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Note: Flooded, AGM and gel are all types of lead-chemistry battery.

How do I know when my lead battery is fully charged?

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 small fraction of its rated capacity.

WTF does that even mean?

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 charger2) is putting out.

Example: the charger is putting out 10A. Loads (laptop, lights) are consuming 6A. The bank's acceptance is 4A. (10A - 6A - 4A). It must be; chargers only put out what the system will take.3)

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 small fraction of its rated capacity

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:

 fraction percentage decimal actual values for 100Ah bank C/200 - C/100 0.5% - 1% of C 0.005C - 0.010C 0.5A - 1A

As you can see the Amps in question are quite small; this is why a battery monitor is important.

putting it all together

Battery X (100Ah) is fully charged when the battery is only accepting 0.5A to 1A at 14.5v.4)

how long does it take?

As your dad might say, “it takes as long as it takes”. The battery bank is driving the bus here and we are along for the ride.

A deep-cyle battery discharged to 50% typically takes several hours at Vabs to fully recharge.

It's fully charged. Now what?

Now you Float as long as possible.

3)
or else voltage would keep rising
4)