So I'm wanting to make a lead acid BMS system. The system would be composed of probably 5-10 batteries to expand to more if I need/want to. I want to measure each battery voltage and temp individually. I then want to output that serially on a per battery basis. This will then go into either one multiplex or some end controller I haven't decided yet. My main concern though is figuring out how to design the BMS becuase I never have before so i'm looking for some help/direction. The large caveat to this is that I want to find a way to do it without having a true ground, or is this even possible? Thanks guys.

 

Yes it's possible but without knowing exactly your voltage and power requirements it's impossible to design something. Start out by drawing the normal battery configuration when power is being drawn from the system then draw out how you want each battery switched in or out of the circuit using simple DPDT relays. If it's a serial string of batteries you might need a hot spare to parallel the switched out battery position first before removing it so power is not interrupted during the switch.

Having a ungrounded system is not a problem if you use a isolated power supply for your measurement system with isolation (opto isolators) for any data links external to the BMS.

 

I'm trying to avoid relays. I would like to go a solid state route. The batteries I intend to use are 12V 190Ah batteries. So I would only want to measure any one battery individually and then those all report to a master controller. So I'm thinking I can have analog measurement as well as a thermistor and then that goes to an A/D and then serial out from there. Any thoughts?

 

You can use anything you like during actual construction but using relays during first paper design just makes it simpler to get the whole concept. A simple 8 bit uC like the PIC18F46K22 should be able to handle all the measurement and BMS functions easily if you can handle the programming requirement.

http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en547757