From http://www.cameronsoftware.com/ev/EV_BatteryManagementSystems.html

Here is one possibility [for a shunt type battery balancer - ed]. Strangely enough, it is currently in my own EV, since I sold its Balancer to Rick Woodbury.

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
• 1 - #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.

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.

Put one of these circuits across every 12v battery. If the charging voltage exceeds about 13.6v, the zeners begin to conduct and the lamp lights. At about 15v, the lamp is fully lit and is bypassing about 0.5 amps.

Use a photocell to detect the light being produced. When any of these battery regulators lights, the photocell sees it and either switches the charger off, or to a low-current trickle charge or float voltage setting.

6 V Version -

Comments and an ansii schematic by Lee (5/06) -

I published the version for 6v batteries a year or so ago. Basically, I suggested the same parts (two 6.2v zener diodes), but with *two* #PR2 lamps. Each lamp has a zener in series. This makes two parallel paths, so it shunts twice the current -- 1 amp max instead of 0.5 amp max. This is more appropriate for >100 amphour batteries.

One zener and one lamp in series, wired across the 6v battery. Use two such regulators across each battery. Physically, you'd put one zener in the ring terminal on the + battery terminal, and the other zener in the ring terminal on the - battery terminal. Like this:

If the 6.8v zener lets the voltage get too high (gel cells, for instance) try a 6.2v instead.

You want a zener that draws negligible current when you aren't charging. The fully charged voltage for lead-acid is around 2.1v/cell, which is 6.3v for a 6v battery, 8.4v for an 8v battery, or 12.6v for a 12v battery.  AGMs tend to be a little higher, and floodeds can tolerate a little more. [The lamp]  provides a roughly constant current once the zeners start to conduct.

I used a pair because two 5w zeners are cheaper than a 10w zener. Also, I could put one in each battery terminal, to split the heat between them rather than concentrating it all at one point. Also (I never do anything for one reason :-) because the temperature coefficient of zeners around 7v is roughly zero.

The resistors act like just a backup system. Someday the bulb may burn out or get broken. The resistor means you don't lose *all* regulation.

Your charger has to be smart enough not to charge excessively. The charger has to be at a low current when the regulators begin working, and has to shut off after a relatively short time. You can do this "open loop" by arranging the charger so it automatically delivers low current for a short time. For example, a transformer-rectifier charger that is adjusted to deliver <1 amp at 2.5v/cell, and has a timer to turn itself off in 1-2 hours.

Or, you can do it "closed loop", where the light from the regulators commands the charger to cut back and turn off.

This is a parts list by Chris Zach, parts from Mouser

• 606-PR2, PR2 Chicago Miniature Incandescent Base Flange 2.38V .5A
• 526-1N5341B, 1N5341B NTE replacement Diodes/Rectifiers 6.2V 5W ZENER DIODE
• 610-CZ5342B, CZ5342B Central Semi Diodes - Zener 6.8V 5.0W
• 660-MF1/2DLT52R10R0F, MF1/2DLT52R10R0F KOA Speer 1/2Watt Metal Film Resistors 10ohms 1% 100PPM

And of course you need potting compound:

• 1 590-832-TC-450ML Epoxy Compound 1 1 0 $29.95

The smaller lugs are standard Home Depot 12-10 gauge yellow lugs; the smaller diode fits in them. For the larger one I used heavy-duty 6ga tinned eyelets, with a 5/16" bolt hole, from Waytek Wire, part# 36472.

Here is a link to a PDF that shows the steps in making the Zener regulator

A light bulb works well as a current limiter, in case there's a shorted cell. The initial current will probably be quite low, but it will g-r-a-d-u-a-l-l-y rise as the battery moves away from 100% dead. Note that it may take DAYS for the current to stop rising.
 

Once the current stops rising, remove this high-voltage current-limited charger. Wait a while, and check the open circuit battery voltage. If it's around 12v or more, it's OK -- continue charging with a normal 12v charger. If it hasn't come up to at least 12v, you probably have a shorted cell -- the battery is shot.

Lee Hart - EVList, 6/07/06