AGM Lead Batteries 4S3P 24V In Solar Off Grid: Cause Of Voltage Imbalance?

[KITROBASKIN]

Inherited this system though did replace batteries January ‘21. The existence of diysolarforum was unknown at the time. LFP batteries were too expensive an upfront cost for us at the time. (Not so now!)

I did tighten the battery terminal connections this early spring, some got a little tiny bit tighter. Did not use a torque wrench. First (not dated) readings were before that in mid February.



Interstate battery vendor gave me (I had to ask for it) this spec sheet; said it was the same battery, different label.



About 15 year old charge controller receiving 15 year old Conergy 175WX4 panels and 8 ancient Arcosolar/Carrizo panels maybe outputting similar to one of the Conergy panels. Operating voltage from array is labeled 68v, maximum 90V. Current 17A, 22A



Readings taken at different stages of absorption I think.


Is there anything I can do to extend their life? Was thinking some batteries were getting worked harder due to their location and maybe rotating them would help but this is not based on knowledge.

We now have two other LFP systems: one covers Viasat internet and summer ventilation, other is mobile providing supplemental and emergency. The lead AGM rarely go below 85% charged and with our sunny location, float maybe 94% of the days.

Thank you for taking the time to look at this and any help you can offer.

Kitro

 

[sunshine_eggo]

I presume the issue arose because the individual 6V were not charged to full individually and then in parallel prior to installation.

Mix and match:



Move 7.32 into 7.42 position
Swap 7.30 and 7.34 positions
Resulting two strings will vary only by 7.30-7.32 on top string and 7.34-7.36 on bottom string
Connect in parallel and charge 7.42, 7.57, 7.22 and 7.26 to 7.50V (temperature compensated). Hold until current drops to 4.2A.
Float at 6.90V (temp compensated) for 24 hours.
If you have a 30V/10A power supply for LFP battery balancing, it's a great tool for this.
Build the 4 parallel charged outliers into a new 4S string and place in parallel with the other two strings.
Continue monitoring voltage near end of absorption cycle and adjust as needed.

The mandatory goal is to keep all 6V in the 7.20-7.50V absorption range factoring in temperature compensation. Going outside this range will damage them. The desired goal is to get them as close as possible.

 

[KITROBASKIN]

You are correct they were not charged individually and in parallel by us; simply took them the old batteries, purchased from Interstate, lugged in and assembled by myself and willing wife.

I did put a meter to measure voltage on each one before leaving the store. There was some variation but nothing big. This was well before learning about diysolarforum. We are only Solar in terms of our source for electricity. Thinking how to do this…

Really appreciate it!

 

[SamDeleted]

You are correct they were not charged individually and in parallel by us; simply took them the old batteries, purchased from Interstate, lugged in and assembled by myself and willing wife.

I did put a meter to measure voltage on each one before leaving the store. There was some variation but nothing big. This was well before learning about diysolarforum. We are only Solar in terms of our source for electricity. Thinking how to do this…

Really appreciate it!

How are they wired together, could you draw a diagram?

Are all batteries being drawn on/charged equally?

SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank

 

[KITROBASKIN]

Sunshine’s image shows the latest readings, with lines between numbers showing the wiring. Negative to battery is upper left, positive is lower right. 3 parallel strings making 24V (each battery is nominal 6V) running horizontally. First image, first post shows battery box where cables are evident. Hey thanks for taking a look!

 

[sunshine_eggo]

I didn't state it explicitly, but you would continue to operate with 2 of the 3 strings while you get the 3 batteries "balance" charged.

 

[KITROBASKIN]

Sounds good and when re-joining the “balance” charge batteries back to the entire group, all the other batteries would already be charged ‘100%’ normally with solar?

We have 2 other Victron charge controllers (MPPT 150/70 and MPPT 100/20 48V) working with separate 2X175W and 3X100W “12V” panels. Too bad those SCC are not able to charge 6V batteries.

When the lead AGM reach the end for our use, we will probably go with 48V LFP and figure out a way to use the hopefully still functioning 24V 3500VA Outback inverter, getting a 48V inverter when the Outback expires. So trying not to put any more money into the lead AGM system, even by purchasing a 6V bench charger?

Your thoughts?

 

[sunshine_eggo]

Sounds good and when re-joining the “balance” charge batteries back to the entire group, all the other batteries would already be charged ‘100%’ normally with solar?

This is a balancing exercise. They appear to be close enough in voltage that when the charger is at peak system voltage, they will all be very close.

We have 2 other Victron charge controllers (MPPT 150/70 and MPPT 100/20 48V) working with separate 2X175W and 3X100W “12V” panels. Too bad those SCC are not able to charge 6V batteries.

Nope. No 6V.

When the lead AGM reach the end for our use, we will probably go with 48V LFP and figure out a way to use the hopefully still functioning 24V 3500VA Outback inverter, getting a 48V inverter when the Outback expires. So trying not to put any more money into the lead AGM system, even by purchasing a 6V bench charger?

Your thoughts?

Anyone who deals with batteries should have one of the 30V/10A benchtop power supplies everybody uses for top balancing. They are incredibly useful, they can even save Christmas:

DIY Solar Forum Saves Christmas!

Wife's enthusiasm for X-mas decorations is waning with age. She is within sight of my X-mas apathy... Her enthusiasm was somewhat renewed by the idea of a "popup" X-mas tree... Oh the glory... open the box, tree complete and decorated in 8 minutes! DISASSAPOINTMENT...

diysolarforum.com

 

[KITROBASKIN]

Point taken and appreciated. That power supply could help with imbalanced cells in our SOK and EPOCH batteries as well.

 

[pollenface]

Consider using a HA02/PowMr 48v balancer. It will work with 4s 6v-24v config as the balancer only requires 2v per leg.

ha02 - Google Search

Anyone who deals with batteries should have one of the 30V/10A benchtop power supplies everybody uses for top balancing. They are incredibly useful, they can even save Christmas:

They are sooo good! And now I'm getting a 60v one as well.

 

[sunshine_eggo]

Point taken and appreciated. That power supply could help with imbalanced cells in our SOK and EPOCH batteries as well.

And it could be used to individually boost lagging 6V in the future rather than breaking down the bank.

Per @pollenface, a balancer for each string would help maintain balance moving forward. You would need one per string.

 

[Zwy]

Interesting that Interstate has equalization listed for an AGM battery. I would only perform EQ if absolutely necessary, in other words as a last ditch effort to bring a battery back.

I would split off one bank at a time and charge those 4 in parallel using absorption voltage setting, then float as Eggo has posted. Then move onto next group of 4 and then final 4. Monitor the battery temps at all times, overcharge on a AGM can lead to electrolyte loss. Keep amps down to minimize heat and possible venting.

I would consider this as part of the maintenance schedule that should be done every few months. This will ensure the bank stays in balance to prevent sulfation of lower SOC cells.

 

[HarryN]

I have had good success putting all of the AGMs for a project in parallel on a charger and doing a few charge / discharge cycles to create essentially "matched sets" from new Lifelines. That said - I have been told by a local battery distributor that they tend to run unusually consistent.

Still - if it were me, I would buy a good 6 volt charger and try it, even if the amps were very low.

Every 5 - 6 hours - unplug the charger and plug back in to reset any built in timers.

After a few days things tend to settle out.

There are some battery pack balancers out there, for example dual pro - but the one that I saw on their site was for 12 volt instead of 6 volt.

The other supplier that I used to see a useful product for your application seems to have stopped building it. (along with several other useful product lines unfortunately )

 

[KITROBASKIN]

Really appreciate the information. Very much.