To answer your specific question, I get to 0.5 Amps/G31 every time I run a full charge on shore power
. And you should too.
First a system description: There are eight Firefly G-31 batteries
in a series/parallel arrangement to produce a 24V bank, rated with a C20 of 464 Amp-Hrs. The charger
used here is a Victron 24/3000/70/16, and a Victron MMPT solar charger as well. Measurements are collected by a Cerbo GX, and data analyzed by Grafana.
I know it will confuse things a bit, but all my graphs are in Watts, not Amps. The difference in energy content between an Amp-hr at 24 Volts and at 29 Volts is, IMO, too large to just ignore.
The the answer to the question about how long in absorption does it take to get to any specific end point VARIES, and varies a lot, depending on the charge rate. The charging
rate make a large difference at what SOC you enter absorption, and therefore how long you need to wait for a specific end point.
But here goes the data...
A routine charge this morning, SOC at the start was 80.5%, charging
at about 1.65kW Depending on the sunshine!) or 57 Amps, or 12% of C20. Absorbtion was triggered at 29.1V at 09:55.
At 12:10, (2 hours, 15 minutes) later, the charge acceptance rate had dropped to 116 W @ 29.1 Volts, or 4 Amps, or 1 Amp/G31, or ~1% of C20.
You can see that further holding at the absorption voltage just continues the exponential curve downward of the charge acceptance, so after another hour it has dropped to 68 Watts, or 2.3 Amps, or 0.52 Amps per G-31 or ~0.5% of C20. I call this "FULL"
This profile is fully repeatable, every charge cycle.
If you never drop down this low I can only think of two reasons: measurement error or you have an internal short in one of your batteries. It is very worth trying to sort this out and not ignore it. As I am sure you know, an internal short in one will gradually destroy ALL your batteries.
Possible mesaurement issues:
- Have you ever "zeroed" your shunt? i.e., checked the reading with the system disconnected and known zero current flow?
- Are you SURE there are NO loads coming directly off the battery that might be reading on the shunt as power going IN?
- There might be others I can't think of... other than the general accuracy of the meter...
EDIT: Ooops! I labeled the wrong spot in the graph as full, but my text above is right! Also just FYI, the green line in the graph is the real time DC load on the system, and the blue line is the 1 minute averaged power into the battery.