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Question: If I have 48 lithium cells connected in series as a pack, how would I best go about physically implementing a system to measure, isolate, and relay cell voltages to a centrally managed microcontroller? I am currently considering the usage of analog optoisolators, but I would be grateful for any advice. However, I would need the ICs and other parts to be hand solderable (ie DIP) as these are the only tools I have available. Thank you!

Context: I am part of a high school electric vehicle team, where we design, build, and test fully electric vehicles converted from gas cars. I joined the team with lots of heavy work already done, but we need a management system for our newly installed LiFePO batteries. Our instructor is considering purchasing a better commercial grade BMS (ironically enough our current one is only consistent in failing), but would rather have our team design a BMS, however rudimentary.

I am looking to design a BMS for a 48 cell LiFePO battery pack. This system would comprise cell voltage readouts and basic cell overvoltage drain functions. I plan on using probes for each individual cell and centrally managing them via some form of microcontroller. In addition to needing to multiplex the data, isolation of the cells and interfacing to a monitor are my major concerns. Because isolation is a major challenge and poses significant safety and reliability implications, I decided to start there.

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I'd consider using switched-capacitor isolation amplifier like (but not) the LTC1043. Basically, analogue switches apply a capacitor across the voltage to be measured, the cap charges up in a milli second or so then the analogue switches return the capacitor to ground so that voltage measured can be determined by a regular ground referenced amplifier.

The LTC1043 doen't have sufficient input voltage range for your application but something like it could easily work.

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Probably your best bet will be to get some battery management analog front end ICs. You'll want them to be cascadeable since most do not support 48 cells in series.

This table lists some options you might look at: http://liionbms.com/html/BMS_IC_table.html. Note that it's incomplete (I definitely notice that it's missing some of the newer TI offerings), and you'll have to investigate which ones are offered in a form factor you feel comfortable soldering, but this should get you started.

You also should take a look at the battery management literature and software from the manufacturers of these devices. TI's documentation is very good, and I've heard good things about Linear's too.

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