Nokia BL-4C / BL-5C are 3.7V, 700-1000mAh (variants) Lithium-ion batteries, which are available in abundance and at very low cost ($4 - $7 a piece), since these are used in entry level Nokia phones, sold in most developing countries.
These batteries have 3 terminals, as explained here, for battery positive-terminal, ground, and a terminal (BSI) which I believe presents a fixed resistance value, that needs to be measured, to determine battery-type (chemistry, characteristics including capacity etc.)
So here are my questions:-
Is anyone aware of any existing open-source hardware project, that has built battery-management circuit around such batteries ? (PS> In my searches so far, I've not come across any such project).
How exactly, can one accurately measure the resistance offered by BSI terminal of the battery ? A very accurate and stable reference voltage would be required, I believe - but if the battery itself is the source of such reference voltage, it could vary with charge-status and condition of battery, so how could it serve as a good reference ?
Is it necessary to have a temperature sensor in proximity/touching the battery ?
What would be some low-cost battery-management IC's that are not too difficult to source or work with, for hobbyists, which can be used to charge such Li-ION batteries safely ?
Have found this question here related to 'resistance measurement', but the mechanisms described are far too cumbersome, expensive & require manual intervention, to be useful in automated battery-type identification. OTOH, it is possible that I've completely misunderstood the purpose of the BSI terminal.
As for whether "temp sensor" is needed, I think the answer is YES, based on this. However, the question then becomes, without access to proprietary battery characteristics information of this Nokia battery, what can one do with the temp-sensor ?