I'm working on a battery charge monitor/controller that goes across each unit in your bank. The idea is that it produces three isolated current loop outputs: an "OK to charge", an "OK to discharge" and a "not OK to charge".
Also, an isolated enable can be used to switch in a resistor for a fixed current charge, switched by the "not OK to charge" sensor. This should enable the batteries in a bank to be charged individually, by applying a fixed 2A current and waiting for them to all show "not OK to charge". As each one shows "not OK to charge", the 2A is diverted through the resistor, so driving only the batteries in the bank that have not charged yet.
There will also be a dual LED - a red one for "OK to charge" and a green one for "OK to discharge", to give an instant visual indication of which battery in your bank is not pulling its weight.
The idea is to wire the outputs together so there are three outputs from the whole bank: "all OK to discharge", "all OK to charge" and "none OK to charge". These should be what is needed to protect the bank and prolong its life, either with clever microprocessor controls or with something much simpler and more computer-free, like a couple of relays and a few transistors for the current regulation.
I'm aiming to keep the bill-of-materials below UK5 - call it $9 in today's money. The monitor will consume a small parasitic current, which I'd like to keep below 5mA. My 104Ah batteries should, by first approximation, supply 5mA for about 28 months - at a rate of 1kWh per unit every two years or so. Of course the batteries will be long flat of self-discharge by then.
I have this in mind as part of my electric car ambitions, but it should be useful for any sort of electronic monitoring of a battery bank. I'll post a circuit when I've finished prototyping it, and there should be a circuit board available in due course.
