thunderstorm80 said:
So I thought, let's compromise. Let's take Arduino which has 6-8 analog inputs, and add an input extension shield like this one so I can read all 24 cells into the same Arduino board:
Problem - they can only handle 5 volts. So let's say you use voltage dividers to divide down the voltage. That gets you to a new problem, which is that different voltages will require different dividers. That potentially means now you have different (and constant) drains on the cells, which means you will force them out of balance. So that's not good.
To get around this you can:
1) Disconnect all the cells when you are not reading them. That requires a 24 pole relay (or 6 4 pole relays, or 12 DPDT relays etc.)
2) Ensure that all dividers present the same load. That means that total resistance has to be the same on all of them. That may drive you to suck more current than you want to meet minimum drive requirements of the topmost cell.
Also via that method you will get excellent accuracy on the lowest cell and terrible accuracy on the top cell.
To get around THAT you can use 24 differential amplifiers to re-center the voltages. But that's a lot of work.
And then I had another idea: I already had my circuit breakers protecting me when one of my controllers became a FULL short-circuit in the middle of a ride. Household circuit breakers are small, reliable, and cheap.
And don't work with DC. Fortunately, Midnite Solar (and several other companies) have DC-rated remote trip breakers like this one:
http://www.midnitesolar.com/productPhoto.php?product_ID=578&productCatName=Breakers&productCat_ID=16&sortOrder=3&act=p
You say I didn't look enough for BMS's? Well, I did. I even got one which is supposed to be for LiFe. The bastard let the cells "fry" to 3.8V before it agrees to cut them off! (3.6V is the HVC)
That's how BMSes work. They only trip when the charger fails. 3.8V will not fry LFP's if it is short duration.
