Cameron Motor Works Electric Vehicle Conversion

Battery Management Systems

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January 2004

Battery management systems are roughly broken into two categories:

  1. Circuits that monitor the batteries (battery monitors)
  2. Circuits that monitor the batteries and somehow adjust the charge going into a battery (battery balancers).

Battery Monitors

Battery monitors are very simple in the data collected, but can be quite complex in how the data is displayed to the driver. The data that is collected is typically:

  • the temperature of each battery or pack
  • the voltage of the pack
  • the amperage of charge or discharge

Battery Balancers

Since one or more batteries in a series string of batteries can get lower than the other batteries, it is important to ensure this weaker battery get extra charging. This will ensure logner life of the pack, as the weaker battery will drag down the life of the other batteries and kill the pack early.

Battery balancers, a.k.a. equalizers attempt to adjust the charge going into the batteries. There are several types of battery balancers, each with their own technique:

Shunt Type Balancer

Shunt type balancers go on each battery. If the voltage gets too high, the shunt activates and directs the current through a load to the next battery.

Pair Balancers

A pair balancer goes between each two batteries of the pack (so if you have 12 batteries, you need 6+1 balancers). A pair balancer works by checking the state of charge of its two batteries, and directing more charge to the battery than is lower. When all these are connected in a big string, the charger will eventually be shuttled along to the weakest battery.

Problem with these chargers is with long battery strings, if the end battery is low, it will require each balancer to detect the condition, and pass the charge on through the series. Some have reported that the end batteries never get enough charge.

Central Chargers

Central chargers detect when one battery in the pack is too low, and using a small charger will suppliment the weak battery. It continuously montitors the state of each battery and directs charge where needed.

Commercial Battery Balancers

PowerCheq Battery Equalizer

These are small units which are connected to each pair of batteries to equalize the batteries continuously. Web Site

  • continous
  • bi-directional
  • $80 CAD each $1920 for a 25 battery pack

    Zivan Battery "Smoother"

  • No longer manufactured
  • has an integrated RS-232
  • $1540 for 25 batteries

    BatOpt battery equalizer

    Web Site
  • Similar to PowerCheq version 1
  • May have a problem if connect incorrectly directly to battery
  • Shunt Type

    Ruddman Regulators

  • Similar to PowerCheq version 1
  • May have a problem if connect incorrectly directly to battery
  • shunt type regulator
  • Not isolated
  • Integrates to the PFC series of chargers
  • $30 CAD each $750 for whole pack

    Ed Ang's Battery Regulators

    Web Site
  • Similar to Rudman regulators

    AC Electric Vehicles

    Web Site

    Lee Hart's Battery Balancer

  • Central charger type battery balancer
  • Web Site
  • Do it yourself

    Lee Hart's Shunt Type Battery Balancer

    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 terminal, 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.

    Badicheq

    ??

    BattPro

    ??

    References

  • Performance Verification of PowerCheq Equalizers
  • Presentation on Battery Balancing
  • Life Extension Through Charge Equalization of Lead-Acid Batteries
  • Energy Efficient Battery Management
  • Switched Capacitor System for Automatic Series Battery Equalization
  • Gordon Stalling's Central Battery Monitoring System


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