Determing when lead acid battery is parallel through embedded systems
Theory proposed in question

A Smart Battery charger ought to know how to regulated charge current from the number of batteries connected to a charge node.

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My logic

In new batteries it is possible to correlate very well between ESR and equivalent capacitance in kilofarads in a 2nd order RC//RC model by testing for any given production line or controlled supplier quality.  But defects or aging effects will present big differences in ESR.

This means the capacity vs ESR=dV/dI and C=Ic*dt/dV can be correlated well with Ah with actual test result.

However it has been confirmed by reports in Battery University that this is not a good correlation between ESR and Ah capacity for many reasons due to mismatch. ( ref. not handy)


If I may rationalize this opinion, we cannot assume the cell-ambient thermal resistance, Rja, of batteries is the same for all types of similar Ah capacities or that all Ah batteries share identical ESR.


The cell temperature rise can have higher ESR in one cell or pack more than the other and thus the current is not shared equally. $$ T(rise)=  Rja * ESR * I^2 = ['C/W] * [W]$$


When heat rise is added to acid, it can boil and create combustible gases (H2, O2)


Busted


Therefore is unsafe for any battery charger to look at a shared node current and voltage and prediction the max current charge rate without the user manually setting the max rate.


Alternative solution

Have separate current regulated ports or a programmable setting.