that if I have a 100ah lifepo4 battery and use 50ah's, then drive the truck for 1 hour, I'll replace 30ah to give me 80ah capacity. So in 2 hours of driving, I'd have a fully recharged battery (in this example).
Yes, basically. There are edge cases where it might not (excessive ambient temps, freezing weather so the batt BMS shuts off, other demands on the alternator, etc).
just looking for a ballpark kind of thing to hopefully figure out the DC-DC chargers "value" to me and my situation.
If you describe your situation we might be able to advise about relative value.
It seems to me the main benefits of DC-DC are:
- the ability to configure charging setpoints higher or lower than the alternator's native voltage (critical with lead, might be useful with lithium depending on alterator voltage)
- the ability to do staged charging (critical for lead, not so for lithium)
- the ability to limit current (possibly useful, see below)
If one has vehicle-mounted solar the solar charge controller can do #1 and #2, if desired/required. With small-to-medium LFP banks #3 might not be required.
It's hard to go wrong with DC-DC charging in the same way that "no one ever got fired for buying IBM". But DC-DC setups typically cost 5x-10x more than direct charging with an isolator. I suspect it is needless expense in some cases.
Before anyone loses their shizz over the suggestion that direct-charging LFP can work, here is
some collected info on
what LFP banks actually draw from the alternator in real life. Including examples from valued members of this forum (
main thread). The sky is, generally speaking, not falling. Yes, it'd be inadvisable to charge 300Ah of
anything from a 90A alternator (looking at you, victron video).
Anecdote: when I went from FLA to LFP in my campervan I assumed I'd need to replace the VSR with DC-DC. But since the VSR was already in place I tested it. Turns out the alternator charges my Li
well within the limits of both the alternator and battery. I put a switch on the VSR's ground wire and disconnect it at will.