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Battery Management System

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A Battery Management System (BMS) is some sort of system for managing the health of the many batteries in an electric vehicle. A BMS can range from a human with a multi-meter, to more automated regulation or power shutteling system. The Battery charger is generally considered a seperate device though it will most likely need to intigrate with whichever BMS is used.


Battery heating is a common practice in cold climates, while the car is plugged in for charging you can keep your batteries warm with some sort of AC heating element. Once you're driving the batteries tend to warm up anyway so there's usually no need for external heating while under way.


The most affordable BMS is a human being, a meter, and a charger! :-)

You just measure each battery's voltage, compare them, and if any are low, charge them a little bit with a small separate charger. Depending on how much you drive, and how good your batteries are, you may have to do this monthly, weekly, or daily! Obviously, this gets to be tedious.

The next step up is to add ways to make this process easier. It could be a single connector or terminal block in a central location, with wires to each battery. It could be a rotary switch and voltmeter. It could be a separate little meter or bargraph per battery, so you can read them without connectors or switches.


Next step up is to automate it somehow. The crudest automatic system is my zener-lamp regulator. It's a couple zener diodes and a light bulb across each 12v battery. If the voltage gets too high, the zeners conduct, the lamp lights, and it bypasses charging current to push the batteries toward balance. These are $5-$10 per battery.

From there, it all depends on how fancy you want to get. Rich Rudman sells regulators for about $40/battery that can bypass more current, provide indications of over- and under-voltage batteries, over-temperature, etc.

BatPro & Rudman Regs

Samples were available to me of both the BattPro regulator designed by Mark Hansen and sold by Wilde Evolutions and the Rudman Regulator designed by Joe Smalley of Manzanita Micro is sold by Rich Rudman, E-Car and others. Since my batteries are placed in tight spacing, and with some on their sides, it was not practical to use the + battery post as the heat sink for the Reg as required by the BattPro. Also living in California with no freezing temperature my battery cases are open to the weather under the hood and in the rear trunk area where the gas tank used to be. Also I wanted to see all the LEDs at once. The blinking LED easier to see on the Rudman than the gradually increasing intensity glow of the BattPro.

At a electronic surplus store I found an IBM accessory box and mounted all the Rudman Regs in it. A fused lead from each battery junction connected through a multi-contact plug and socket connects to the regs. A pair of wires from each Reg then go to an external 5 ohm 25 watt resistor on a common heat sink. For final equalization battery charge every 2 weeks or so the cable connector can be disconnected to remove the regs from each battery and then fully equalized Optimas charged according to Optima recommendations for the final full charge of 2 A for 3 hours. I will not do this for more than 1/2 hours so as not to over gas any of the sealed batteries.

In operation the Rudman Regs operate only while the LED is flashing slowly at first, then bypassing that battery by sending the current through the other Regs through the on board power resistor and the heat sink and the external shunt to next batteries in the series string. Rich Rudman says 'You don't want the LEDs to almost lock full are driving the Regs beyond their bypassing abilities. A good fast flash is ok, about once a second. While the Regs LED is off ALL the power is going through that one battery, and no current is being bypassed. When the batteries voltage rises to the Regs trip point then the Reg flashes on to bypass another pulse. '

The Charger should reduce it's charge current when the entire strings voltage is just at the combined Regs set should trickle charge at this voltage, and not over amp the batteries and the Regs. If the Regs are no longer blinking...they are no longer regulating...they must blink or else you battery voltage will be above 15.5...or about 1 volt over their set point. With a loadless Regs and 5 ohms of bypass on a external heat sink, the Regs are safe from overload...but they can't keep the battery voltage under control if you drive them to full on. To test this put a volt meter across a Reg protected battery and watch the voltage as the Regs start to work... Once you have them blinking...if the charge current is still to high for the Regs..the voltage will stay in the regulation voltage area for a while and then the blinking will increase in rate and duration, at a certain point the flashing will increase and the the voltage will then start to rise again. At this point reduce your charge current, or double up your 5 Ohm external load banks. The stock Regs will stand about 10 amps of continuous current...or 120 watts of waste heat. This would take 4 or 5, 5 ohm 25 watt resistors. Better yet crank down your charger to stay below 2 amps of finished float charge current.'

The charger must be set to turn off just before all the LEDS are full on and no longer blinking. That is exactly the way my 220 Vac Zivan charger operates. l am beta-testing a RUSSCO separate on-board 110 Vac for charging on the road when 220 Vac is not available.

Zivan Smoother

Greg McCrea, U.S. distributor for Italian-made Zapi controllers and Zivan chargers, at Electric Conversions in Sacramento CA. My test is on my 20 Optima YTs buddy pairs at 120 VDC. I can tell you that I am very pleased with its performance. I have moved the Smoother display and control panel from the under hood box and mounted it in the speaker opening on the dashboard so I can watch the equalizing process directly

I like the price on the Smoother, about 1/3 of the Badcheq. I have seen the Badicheq but do not know enough of the details to compare it to the Smoother. The Smoother really works as promised and I am going to keep it.

I have SPDT toggle switch on the dash to change the Smoother operation to the 5 minute period on Drive and 30 minute on Charge. Normally this command is automatic by wiring the 12 VDC supply thru the ignition switch for Drive or for Charge through a two wire cable from a relay contact in the Zivan NG3 charger. Since I have two separate chargers, one 120 Vac and 240 V, I will have to a have a relay on the Russo also.

I have an analog ammeter on the dash which I usually drive by that indicated pulse charge by the NG3 near the end of the regular charge and during finish charge. Pulses are at about one second which explains why the E-Meter is reading charge current at 0, 2.5, 5.6 A and points in between depending on the instant of of the sampling.

Individual battery regulation occurs for each buddy pair separately, one at a time (or for each battery if in a full series string) This 5 A equalizing current, in my case, is added to the regular charging current and is switched between each pair depending on the battery voltage test at the start of each time period. The charge current is provided by the Smoother by a 120 VDC/12 VDC converter and switched to the individual batteries one at a time, depending on which ones are in the lower half of the voltage check.

The time period determines when the next regulating voltage will be measured. If all the batteries are with 0.5 volt the system decides not to operate and shows the Wait Phase LED.

The Italians designed the Smoother box with no visible means of attachment to the EV as if it was a VCR sitting at home. I did mount mine very neatly by installing thin metal mounting straps between the rubber feet and box to fasten to the top of the battery rack hold downs.


  • Minimum voltage input 80 V Nominal
  • Max voltage input 540 V (for 55 Ahr capacity AGMs)
  • Max number of 12 V batteries or buddy pairs - 32
  • Internal DC-DC converts 75 W at 5 A max
  • Red LED shows polarity reversal
  • Current sensor to 300 A
  • Baud rate RS-232 9600 Baud
  • Dimensions 11 13/16 x 10 5/8 x 4 inches.

It transmits V, A, and unit number, number of individual times charged during that drive or charge period, to my laptop via RS-232 making maintenance more simple by providing data which can be converted into graphs showing immediately imbalance of the batteries. It also displays the weakest batteries each time the unit is activated for those installations without data dumping capacity.

The Smoother sells for $950 for a 120 VDC battery pack, $10 extra for each 12 V battery relay circuit added.


My (Lee Hart) Battery Balancer is an elaborate system that uses a microcomputer to monitor every battery, and charge it as needed to keep them all in balance. It will cost you around $1000, and is open source (see it at

Battery Management System Validation and Testing

Fully testing and validating the operation of a BMS (specifically advanced BMS designs used for electric vehicle battery packs), usually involves simulating and controlling all inputs the BMS encounters during normal vehicle operation, subjecting it to any possible conditions it would encounter in the real world, and making sure it responds correctly in all cases. All required functions of the BMS (monitoring cell voltages, cell temperatures, pack voltages, and pack temperatures, performing cell balancing operations, reporting diagnostic information over interfaces such as CAN to external control systems, etc), can be fully verified using a Hardware in the Loop (HWIL / HIL) style of testing. An example of this approach can be found here:

See Also