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EV Circuits: The Zener-Lamp Shunt Regulator for Lead-Acid Batteries

The Batt-Bridge tells you that some batteries in your pack are less charged than others. With flooded batteries you can overcharge to bring up the low batteries, and replace the water lost in the ones that were already full. But if you overcharge sealed batteries, it shortens their life.

The Zener-Lamp regulator is intended for sealed 12v lead-acid batteries (though it can be adapted for other types). It bypasses excess charging current on full batteries, so the weaker ones can finish charging without overcharging those that are already full. It's a budget regulator, so even cheapskates won't have an excuse to murder their batteries because a real BMS (Battery Management System) would cost them too much.

How it works: Two zener diodes set the voltage above which it bypasses current. 6.2v and 6.8v zeners will bypass above 13v (full charge for a 12v battery). A #PR2 or #43 2.5v 0.5a lamp limits the current, provides a visual indication when it is bypassing, and acts as a fuse in case of overvoltage, miswiring, or component failures. The 10 ohm resistor is a backup in case the lamp fails.

Zener-Lamp Regulator Photo
Construction: The zeners get hot! Mount them in large copper ring terminals for #6 wire. Fill the body with a thermally conductive epoxy (like JB Weld) to transfer the heat to the battery terminals. Solder the lamp and resistor in parallel. Connect them to the other ends of the zeners with short wires. Cover the connections with heat shrink tubing and epoxy to waterproof them.

Use: Near the end of a charge cycle, the lamps will begin to light. Set up your charger to be at a low current (like 0.5 amp) at this time. At this current, each hour you charge puts an extra 0.5 amphours into the low batteries whose lamps aren't lit. The pack is balanced when all the lights are glowing to some degree. This may take many hours on an out-of-balance pack. Once balanced, subsequent charges should take under an hour to reach this state.