• Have you tried out dark mode?! Scroll to the bottom of any page to find a sun or moon icon to turn dark mode on or off!

diy solar

diy solar

2S2P 24V AGM setup issues/problems (low voltage on end of string), recommendations?

caveman.b

New Member
Joined
Oct 8, 2024
Messages
2
Location
Corpus Christi, TX
I was gifted 4 used 12V 126AH AGM batteries a couple of years ago when a friend upgraded his (new to him) sailboat and they have been sitting in my garage (being charged every once in a while). I am not sure what configuration they were used in before, but I suspect it was 2S2P. Two have a date of 6/2020 and 2 of them have date of 2/2020.

I wired them into a 2S2P configuration (in dated pairs) in my cargo trailer after charging all batteries within .01 V of each other to power a 24V system in a test situation temporarily until I get permanent location for all of the equipment (inverter, solar, shore power, transfer switch, charger all separate). The batteries are all connected direct without bus bar (lead battery for one parallel string directly connected to main pos and the other to neg bus). I don't have any individual monitoring set up on the batteries, currently. After operating (mostly sitting with charger attached - wired to main pos and neg bus) and using some 12v led lights (with 12V converter) infrequently for a couple of months I tested each battery individually. The end battery (ie not connected to the main pos or main neg) on each series string was very low (2.0 V and 3.2 V). The lead battery on each string had voltage of 13.1.

I disconnected them immediately and have since charged them up and let them normalize a few times. The batteries that were lead on each string seem fine (rest at 12.99-13.0 after 24 hr) and the other two that experienced the low voltage normalize at 12.85 V. I have seen different charts say different things for expected voltages but majority say resting 100% for AGM is 12.80 so I am inclined to think that something has changed but they are probably still good.

What might have happened to the batteries to cause the low voltage on the end string battery? Can I still use them without causing issues? If so should I pair the similar resting voltage batteries together in series (instead of date pairs)? If not is there any use for them still? Will they cause problems if I use them all together in the future in a 12V parallel system (upgrading to LiFePO4 soon).

I also have a couple of HA01 battery equalizers I can use but I was skeptical about the instructions for the 2s2p instructions. How do I wire these? Do I truly only need one for the entire setup and wire the white to all four batteries as shown in the pic attached? I did see this thread and it sounds like I should wire like the first image (showing 2 batteries in series) on each series string. https://diysolarforum.com/threads/battery-equalizer-for-2s2p-agm.52992/ - my battery setup looks just like the poster's first diagram.

Thanks in advance for any assistance/advice!
 

Attachments

  • Media (1).jpg
    Media (1).jpg
    274.8 KB · Views: 2
  • Media.jpg
    Media.jpg
    319.1 KB · Views: 2
  • Media (2).jpg
    Media (2).jpg
    206.2 KB · Views: 2
Last edited:
I was gifted 4 used 12V 126AH AGM batteries a couple of years ago when a friend upgraded his (new to him) sailboat and they have been sitting in my garage (being charged every once in a while). I am not sure what configuration they were used in before, but I suspect it was 2S2P. Two have a date of 6/2020 and 2 of them have date of 2/2020.

I wired them into a 2S2P configuration (in dated pairs) in my cargo trailer after charging all batteries within .01 V of each other

Charging 12V properly depends on both current and voltage, and you need to FULLY charge them before placing them in series.

Generally speaking, your batteries are fully charged once they've reached 14.4-14.8V, and the current flowing to the battery has dropped to about 2.5A.

to power a 24V system in a test situation temporarily until I get permanent location for all of the equipment (inverter, solar, shore power, transfer switch, charger all separate). The batteries are all connected direct without bus bar (lead battery for one parallel string directly connected to main pos and the other to neg bus). I don't have any individual monitoring set up on the batteries, currently. After operating (mostly sitting with charger attached - wired to main pos and neg bus) and using some 12v led lights (with 12V converter) infrequently for a couple of months I tested each battery individually. The end battery (ie not connected to the main pos or main neg) on each series string was very low (2.0 V and 3.2 V). The lead battery on each string had voltage of 13.1.

Curious how 13.1+2.0 and 13.1+3.2 are a full 1.2V different, which is not possible when you place batteries in parallel. I'm going to assume you are making approximations.

I disconnected them immediately and have since charged them up and let them normalize a few times.

What do you mean by "normalize"?

Did you charge them consistent with the process I describe above?

The batteries that were lead on each string seem fine (rest at 12.99-13.0 after 24 hr) and the other two that experienced the low voltage normalize at 12.85 V. I have seen different charts say different things for expected voltages but majority say resting 100% for AGM is 12.80 so I am inclined to think that something has changed but they are probably still good.

"lead on each string" = ?

12.9-13.0 is more accurate

The fact that they're .15V different means something is different (worse), or they weren't fully charged.

What might have happened to the batteries to cause the low voltage on the end string battery?

Unknown. what you describe in an operational system is essentially impossible.

Were you drawing 12V loads from half of the 24V battery?

Can I still use them without causing issues?

Unknown. More evaluation needed.

If so should I pair the similar resting voltage batteries together in series (instead of date pairs)?

If they seem to improve following the recommendations below, this is the most sensible config.

If not is there any use for them still? Will they cause problems if I use them all together in the future in a 12V parallel system (upgrading to LiFePO4 soon).

Unknown. In parallel, if they're unhealthy, they will pull charge from the "good" batteries.

I also have a couple of HA01 battery equalizers I can use but I was skeptical about the instructions for the 2s2p instructions. How do I wire these? Do I truly only need one for the entire setup and wire the white to all four batteries as shown in the pic attached? I did see this thread and it sounds like I should wire like the first image (showing 2 batteries in series) on each series string. https://diysolarforum.com/threads/battery-equalizer-for-2s2p-agm.52992/ - my battery setup looks just like the poster's first diagram.

You would wire each HA02 to each string of 2S 12V without any crossover in the center.

Very confused how you had an operational 24V system when the battery bank is at 15-16V.

Is this a correct visualization of your config:

1735593185496.png


Recommend you hold all 4 batteries at 14.8V for 24 hours individually or in parallel. This is analogous to an "equalization" charge for flooded lead acid.

After holding them at 14.8V for 24 hours, let them rest individually for 24 hours and record their resting voltage.

I suspect your friend operated these batteries out of balance for an extended period, which caused damage to the two.

At nearly 5 years of age, dubious service history, and unknown state of health, it may be time for the lead recycler. AGMs may be maintenance free, but they have about 1/2 the life expectancy of well-maintained flooded, and they still must be treated well and operate within their ratings.
 
Curious how 13.1+2.0 and 13.1+3.2 are a full 1.2V different, which is not possible when you place batteries in parallel. I'm going to assume you are making approximations. I measured the voltage several times each because I thought it was an error. It could be that the cheap multimeter is faulty. I have ordered a better meter (Klein) for future use. The voltages were as noted in the image attached.

What do you mean by "normalize"? Resting (24 Hr).

Did you charge them consistent with the process I describe above? I charged them individually - I will try again charging in parallel, then disconnect them and let them rest for 24 Hr as you describe

"lead on each string" = ? Per the diagram attached I meant the battery that had the connection running to the (neg or pos) bus. For neg it is a bus bar for positive it is a breaker

Were you drawing 12V loads from half of the 24V battery? No. I have a step down converter that takes 24V to 12V for 12V loads.

After holding them at 14.8V for 24 hours, let them rest individually for 24 hours and record their resting voltage.
Can you recommend a charger that I can achieve the equalization charge with? It seems my charger only does 14.4 for AGM and a pulse/repair mode (that does not maintain). It's this one: https://www.amazon.com/Battery-Charger-Lead-Acid-Maintainer-Motorcycle/dp/B0B3CV4S2Y/ref=sr_1_10.

Thank you for all of the detailed information.


1735598657366.png
 
Curious how 13.1+2.0 and 13.1+3.2 are a full 1.2V different, which is not possible when you place batteries in parallel. I'm going to assume you are making approximations. I measured the voltage several times each because I thought it was an error. It could be that the cheap multimeter is faulty. I have ordered a better meter (Klein) for future use. The voltages were as noted in the image attached.

Okay, so it's likely due to variation over time. The tool doesn't matter THAT much as long as it's used consistently.

What do you mean by "normalize"? Resting (24 Hr).

(y)


Did you charge them consistent with the process I describe above? I charged them individually - I will try again charging in parallel, then disconnect them and let them rest for 24 Hr as you describe

I was asking if you charged them to 14.4-14.8V and HOLD that until about 2.5A. Fully charging lead acid must meet a voltage and tail current criteria.

Parallel charging is fine. Just make sure you disconnect them at the end of the charge before the 24 hour sitting clock starts.

"lead on each string" = ? Per the diagram attached I meant the battery that had the connection running to the (neg or pos) bus. For neg it is a bus bar for positive it is a breaker

Okay. Position within the string doesn't matter unless there are external influences, like a hot wall or something that might make one battery run hotter than the others.

Were you drawing 12V loads from half of the 24V battery? No. I have a step down converter that takes 24V to 12V for 12V loads.

Just wanted to make sure. This has screwed up 2S banks before.

After holding them at 14.8V for 24 hours, let them rest individually for 24 hours and record their resting voltage.
Can you recommend a charger that I can achieve the equalization charge with? It seems my charger only does 14.4 for AGM and a pulse/repair mode (that does not maintain). It's this one: https://www.amazon.com/Battery-Charger-Lead-Acid-Maintainer-Motorcycle/dp/B0B3CV4S2Y/ref=sr_1_10.

Thank you for all of the detailed information.


View attachment 266690

"Canned" chargers are mostly ass, unless you're investing big $ in something like a Victron bluesmart charger or other programmable charger.

I would recommend you get a benchtop power supply.

Another forum user recently purchased this one, and it looks like a pretty good deal:


You can set the exact voltage and see the current. It's not a charger, but they're crazy useful.

Any 30V/10A power supply that fits your budget and has decent ratings is probably fine.
 

diy solar

diy solar
Back
Top