Battery Balancers

Posted by admin May 12th, 2010

Lee Hart’s Shunt Type Battery Balancer
12 V Version –

Each battery has a regulator which consists of

2 – ring terminals, with hole for 5/16″ bolt, and crimp for #6 wire
2 – 6.8v 5watt zener diodes
2 – #PR2 flashlight bulb
Solder a zener diode into each ring terminal where the wire normally goes. Solder a 6″ piece of wire to the other end of the zener diode. Solder the flashlight bulb between the free ends of these two wires. Now you have the two zeners and flashlight bulb all wired in series.

Lee added on 8/09 – Since there are two zeners in series, and they have a 5% tolerance, I measure their actual zener voltage, and match them up in pairs with the same total. I have a bench power supply with knobs to set the voltage limit and current limit. I set the current limit to some value like 100ma, and set the voltage limit higher than the zener voltage (like 10v for a 6.8v zener). Connect the zener, and the power supply automatically supplies a fixed 100ma. Measure the voltage across each zener. Sort them into bins (6.5v 6.6v 6.7v 6.8v 6.9v 7.0v 7.1v). Use pairs that add up to the same total
6.5v + 7.1v = 13.6v
6.6v + 7.0v = 13.6v
6.7v + 6.9v = 13.6v
6.8v + 6.8v = 13.6v
Fill the space between the zener and ring terminal with epoxy glue. Likewise, dunk the bulb and its wires in epoxy glue. This makes everything waterproof and acid proof, and helps conduct heat. The zeners get their heatsinking from the large ring terminals and battery posts they are bolted to.

The diodes are all sorted based on the voltages.

The bulb is soldered to the wire and the lead of the diode.

The whole thing is wrapped in heat shrink tubing.  Each end is the same except the diode is reversed.  The positive ends are marked with a red marker.

One of these is made for each battery.

As the pack charges the lights slowly begin to light up on the batteries that reach capacity.  This shunts about a half an amp around that battery to the next which helps to balance the batteries out.  Each time you charge the pack the batteries should get closer and closer.

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