backchannel - misinterpreting expert advice about alternators and charging deep cycle banks

from this reddit thread

OEM alternators are famous for over heating and burning out the coils when used for charging deep cycle battery banks. [OP links to one of Rod’s excellent articles]

Yes, Rod is an expert. No, the article is not about alternator failure and, no, Rod made no such claims.

what the article is about

The article is about damage to deep cycle lead batteries caused by chronic undercharging by automotive alternators. The title is A Deep Cycle Battery Assassin not An Alternator Assassin.

The culprit is an overly-simplistic implementation of alternator protection:

  1. that it derates voltage too early, thereby damaging lead banks
  2. but cannot adquately protect from actually-damaging alternator temps

but they fried an alternator!

Yes, the Cooked…!! section decribes an alternator damaged when it overheated:

the battery bank was simply too large [“about 900Ah”], and regularly over-discharged, for the alternator to adequately self protect itself.

That was a grievous error by the owner, like trying to tow a Space Shuttle with an F-150. F-150s can tow just fine within their limits.

Rod goes on to say that even with a discharged monster bank in 145F ambients (closed marine engine room) that external regulation could have saved the alt:

In this case a properly installed external regulator with an alternator temp sensor, would have saved this alternator.


  1. don’t massively overtask your alternator <- this is the takeaway for vanfolk
  2. if you want to overtask, install external regulation that can actually deal with the problems overtasking creates