parallel and
they will operate in concert to keep the batteries balanced.Balancing operation
There are two sections to the circuit; the bal-
ancer and the low-battery cut-out. The entirediodes D1 and D2 are connected in series with
the two batteries so that no damage should occur if
they are wired up incorrectly. These diodes are then con-
nected to MOSFETs Q1a and Q2b at the right-hand side ofShown rather signifi cantly oversize for clarity (the PCB
measures only 31.5 x 34.5mm) – see the coin for reference –
all components mount on this single board.