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.

• DC-DC charge rate is usually the rated output, so roughly speaking hours driving x rated output = Ah of charging * charging with a relay is less predictable because current and voltage are not actively managed * The charge rate will likely start off ≤0.33C (33A per 100Ah of capacity) and taper off as the battery charges. * the shape of the taper hinges chiefly on battery chemistry ===== what limits the current? ===== ==== the battery bank ==== 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: * AGM - 0.33C (33A per 100Ah of capacity) * Flooded - 0.2C (20A per 100Ah) * lithium - this one is tricky. In theory (like on a bench as in the Victron and Sterling videos) LiFePO4 can pull 1C (100A per 100Ah). In normal split charger installs¹⁾ it tends to pull about the same as AGM. 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). ==== factors outside the battery bank ==== Sometimes other factors will intervene: * DC-DC chargers have a set output limit (20A, 40A, etc) * resistance in the wiring and connections can limit current * if the bank is oversized and current otherwise-uncontrolled the alternator's output can be maxxed out. So don't do this unless you want to buy a new alternator ==== charging method ==== 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.