How much will my battery charge by alternator?

This should be easy, right?

time charging x charge rate in A = Ah replaced in the bank

We know the time part because we're the ones driving. But the charge rate part can be hard to predict with any certainty; this is especially true for combiner (“split charger”) charging.

• 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 will vary over time
  • 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
  • this unpredictablity is less important when solar is added.

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).

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

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 1/2 hour driving x 20A = 10Ah returned to the battery bank. The price of this predictability (and other features) is… price.

If you need predictable charging then DC-DC is likely the answer. There are other scenarios where DC-DC is effectively required.

This is the tough one. We know roughly where the charge rate will start out for discharged batteries (≤0.33C) but the taper complicates things. All the batteries would eventually charge to full²⁾ given enough time, so shorter charging periods are what interest us. Let's assume a short drive = 30 minutes.

NOTE: this section contains guesswork from observations and theory. You will learn how your particular setup behaves in actual use.

• FLA taper is steep; charging current starts at ~0.2C and quickly falls off within seconds or minutes. Current will likely stabilize around 0.1C by the end of the drive. So we might use an average of 0.12C for our charge rate when estimating.
  • 100Ah x .12C = 12A.
  • 0.5hours of driving x 12A = 6Ah replaced.
• AGM taper is more gradual, so charging currrent stays higher on short drives. It might only taper to 0.15C by the end of the drive. We might use 0.175C as our average rate. 8.75Ah replaced.
• Lithium has an odd taper because the charging voltage curve is not linear.
  • in the broad “flat” portion of the voltage curve (20%-80 state of charge) current will likely stabilize around 0.2C. We could use 0.2C as our average rate. 10Ah replaced.

In most installs this variability isn't a major issue:

• combiner charging is commonly paired with solar charging (also highly variable!)
• alternator charging is often a backup or “nice when you can get it” charging source. Exception: there are scenarios that depend on it.