I have a product that is in thousands of installations. It has a 12v 1.3AHr gel cell and a charger that charges it to 14v and holds 14v for 30 minutes, then switches over to float charge with a constant current of 5ma. The board draws between 60-100ma. The problem is that it cannot detect a battery that has a really high internal resistance (i.e., dead). It can if the battery gradually loses its capacity but a sudden failure goes unnoticed. My mission is to design an add-on PCB that will go between the battery and the board that will detect ALL possible battery failure modes.

My initial plan consists of an 8-pin uC, a relay, a series R with the battery of 0.5 ohms to measure charge/discharge current, a 24 ohm, 10w load that is slapped across the battery once a day for a second or so to roughly check capacity.

I should be able to sense if a battery fails to attain a full charge and a battery with high resistance. Shorted cell - check, won't accept a charge - check. What else?

 

At a time control systems was elegant; you fine tune supply charge current based on feedback current sense. Your time around current supply must be based on a lookup table..

 

I would go here.
http://batteryuniversity.com