Charging an AGM battery

[-robw-]

The charge will follow the lowest resistant string. Anytime you wire batteries in parallel you run the risk of one string getting more of the charge. I feel that is why some of your batteries are not getting full.

Ok, gotcha on all that, but that is not the issue here. Coincidentally, I just got in from taking my voltmeter to every battery. They're as good as month old batteries should be: all voltages were right around 13.55 after 5 hours of EQ and a rest. And both strings had exact same voltage.

I've owned floodeds, lifep04's and, now, AGM's. With flooded we can jam as much current as we want passed them (CC outputting max amps), they don't care. With lithiums we cannot because they have imbedded balancers that kill the current flow once they're full. Are Renogy AGM's like that, with some kind of balancer? cause that's how they're acting. The tech at Renogy kinda inferred that when she said the battery would shut down if its voltage is too low., but I didn't believe her. She didn't come across as being particularly bright.

[SunEagle]

A resistant battery/string would support a higher current need though? I mean, if a battery or string is not charging properly it would require more charging, not less. The issue I'm facing seems to be either 1) a battery string which is not 200AH, or 2) too low an absorb voltage to charge properly.

The charge will follow the lowest resistant string. Anytime you wire batteries in parallel you run the risk of one string getting more of the charge. I feel that is why some of your batteries are not getting full.

[-robw-]

It really comes down to the resistance in each of the strings. The one that has a higher resistance will fight the current to charge it. Believe me when I say it does not take much of a resistance to slow down the charging of that string. It only has to be more then the other string to cause an issue.

A resistant battery/string would support a higher current need though? I mean, if a battery or string is not charging properly it would require more charging, not less. The issue I'm facing seems to be either 1) a battery string which is not 200AH, or 2) too low an absorb voltage to charge properly.

[SunEagle]

HI. Two strings @ 48vdc (2p/4s). All batteries were fully charged using a NOCO before activation, and yesterday I took a voltmeter to each one: One battery was .2 volts off before I started EQ, the rest were closer. All pretty legit especially since my ammeter indicated the bank was only 75% charged.

Am I right to assume a higher absorb voltage would help?

It really comes down to the resistance in each of the strings. The one that has a higher resistance will fight the current to charge it. Believe me when I say it does not take much of a resistance to slow down the charging of that string. It only has to be more then the other string to cause an issue.

[-robw-]

My fear is how those 8 batteries are wired. I presume that there is unequal charging currents so one or more of those batteries will never see a full charge. One or more may also be damaged so it can't every get back to full charge again.

I would try to charge each 12V separately using a single charger and then see if they all come back to being full.

HI. Two strings @ 48vdc (2p/4s). All batteries were fully charged using a NOCO before activation, and yesterday I took a voltmeter to each one: One battery was .2 volts off before I started EQ, the rest were closer. All pretty legit especially since my ammeter indicated the bank was only 75% charged.

Am I right to assume a higher absorb voltage would help?

[SunEagle]

My fear is how those 8 batteries are wired. I presume that there is unequal charging currents so one or more of those batteries will never see a full charge. One or more may also be damaged so it can't every get back to full charge again.

I would try to charge each 12V separately using a single charger and then see if they all come back to being full.

[-robw-]

I'm thinking Renogy voltage specs are wrong.

I'm having the damndest time charging 8 Renogy 12v/200AH batteries running for a few weeks now. I noticed something odd before, in that once they get to about 70% charge they won't accept enough current to finish charge by EOD. One of my FM-80's will drop off ("charged") and the other supplies only about 20 amps when far more is available via my array. But now I'm really lost. Us Texans were just hit with a bad storm and, for the first time since living here, we actually had 4 straight days of very dark skies. These batteries got as low as ~25% (47 and change), but when charging them today I had to put my CC's in EQ mode at around 60% SOC to finish charging, and even then my they wouldn't fully accept what my CC could supply.

That seems to me to be the result of too low an absorb voltage, but I've read that AGM's are susceptible to high voltages and don't want to kill them. Renogy specs call for 57.6 vdc absorb, which is 1/2 volt lower than my prior floodeds. I'd like to experiment with higher voltage but before I do I'm wondering if anyone else has had such an experience with Renogy/AGMs and found a solution. Or if this just how AGM's behave.

I've communicated this problem with Renogy but their team is utterly useless.

Thank you.