Current regulators and limiters

[akeeton]

I have a couple of ALM 12v7 batteries from A123. They are rated at 20 amp continuous discharge and have a 30 amp fuse for over-current protection.

My intent was to have a parallel setup to get a better current discharge. For the sake of clarity, let's assume I'm running the two batteries in parallel.

In a parallel setup, I can't rely on an ESC to limit the current discharge to safe levels. After all, one battery in parallel will inevitably fail before the other meaning the current limit at the ESC would have to automagically drop to compensate (not gonna happen).

I've considered using an extra BMS on each battery (ie, in addition to the one already in the battery pack) just to limit the current. Another option would be to use a cheap ESC that would limit the current draw to less than 30 amps (in addition to the existing ESC the controls the motor). Maybe the best option is to use an auto-resetting circuit breaker and a big capacitor to smooth the cut-off/on.

These all seem like really shoddy heavy-handed solutions.

This seems like a problem that must have been solved over and over and over, but it looks like the cleanest solution might be to build my own off of one of the many existing circuit diagrams.

How do current limiters normally work? How do you deal with over-discharge protection? Any reason not to use an auto-resetting circuit-breaker?

 

[Punx0r]

The documentation from A123 is a bit vague on the subject of over-current protection. It has overdischarge (voltage) protection, but doesn't mention over-current, which is a common feature on other BMSs.

Even if it doesn't have it, you can safely connect your two batteries in parallel and use the controller to limit current. The batteries will share the load so that they discharge evenly. Due to manufacturing tolerances, one BMS will cut out from under-voltage slightly before the other, but you don't want to make a habit of running them down that low. If it makes you feel better you could use a diode and bulb on each battery to indicate whether each is still supplying current. However, I suspect the reduction in available power when one battery cut out would be very noticeable.