Lead batteries are killed by 'dwellers undercharging them: not fully, and/or not often enough. To reach the expected cycle lifetime¹⁾ a lead battery must be charged:

1. fully
2. regularly (daily or near-daily is optimal)

So how do we know when the battery is fully charged? When I have topped it off for 15 minutes from the alternator? (bzzzt) When the controller drops to Float? (bzzzt) When it is above 12.7v? (bzzzt)

A lead battery is charged when acceptance at Absorption voltage has dropped to a specified value.

Let's break it down.

A lead battery is charged when acceptance

Acceptance is the amount of charge current the battery bank is accepting. Since there may be loads this may not be the same amount the charger is putting out.

Example: the charger²⁾ is putting out 10A. Loads (laptop, lights) are consuming 6A. The bank's acceptance is 4A. (10A - 6A - 4A).

The presence of loads can make assessing acceptance tricky, which is why we watch current going in/out of the bank with a battery monitor (shunt or clip-on meter).

at Absorption voltage

The voltage specified by your battery manufacturer for charging batteries. Typically in the mid-14v range, adjusted for temperature.

has dropped to a specified level.

This is where the battery monitor shines. Lead battery manufacturers typically specify Absorping until acceptance is some small percentage of bank capacity (C).

These values can be expressed in different forms but they all mean the same thing:

Battery X (100Ah) is fully charged when the battery is only accepting 0.5A to 1A at 14.5v.

Now you Float as long as possible.