Ok. I'll feed that into my knowledge base about batteries. However the original statement was too extreme. I would never buy a new battery because the alternator failed and I kept driving the TRUCK until it wouldn't run anymore. I only replace batteries if there is good evidence that its degraded. Like ... a battery tester says its no good anymore. Or it won't hold a charge or the voltage drops too low while cranking or other things. I always give a battery a chance to prove itself but when its not up to ***** it needs to go.
And I think it depends on how much of a discharge the battery was subjected to. If it only goes to 8V ... I wouldn't think that would be too traumatic. If its shorted to ground and run down to 2V then that's worse. But I'm just guessing
Also ... What happens that ruins a battery if it discharges? There has to be some physical phenomenon that transpires. What is it? Does it matter if its an AGM battery or any other technology ... Like a marine battery?
There are significant differences in the lead alloys that make up the plates in a starting battery vs. a deep cycle. AGM & Gels add another layer of complexity. A starting battery contains higher levels of calcium when compared to a true deep cycle. This allows the higher cranking amps for turning over an engine. A deep cycle has a higher lead content which allows the repetitive discharge/recharge cycles and reserve capacity. If you completely discharge a starting battery such as the one in your truck, the chemical reaction that takes place causes the plates to harden a bit. Do this more then once or on an older battery and it will never come back. It would come back voltage wise for a while but the cranking amps would decrease significantly (think run stereo, turn on headlights but only click the solenoid when you turn the key)
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