Minimalist Battery Regulators
by Lee Hart, with modifications by Cor Van de Water
Batteries in a long series string often become imbalanced, with some staying at higher states of charge than others. This is partly because of manufacturing variations among batteries, and partly because multiple batteries in an EV are always at different temperatures. Unless you find a way to equalize or balance the batteries, over time, they drift further and further apart. The lowest batteries can begin to develop sulfation from chronic undercharging, and there's a risk you'll reverse them as you discharge the pack. An unequalized pack will usually have a short life.

With flooded batteries, you can safely equalize by overcharging a bit - the batteries just use more water. However, you can't overcharge valve regulated (sealed) batteries the same way.

Lee Hart designed these simple, inexpensive bypass regulators to help prevent battery imbalance. They'll help flooded batteries too, but Lee mainly meant them for valve regulated batteries.

Lee says, "There are hundreds of EVs out there with [these regulators]. It's one of those KISS (Keep It Simple, Stupid) designs that no one could make money from, because it's too easy to make yourself. The whole idea was to provide a budget battery regulator, so even cheapskates wouldn't have an excuse to murder their battery packs because a real BMS (battery management system) would cost them too much."

How They Work: These regulators use a #PR2 lamp (0.5 amp flashlight bulb) as the current limiter. I chose the zeners shown so they begin to bypass as soon as the battery goes above a float voltage and you are into the finishing or equalizing phase of your charging cycle. The batteries are likely to spend hours in this region as they are slowly brought from 80% to 100% SOC.

I use a lamp rather than a resistor because lamps behave more like a current source. The PR2 lamp draws 0.5amp at 2.38v, but 0.2amp at 0.238v. In other words, a 10:1 change in voltage only causes a little more than 2:1 change in current. Although the lamps aren't likely to ever burn out - they spend most of the time off or very dim - a lamp could get broken or otherwise fail. So I add a 10 ohm resistor in parallel with the lamp. If something happens to the lamp, the regulator will still work, though not as well.

I use two zeners rather than one for several reasons.

The zeners are encased in large (#6 wire) copper lugs and potted with thermally conductive epoxy. This helps them dissipate their heat. Some examples of thermally conductive epoxy are Aavid TherObond and Thermalbond, MG Chemical 832TC (thermally conductive), and 3M TC-2707 and TC-2810. These are expensive, however. A thermally conductive epoxy is really just normal epoxy with a more heat conductive filler material, like silica (sand), gypsum (talcum powder), carbon (graphite), or metal dust (zinc, silver, etc.) JB Weld is a common example (it is zinc dust filled).

These regulators draw essentially zero current (less than 1ma) below 13v, so they won't run down a fully charged battery no matter how long you leave them connected. They only conduct when you are charging and the voltage is high.

The whole idea of these regulators is that they don't suddenly switch on at some voltage (i.e. try to reduce the charge rate of the fullest battery to zero). Instead, they gradually taper on as the battery voltage rises above the gassing point. The battery that gets there first starts having some of its charging current bypassed, so it is getting fewer amp-hours. The less-charged batteries keep getting 100% of the current you are applying.

As a rule, the voltage goes above float voltage (13.2 to 13.8v, depending on the battery's design) when the battery is about 60-80% charged. You then keep charging it for several hours, until the current falls to 2%-4% of its amphour capacity. During this time, these regulators are creating up to a 0.5amp difference in charging currents between batteries. Two hours at 0.5a is 1amp-hour, so they can compensate for a difference of 1ah or so between batteries.

This has so far been fine for good batteries in reasonable condition; it is enough to hold them in balance. But if your pack were seriously out of balance, you'd have to set your charger for a lower float voltage, and leave it on for days. Then these regulators would eventually nudge the batteries back to the same voltage.

These regulators can only bypass up to 500ma. They can't do much if the charger insists on continuing to charge at high current at high voltage. For them to balance the battery pack properly, you need some mechanism to get the charger below 1 amp when the pack is fully charged.

To do that, I use an inexpensive children's night light to sense light inside the battery box. It operates a relay to turn the charger off. It could also be set up to reduce the charger output to a very low level.

If you don't have any way to shut off or slow down the charger, use CM43 lamps instead of PR2 lamps. A #43 is a tougher part, and will last longer if the regulators are forced to run at higher currents and for longer periods of time.

Lee Hart Battery Equalizers

Schematic courtesy of Mark Hanson
Parts List: (per battery)

2 - heavy duty copper 5/16" ring terminals, for 6 gauge wire (Waytek 36472)
2 - 6" lengths of #18 or so insulated wire
1 - 6.8v 5W zener diode, type 1N5342B1
1 - 6.2v 5W zener diode, type 1N5341B1
1 - PR2 lamp, 2.38v 0.5a, Mouser 606-PR2; or CM43 lamp, 2.5v 0.5a, Mouser 606-CM432
1 - 10 ohm 1/2W resistor
Heat shrink tubing to fit the barrels of the terminals
Thermally conductive epoxy filler (available from electronics vendors)

Note 1: The zener types and values shown above are for typical 12 volt AGM batteries, such as Hawkers, or for flooded 12 volt batteries.

Note 2: CM43 lamps cost about 75% more than PR2 lamps, but are more rugged. See the paragraph just above the schematic to decide which you should use.


  1. Drill a small hole in the ring terminal.
  2. Slide the proper wire from a zener diode through this hole, until the zener body is where the wire would normally go.
  3. Solder the zener connection.
  4. Solder a 6" piece of wire to the other end of the zener diode.
  5. Slide a piece of heatshrink tubing over the terminal barrel.
  6. Repeat for the other ring terminal, using the other zener's opposite lead wire.
  7. Solder the flashlight bulb and resistor between the free ends of the two zener wires.
  8. Fill the space in the ring terminal barrels, around the zeners, with the thermal epoxy.
  9. Shrink the tubing on each terminal to squeeze out the excess epoxy.
  10. Mark the battery positive terminal (see schematic), so you can identify it when you intall the regulator.
  11. Dunk the bulb, resistor, and wires in the epoxy.
Performance: Measured for two 1N5342B 6.8v 5w zeners in series with a PR2 lamp, with a 10ohm 1W resistor in parallel with the lamp:

13.85v at 1ma
14.2v at 10ma
14.5v at 100ma
14.6v at 200ma
15.6v at 500ma
16.6v at 730ma

At 15.6v and 500ma, each zener has about 7v across it. 7v x 0.5a = 3.4 watts, which is OK for a 5w zener. The lamp has 15.6v - (2 x 7v) = 1.6v across it, which is fine for a 2.38v lamp. You need to exceed 16.6v to reach rated voltage on the lamp and 5w on the zener.

With one 6.2v and one 6.8v zener, reduce the voltages by 0.6v. With two 6.2v zeners, reduce the voltages by 1.2v.

Modification: Cor Van de Water's design (below) replaces the lamp and 10 ohm, 1/2 Watt resistor with a 4 ohm, 2 Watt resistor; a 56 ohm, 1/8 watt resistor; and an LED. This eliminates the risk of damage or burnout to the lamp, at the expense of losing the lamp's more nearly constant current characteristics. Cor says this makes the design more robust and will help to avoid possible battery damage in the event of a lamp failure.

On the failure of any component, the circuit stops drawing current from the battery to avoid draining it. Should a zener diode fail shorted, the resistors will act as fuses and open to prevent fire hazard.

Lee Hart Battery Equalizers, modifications by Cor Van de Water

Schematic courtesy of Cor Van de Water

This circuit can be assembled the same way you would build the original regulators (see above).