One cell 0.025 lower than the rest...

[BotanyBay]

Just finished equalizing a 24 volt bank of wet cell golf cart batteries.

This bank is two strings of 6V wet cell batteries, no cross connections. Recently rewired to work with upgraded electrical system. All wires are within a quarter of an inch of each other.

One cell remains slightly low (about 0.025 on the hydrometer) all of the rest of the cells are +/- 0.005...

The voltage of that 6 volt battery is about 0.05V low compared to the other batteries. Using two Victron 702 battery monitors with midpoint monitoring so I can see the difference across the run when pulling the batteries down and charging. Never more than about 0.4% top of bank to bottom.

One bank is extremely flat with no difference top to bottom across the entire discharge / charge cycle. Current AH when run down to 10.5 volts for load testing at .2C is 225AH

One bank when approaching 165 AH used has one battery which falls off the cliff, top to bottom ratio goes to about 4% and the voltage on the one battery rapidly falls below 5V. The middle cell of that battery shows a very low hydrometer reading. Battery recharges and does not consume water.

The bank is almost 5 years old (I know, count my blessings).

The question is in the short term do I

1) Consider it close enough and keep running as is (perhaps isolate the string with the weak cell when I am not on the boat)

2) Replace just the one battery with another identical new battery (knowing that the battery will be throw away when I replace the whole bank)

If putting a new battery in just results in the new battery coming down to the capability of the rest of the bank then that would be fine if there is some value in getting the weak cell out of the bank. The $100 for one new battery as a throw away is not a big deal if there is no other downside.

Thanks!

 

[BotanyBay]

A note about the equalization applied

For equalization my battery charger provides up to 8 amps for 1 hour (100Amp Skylla-I 24v from Victron)

I ran 1 hour of equalization on the bank with the weak cell (220ah bank) ~3.6% of capacity, and then 1 hour of equalization against the whole bank (440Ah bank) ~1.8% of capacity.

This is a temperature compensated charger with the probe hooked up.

I saw the low cell come up from 0.040 low to about 0.020 - 0.025 low after this process.

Reading here it would appear that a longer equalization of the single bank is inorder. I do have the ability to program the charger to equalize at 25A max current also which would be 5.7% of capacity until it hits the upper voltage limit.

From memory, at the end of the equalization cycle the sg was 1.300, after cooling it was 1.275 +/- 0.005 for all of the cells except the weak one. There might be slightly larger variation across the string with the weak cell +/- 0.010 would be the worst case though.

 

[Maine Sail]

Something seems off? I can't see any possible way a 5 year old 220Ah bat can put up 225 Ah's at a .2C load (44A)? The load should be:

CAPACITY DIVIDED BY 20 or 220Ah/20=11A

For your bank this should be a 0.05C or 11A load for 20 hours or until the battery hits 10.5V. A capacity test is only valid & useful if the batteries are discharged at the correct load, temp and the load is held as steady as possible.

Series cells can get out of balance if not regularly pushed to equalization voltages. When ever I place cells in series I parallel balance them first. This helps keep them at a similar SOC so when discharged they are all seeing a similar discharge floor. This is a practice that used to be common place but folks rarely do it today.. I see many series batteries out of balance because they simply were not properly commissioned.

The amperage for an equalization is not what is important. Ideally the current should be the bare minimum to hold the batteries at 15.5V to 16V (31V - 32V for 24V) and no more. Equalization can easily need 8 hours at voltage to be effective..

If you have a bad batt, and all others are good, throw in a new one to get you another year of so... Parallel balance the new pack before installation...

 

[BotanyBay]

Something seems off? I can't see any possible way a 5 year old 220Ah bat can put up 225 Ah's at a .2C load (44A)? The load should be:

CAPACITY DIVIDED BY 20 or 220Ah/20=11A

For your bank this should be a 0.05C or 11A load for 20 hours or until the battery hits 10.5V. A capacity test is only valid & useful if the batteries are discharged at the correct load, temp and the load is held as steady as possible.

Series cells can get out of balance if not regularly pushed to equalization voltages. When ever I place cells in series I parallel balance them first. This helps keep them at a similar SOC so when discharged they are all seeing a similar discharge floor. This is a practice that used to be common place but folks rarely do it today.. I see many series batteries out of balance because they simply were not properly commissioned.

The amperage for an equalization is not what is important. Ideally the current should be the bare minimum to hold the batteries at 15.5V to 16V (31V - 32V for 24V) and no more. Equalization can easily need 8 hours at voltage to be effective..

If you have a bad batt, and all others are good, throw in a new one to get you another year of so... Parallel balance the new pack before installation...

Oops, typo, intended to put down 205AH for the good bank, hit the '2' key twice and did not catch it! Thanks!!!

The question is how far out of balance is too much. With the bank average being 1.275 and the lowest cell being 1.250 is that too far out of balance? The highest cells are 1.285 and other than the low one the next lowest cell is 1.265. Most of the cells are between 1.270 and 1.280.

I will definitely run a longer equalization and then determine what the capacity of each bank is again.

Thank you very much for the information!