AGM - Mixing brands or voltage or Amp-hours

[golis2020]

I purchase a used 48 volt Schneider system with 60amp controller with eight Trojan AGM 6 V , 225 AH batteries. The voltage for each of them is between 5.7 and 5.9 but unfortunately one of them is not holding a charge and needs to be replaced from what I've been told. Can I replace it with any brand 6 V AGM battery and can the amp hours be a little bit different than 225? I'm just running a small refrigerator so I'm not too worried about performance but I don't want to damage my other batteries.

 

[sunshine_eggo]

5.7-5.9V resting is essentially a dead 6V battery, and a 0.2V variation is not good at all.

I would conclude that your bank is in DIRE distress likely due to over-use/abuse, and a complete evaluation is needed.

What is your bulk/absorption voltage?
What is your float voltage?
When at absorption voltage, what do all 8 6V batteries measure (list all 8)?
When at float voltage, what do all 8 6V batteries measure (list all 8)?

 

[Brett V]

It sounds like all your batteries are toast.

 

[golis2020]

Thank you Sunshine. I'm sorry, I'm brand new at this and my system isn't hooked up yet and I don't know how to measure the bulk / absorption voltage or float voltage or float voltage. But because my resting voltage is low, it sounds like my batteries are no good. I guess I'm going to have to buy new ones. I wish I would have posted this before about them. Live and learn... I have six solar panels that are 325 watts each and all I'm trying to do is power a freezer chest that requires 1.7 amps. I was hoping these batteries could limp along and do the trick because I'm in Puerto Rico in the tropics, but it does get cloudy here sometimes as well.

 

[sunshine_eggo]

Thank you Sunshine. I'm sorry, I'm brand new at this and my system isn't hooked up yet and I don't know how to measure the bulk / absorption voltage or float voltage or float voltage. But because my resting voltage is low, it sounds like my batteries are no good. I guess I'm going to have to buy new ones. I wish I would have posted this before about them. Live and learn... I have six solar panels that are 325 watts each and all I'm trying to do is power a freezer chest that requires 1.7 amps. I was hoping these batteries could limp along and do the trick because I'm in Puerto Rico in the tropics, but it does get cloudy here sometimes as well.

So these are batteries that are sitting doing nothing and reading 5.7-5.9V?

6V AGMs sitting at < 6.25V are in desperate need of a charge. If left that way for extended periods, irreversible damage occurs.

Given that you seem to have a relatively small requirement, that's a lot of lead to throw away. Personally, I would charge each 6V individually and let them sit for a few days to see what their voltage does.

 

[golis2020]

Thank you, my car mechanic charged them all up and they've been sitting for a couple of weeks after he charged them and they are remaining at the resting voltage I mentioned. I'm going to have them recharged just before getting my system installed later this week. Hoping it will work just to power my little freezer. Do you think it'll work? Being as one of them is not holding a charge, I need to replace it. Does it have to be another battery that is 255 amp hours or can it be a little bit different? Obviously it needs to be 6 volts because I need 48 volts total. I found a relatively inexpensive new Trojan 6 volt 255 amp hour battery, it is lead acid but it is not AGM. Would that one work? I'm sure I can find a decent used AGM battery that is 6 volt but I doubt it will be exactly 255 amp hours. Trying to figure out if I can use that one. Thanks so much! How did you learn so much about this? I really appreciate your help. Greg

 

[sunshine_eggo]

If they've been sitting for a couple weeks after a full charge, and they are 5.7-5.9V, then:

1) your mechanic didn't charge them.
2) They are beyond hope.

NONE of them are holding charge at 5.7-5.9V after two weeks. They are done.

You should replace all of them.

You could also get 4X 12V batteries and make 48V as well.

 

[golis2020]

Ok, got it, thank you!

 

[yodamota]

so a full 6v battery at resting voltage would be 6.3v? I also can't imagine what your car mechanic did to charge 8 - 6v batteries....

 

[sunshine_eggo]

so a full 6v battery at resting voltage would be 6.3v? I also can't imagine what your car mechanic did to charge 8 - 6v batteries....

That's as low as I would accept on an AGM sitting for two weeks. AGM resting 100% is 12.9-13.0 where FLA is 12.6-12.7 (24 hour sit).

I wonder if the mechanic charged them as 12V... overcharging half and undercharging the other half...

 

[Brett V]

It really sounds like those batteries are sulfated beyond recovery. If you have a light load, a single 100 Ah rack mount LFP battery is likely more cost effective than replacing 8 AGM batteries.