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This should be easy, right?
time charging x charge rate in A = Ah replaced in the bank
We know the time part. But the charge rate part can be hard to predict with certainty.
In most cases the battery itself will be the limiting factor; it wants what it wants.
These are common maxxes for batteries discharged to their deepest normal state of charge:
Since current is a function of C, bigger banks will pull more current than small ones. At 0.2C a 100Ah bank will pull 20A and a 200Ah bank 40A.
Banks will pull more current at lower states of charge and less current at higher states of charge. This affects combiners more than DC-DC chargers (see below).
Sometimes other factors will intervene:
There are two main ways to charge from the alternator: [combiners](https://rvwiki.mousetrap.net/doku.php?id=electrical:12v:alternator#combiners) (relays, split chargers) and [DC-DC chargers](https://rvwiki.mousetrap.net/doku.php?id=electrical:12v:b2b).
Combiners are simple connections, and so the charge rate will vary based on alternator voltage, bank voltage, and resistance in the circuit (including the battery itself). We can safely say the bank will draw more current at low states of charge and less current at higher states of charge.
T DC-DC charging rates are more predictable because of how they work. A 20A DC-DC will likely pump 20A into the bank most of the time. So 3 hours driving x 20A = 60Ah returned to the battery bank. The price of this predictability (and other features) is… price.