If I get/have xyz brand DC-DC charger rated at 12vdc, 30 amps ... what does that mean in terms of recharging the camper battery using the truck and xyz brand DC-DC charger? Does that mean, in very basic simple terms, if I drive, or run the engine, for 1 hour I put 30AH into the camper batteries from the trucks charging system?
DC to DC charger basic questions
- Thread starter johnm1
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sunshine_eggo
Happy Breffast!
If the charging occurs in the bulk phase, yes.
Checkthisout
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Its hard to say. 30 amps at 12V = 360 watts.
30 amps at 14.4 (most vehicles charge at 14.4) = 432 watts.
You only need a DC-to-DC charger to limit current to something like a lithium battery or maybe an AGM and to protect the wire running to the batteries if you want to use smaller than the vehicle charging system is capable of. If you're using FLA batteries for your house loads then I wouldn't bother
sunshine_eggo
Happy Breffast!
OP's not asking about Wh. He's asking about Ah.
30A for 1 hour is 30Ah - regardless of voltage.
Thank you and yes, this is what I'm asking ... a very basic, simplistic view of what it's doing.
I was thinking, and it appears I was correct, 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).
I know this is very approximate and I'm not basing any real calculations on this ... just looking for a ballpark kind of thing to hopefully figure out the DC-DC chargers "value" to me and my situation.
sunshine_eggo
Happy Breffast!
Yep. There are other considerations. I mentioned bulk phase because that's the portion of the charge profile where the charger can output max amps. It's pretty common for the wiring between the charger and the batteries to contribute to a voltage drop. This means the charger "sees" a voltage higher than the battery because of the resistance in the wiring. This can cause the charger to taper current so as not to exceed the charge voltage and deliver less than max amps.
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fratermus
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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).
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)
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.
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