Originally Posted By: Jetronic
Originally Posted By: Cardenio327
If the battery has a shorted cell, yes, very hard on the alternator.
If the battery is just weak, no, it is much easier to charge than a strong battery.
A weak battery, however, is harder on the electrical system in general, especially the starter, as the lower the voltage drops while cranking, the higher the current flow and more heat is produced.
erhm, the lower the voltage, the lower the current aswell... P=U².R. If the voltage drops, power drops dramatically...
The issue with applying electrical engineering to a failing car battery is that there may be more than one symptom, eg, low voltage. You need to know all your symptoms and continue on an assumption that the half-dead battery will stay the same until it's replaced. You've got busted or bridged plates rattling around in there and who knows what else.
Best case scenario, the kaput battery just boils itself, and its impedance is high enough the alt keeps on trucking.
Medium case, the battery has nothing to absorb, nothing to contribute, and it's the same as driving around on the alt with the terminal disconnected.
Worst case, the battery is heating itself up with all the amps the alt can make, and wreaking havoc on the PCM and anything else touchy on the system.