The BMS does two things usually:
- It monitors the individual cell, or paralleled cell sub-group, voltage during discharge and either provides a warning, or disconnects the power, if any single cell or paralleled cell sub-group drops below the set low voltage cut off (which will be different for different cell chemistries). This function is usually just abbreviated to LVC, for Low Voltage Cut-off.
- It monitors individual cell, or paralleled cell sub-group, voltage during charge and shunts excess current across any cell, or paralleled cell sub-group, that exceeds the maximum allowable full-charge voltage (which will also be different for different cell chemistries). This function may also control the charger, or provide visual indication that cell shunts are active, in the form of LEDs. This function is usually just abbreviated to HVC, for High Voltage Cut-off.
It's perfectly possible to design and build your own BMS, but it's time consuming to get it to work properly and really only something I'd recommend for someone who's work or hobby is playing with electronic stuff. The easy option is to buy a BMS for the type of pack you want to build, then just wire it up. It's unlikely to be wholly plug-and-play, even then, as the cell taps will need to have the right connectors fitted to match the BMS. There are a few BMS suppliers around, both mass produced Chinese ones and one or two designs from members of this forum.
There are several threads here describing DIY BMS designs, going back several years. The best ones to look at if you want an insight into the design and some of the challenges are those from the time when GGoodrum, fechter and bobmcree were developing designs (like this one: http://endless-sphere.com/forums/viewtopic.php?f=14&t=3345). There are also some BMS design snippets (along with a lot of other stuff) in the old "Circuit schematics" thread, here: http://endless-sphere.com/forums/viewtopic.php?f=2&t=278
List of relevant BMSs
|Manufacturer||Model||Technology||Chemistry||Method||Topology||Cell Format||Balancing Current||Cell # Range||other relevant details|
|ES: Goodrum/Fechter/Hecker||Zephyr||Analog||LiFePo4, LiPo||top balancing||Centralized, modular||all||1A?||4-100||Threads|
|SingalAB/Ping||Varies||Analog||LiPo or LiFePo4||top balancing||Centralized||30-60A||discharge||7-24 (chainable)||Threads|