Hi hollisterco. I am certainly not the expert that dogman is, but there are some good questions in your post. Here is some info I previously posted on Motorbicycling.com: "BMS simply stands for battery management system. In a practical sense that means all the cells get charged to their maximum without overcharging and no cell gets discharged below its minimum. My understanding is that most packs that use multiple batteries have a BMS of some sort. That includes NiCad tool packs, etc, not just Li. Some ebike packs come with them but if you are making a pack or buying individual cells, then you have to devise your own. There are also aftermarket ones and long threads discussing the relative merits of factory-built vs aftermarket on Endless-sphere. Problems arise when a BMS fails, as then the BMS may prematurely shut down a perfectly good pack or, more importantly, a rider may ruin an expensive pack by overdischarging or by overcharging it. Some argue that for various reasons (poor soldering, poor quality parts, rough shipping) factory-built BMSes are more prone to failure.
"I bought my LiFePO4 packs without a formal BMS, as some riders propose that LiFePO4 are the most forgiving and that, with care, a rider can be his own BMS. While that may be true, I use a pair of relatively inexpensive devices to be my BMS and they seem to work well. The first is a Celllog 8M, which is essentially an 8 channel voltmeter with fully adjustable alarms for high voltage (pack/individual cell), low voltage (pack/cell) and differences in voltage between cells. The alarm beeps and the affected cell indicators flash when the alarm is triggered. The Celllog also has an external alarm circuit that will trigger a louder alarm or LED. The alarm circuit can also be used to turn off the charger when any cell or the whole pack reaches a preset limit or turn off the controller if a cell or the pack gets too low. As I use an 8 cell (24V) pack, I use one of these. I don’t bother with the external alarms/shutoffs at this point. If I know I might be running low, I keep a closer eye (and ear) on it while riding. They are under $15 from HobbyKing Online R/C Hobby Store: http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=10952. It requires a 9 pin JST-XH connector. I got 5 connectors for $10 (inc s&h) on ebay.
"The other device I use is a Turnigy Accucel 8150, $42.44 from Hobby King as well - http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=7523. This is a balancing charger that uses the same JST connector as the Celllog to monitor the battery while charging. It is fully programmable for charging all kinds of Li batts as well as lead, NiCad and NiMH. It monitors each cell (up to 8 ) while charging and brings them all up to the proper voltage (3.60V for LiFePO4) without overcharging. It requires an external 11-18V power source. I originally used an SLA and charger for its power but since have adapted an old 300W computer power supply. It charges at up to 7A, so if I use 16 ah I can get it mostly charged in a little over 2 hours, although a full charge with balancing takes longer.
"Hobby King is in Hong Kong and shipping is slow and relatively expensive, or faster and more expensive. I ordered them separately and they took 2-3 weeks each to eastern US. S&h for the Celllog was about $5 and the charger was about $19 s&h. Add a few bucks for standard connectors and wire and you’re all in for under $100. If you use 36 or 48 volts, buy 2 Celllogs and divide the pack for charging." Only you can decide if you would be comfortable with this type of system but it does give you some flexibility - jd
