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Old 08-27-2014, 03:20 PM   #1
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Join Date: Aug 2010
Location: Minnesota
Posts: 12,099
Default Recharge after deep discharge

One of the things that you hear from the wet cell manufacturers is that if you have deep discharges of your batteries (20-30% SOC) you should equalize much more often. They are pretty shaky on how often a deep discharge, or how many, generate the need to equalize. Some say "whenever" deep discharges are happening, some say after several, etc. Some say once a week if cycling deep, etc. Throw on top of that the question of needing equalizing from not getting full on each cycle and it is more confusing.

The easy answer is to just keep an eye on the specific gravity. If the fully charge SG starts to drop or get uneven, it is time to equalize. PITA, but is the gold standard.

Since we had just done a deep discharge to test the alternator output to low batteries, I just ran a single charge cycle with the shore charger. The Blue Sea is multibank with ending amps on each bank taking that bank to reduced voltage prefloat (14.1v), and then to float when all banks hit ending amps. Absorption was at 14.6 and temp compensated. I set the ending amps a bit high on the 12v battery as it always is done first, and I wanted to see if the 14.1v would get it finished while the 6 volts were still charging. 40 amp charger, 375AH of batteries of mixed size and voltage (2X6 and 12). SG started at about 1125 for both banks, surprisingly because they had gone uneven in the middle of the discharging, but got back even at the end.

Just checked them, and both banks are at 1280 within a couple of points, which is exactly where they were before our last trip even, and the max we can get them to with charging or equalizing.

This would lead me to believe that the manufacturers are counting on chargers not getting batteries full on recharge, especially deep ones that need longer absorption time, and/or the higher 14.6 volts is helping some. I see no indication they will need equalizing anytime soon, even with more deep discharges. We settled on 14.6v as a good spot to reduce gassing intensity, while still getting totally full, even though it takes a little longer to charge.

This also seems to show that the ending amps is still an accurate indication of full charge when coming up from a deep discharge, which is a very good thing.

I think most of this also applies to AGMs, but you don't have the ability to see if the batteries are in need of help or not, and equalizing them is hard on them, compared to wet cells. If AGMs react the same to deep cycling and recharge as our wet cells did, it would indicate that just getting them totally full pretty often, based on ending amps, probably would significantly increase the life of them, and be all you would need to do. We have heard this a lot, but you don't hear much about how you tell if an AGM is full, as you can't check SG, and it is a pain to let them sit a day to check voltage. Most chargers don't get them full, or overcharge them, so it is difficult. If AGMs do really react like the wet cells did, testing would really not be necessary, and I would say the same for the wet cells.

We will be going on a longer trip soon, with lots of varying conditions of shore power, off grid, etc. I think I will keep track of the SG to see if it continues to come fully back each time we eventually get a shore power full charge. Should be interesting.
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Old 08-27-2014, 04:20 PM   #2
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Join Date: Dec 2010
Posts: 2,058
Default Re: Recharge after deep discharge

Booster-i have no clue on this! you know everything
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