AGM battery voltage questions and concerns

LGJohnVT

New Member
Joined
Mar 6, 2021
Messages
16
Hey folks, so I finally got around to setting up my solar setup in my off grid cabin. I kind of copied one of Will’s 24v systems. I’ve got 8 155ah Vmax tanks wired as pictured. I wired them that way so that the batteries could be arranged long ways without variance in cable distances.

Anyway, I’ve got my 4 415w q.cells wired 2 in parallel then in series into a Victron 150/70 charge controller. I ran the batteries down a bit last night to see how they charge. I didn’t catch any readings when the controller was in bulk mode, but when it was in absorption I started to hear some bubbling coming from the batteries I got down and listened and it was specifically batteries D and F. I brought the charging voltage down from the default settings for AGMs from 28.8v to 28.4v. When I noticed it was still specifically those two batteries I took out my multimeter and checked the voltages. Across all 4 sets it was consistently just under 28.4v. But when I checked each battery’s individual voltage batteries A, B, G, and H we’re all just about 14.2 but batteries C and E were at 13.8 while their pairs D and F(the ones making noise) were up at 14.6v. I brought the charging voltage down to 28v even just to see and C and E remained at 13.8 while D and F came down to 14.2v.

The charge controller has since kicked down to float at 26.8 and all batteries individually are reading about 13.3-4 and none of them are making noise.

I’m confused about what this means.
Should I should be concerned?
Are there two faulty batteries?
or is this due to my weird wiring of the batteries?

I’ve read a lot that AGMs should not make noise, so I’d like to get ahead of this before damaging anything.
 

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Q-Dog

¯\_(ツ)_/¯
Joined
Dec 22, 2020
Messages
637
I would check to see if any of the connections are getting hot or making poor connections. Also, it looks like your battery monitor sense/power leads are under the bigger battery interconnect cables. Biggest connections should be directly on the battery terminal and smaller connections on top of those.
 

LGJohnVT

New Member
Joined
Mar 6, 2021
Messages
16
Thanks Q dog. Nothing seems warm at all, but I have been concerned with some of the bolts that have multiple lugs on them might not be deep enough. Might get some longer bolts. Are zinc bolts ok for this application?

As for the wire gauge: all battery cables are 00 as well as the cables running to and from the shunt and battery shutoff.
2 gauge wire running from the charge controller to the fuse and I’m going to replace the wires running to the inverter to be either 2 gauge or 00, but I ran out of lugs so I have the wires that came with the Giandel inverter for now.
 

Q-Dog

¯\_(ツ)_/¯
Joined
Dec 22, 2020
Messages
637
If connections are not getting hot, you could label each battery and write down the voltages, then move the odd ones to a different spot in the pack and see if the problem follows the battery.
 

Substrate

Solar Addict
Joined
Apr 28, 2021
Messages
871
Location
SoCal
STOP - RIGHT NOW!

Your batteries are severely out of balance and they are screaming for attention. You \may\ have already done major damage by frying them, or out-gassing them.

Before assembling your multi-battery bank together, did you INDIVIDUALLY CHARGE each one to full using an AC powered charger first? This "first birthday" so to speak insures that not only are each battery at the same relative level in balance before assembling them into a large bank, but also ensures that each CELL inside those individual batteries are balanced.

If you don't have an AC charger at your cabin, then repairing this imbalance is going to take some time. We can do it two ways:

1) Those agm's are "conventional" agm's and have an inrush current limit of typically 0.25C. Which for a 155ah agm, means that you shouldn't exceed (155 * 0.25) = 38.5A individually. Can you limit your solar array to say 40A or less to give each individual battery the very first and most important charge it needs?

Depending on your solar array's output, we're talking weeks to maybe a month or more with solar to charge individually once for all 8, before you try to use them as a larger bank.

2) The other option is to FLOAT CHARGE the entire bank at a lower voltage of 13.6v for a few weeks, before trying to use them again with a more normal 14.6v cv/absorb.

I think your best option is to somehow find / drive to a buddie's place with a source of AC, and get a charger set for AGM (typically 14.6v CV, and 13.6v absorb) and get all 8 of those charged individually.

Mark and identify those two screaming batteries, which I think were "D" and "F". When the imbalance is corrected, they are likely to be the ones to go again out of balance first. Ideally, when charging there should be no more than a 0.1v difference between batteries. If not, then either those batteries are damaged, or there is an infrastructure issue (loose cables etc) between them.

I see this a lot - even on singular vehicular installs - where the agm battery never once in it's lifetime gets that all important "first birthday" full-charge - nevermind being put into a large bank array.
 
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