I have everything here to do a sample system experiment except the 3ah lithium battery which is supposed to show up tomorrow. I have started to do some preliminary data collection on AGM batteries so I know what to expect. Glad I did, because I did get some surprises.
Since the van was sitting right there and has shore power and the Magnum charger with built in monitor, I decided to use it to see how much the 440ah bank would accept when at 80% SOC. The results were not what I expected.
I ran the batteries down to 80% on the meter, which confirmed as being down 88ah, let the batteries rest for a few hours and checked the voltage, which came in at 12.9v which is way over the 12.54v that Lifeline says that 80% should be. I have run into this before with other tests, but not quite as extreme a difference. I think it is because Lifeline is basing the chart on "normal" charging that doesn't take the batteries to very full condition of .5%C amps at absorption. The batteries are few years old, but still appear to be in very good shape.
Anyway, I put in on charge with the Magnum 100 amp charger, and it pinned it at 99 amps which was surprising because in the past I have seen tapering by then. Probably has to do with starting at higher SOC than before. This is .23C amps of charging, so it certainly would indicate that a good sized DC to DC charger would be needed or one that safely current limits without just shutting down like the Powerstream ones say they do. It didn't reach absorption voltage and start to taper until nearly 90% on the SOC meter.
This is kind of contrary to the normal knowledge about how long faster charging continues before you get to the long haul tapering that the AGMs need to get full, as running a generator to 90% still be charging at high rate.
I let the charger complete the charge, which did take the normal 6 more hours approximately. When I looked at returned AH screen it got obvious what is going on as the returned amp hours were nearly 94AH to get full. This is the charge efficiency showing up so you need more amps back in than you took out. If it were a deep discharge the difference would be even more than 10% of the discharge AH from what I have seen in the past. The monitor is supposed to auto adjust over time to keep the SOC % correct at recharge by using charge efficiency, but it can't keep up with varying discharge amounts and is never correct on recharge, but is correct on discharge.
All this really says is that when you are 80% on recharge you actually have over 20%C left to charge and would help explain the no tapering right away.
Because of this, going be SOC on recharge is probably not the best way to determine when to shut off the charge source and let the lithium battery finish off the charge. I think I would wait until the absorption voltage is reached and the amps start down, regardless of what the SOC% is. The exception would be if you find you always have lithium capacity left when the charging tops off and want to shorten the charging time as much as you can.
I then did the same test on the 12AH AGM that is going to be used in the test and at 80% it took nearly 10 amps of charge which is over .8C amps. It held absorption voltage right away because it was on a 40 amp charger. If you have a hugely oversized charger like an engine generator would be, I think I would just test what the right charging amps are, based on the lithium finishing the charge, and stop charging that way.
All this will complicate my testing a bit because the DC to DC charger I got is not current limited and would be beyond capacity at 10 amps by quite a bit. I will either have to use just the lithium without the charger initially or ramp up the voltage slowly on the charger. Neither should alter results, though, as the test is if the lithium will have the capacity to adequately finish the AGM charge, and by the end of charge the amps will be down so the charger can hold the voltage up.
Hopefully, in the next couple of days I should have more complete information.
