Battery voltages unbalanced

[RoyceC]

I have six L16 (6-volt) flooded lead/acid batteries that are over seven years old now and on their last legs. They are connected in pairs so that each pair is 12 volts. When I measured the battery voltages this morning, two of the pairs were out of balance. In the worst pair, one of the batteries read a little over 7 volts and the other battery read under 5 volts. The 7-volt battery was bubbling a lot more than any of the other batteries. What is the cause of this imbalance? Should I remove the batteries that read low from the system?
Another of the pairs reads about 6.5 volts vs. 5.25 volts. The other pairs are balanced at 6.25 volts each (or very close to that). When I measured the voltage for the pairs of batteries, each of the three pairs reads about 12.5 volts.
I took the voltage measurement during the morning, overcast sky, with a 700-watt solar array.
I know the batteries are way too old and need to be replaced. We are hoping to nurse them along for a few more months until funding becomes available so we can buy new ones.
Any suggestions?

Thanks,

Royce
Manager at Walnut Creek Center for Education and Research (WCCER)

[inetdog]

I have six L16 (6-volt) flooded lead/acid batteries that are over seven years old now and on their last legs. They are connected in pairs so that each pair is 12 volts. When I measured the battery voltages this morning, two of the pairs were out of balance. In the worst pair, one of the batteries read a little over 7 volts and the other battery read under 5 volts. The 7-volt battery was bubbling a lot more than any of the other batteries. What is the cause of this imbalance? Should I remove the batteries that read low from the system?
Another of the pairs reads about 6.5 volts vs. 5.25 volts. The other pairs are balanced at 6.25 volts each (or very close to that). When I measured the voltage for the pairs of batteries, each of the three pairs reads about 12.5 volts.
I took the voltage measurement during the morning, overcast sky, with a 700-watt solar array.
I know the batteries are way too old and need to be replaced. We are hoping to nurse them along for a few more months until funding becomes available so we can buy new ones.
Any suggestions?

Thanks,

Royce
Manager at Walnut Creek Center for Education and Research (WCCER)

Hi Royce.

If the two cells at 5 volts and 7 volts are in parallel, you must have disconnected them from the string to measure two different voltages, or else one of your connecting wires is broken.
If the 5 volt battery is out of the circuit, its voltage will go steadily down from self-discharge and all of the charging current will be forced through the other battery, causing it to overcharge and boil.
Any system with batteries in parallel requires careful monitoring (voltages and SG) to make sure it stays balanced. You need to correct this ASAP even if it means cutting back to only one series string containing only the good batteries. (Be careful that the charging current is not too high for that string!)

OOPS, let's try that again. My assumption above is that you had the batteries in parallel and then connected pairs in series. Since you really have series pairs in parallel, my answer is different:

The battery which shows 5 volts probably has an internal short circuit of one of the cells. That is causing the voltage of that series pair to be low and hog the charging current, boiling the good battery.
The solution remains the same: construct as many pairs as you can using only good batteries and put those in parallel while you limp along.

And for your new batteries, look at getting 2 volt batteries of the total amp-hour rating you need and putting them in series.
Avoiding having batteries in parallel will make their life and yours easier. There are also better battery form factors available now than L16.

BTW, you must have been doing something right to get 7 years out of them!

[Sunking]

Well you just learned an expensive lesson the hard way. Never put secondary batteries in parallel. If you do it will destroy them in short time. At 7 years you are already 2 years over due for replacement. Assuming these are 350 AH L16 means you have a 3 x 350 AH = 1050 AH battery @ 12 volts. Bottom line it is time to replace the batteries, yours are toast.

If you insist on a very inefficient 12 volt toy you need to replace the batteries with 1050 AH batteries like Rolls S-1380 model which is a 2 volt 1050 AH battery and you would need 6 of them for a 12 volt system. But if you are smart you will move up to a 24 or 48 volt system and use 500 or 250 AH cells respectively to voltage.

Have a great day and hope that helps.

[RoyceC]

Thanks for the info. I will remove the low batteries and reconfigure the good batteries into two series pairs. Then we need to find some money quick! I've got to replace these batteries sooner rather than later! I used to have eight batteries in the system but the same thing happened to them and I removed the low ones and re-configured to run six. Now I have to reconfigure again.

Another related question:
We are planning on upgrading the whole system sometime within the next couple of years. I don't know exactly when. The new system will require at least twice as many batteries as I am using now. If I bought six new batteries now, and then another six in a year (maybe two), would that work out alright? I know I am supposed to buy all the batteries at once but I don't think I can wait until the new system funding becomes available. Batteries are expensive and our funding is minimal.

Hi Royce.

If the two cells at 5 volts and 7 volts are in parallel, you must have disconnected them from the string to measure two different voltages, or else one of your connecting wires is broken.
If the 5 volt battery is out of the circuit, its voltage will go steadily down from self-discharge and all of the charging current will be forced through the other battery, causing it to overcharge and boil.
Any system with batteries in parallel requires careful monitoring (voltages and SG) to make sure it stays balanced. You need to correct this ASAP even if it means cutting back to only one series string containing only the good batteries. (Be careful that the charging current is not too high for that string!)

OOPS, let's try that again. My assumption above is that you had the batteries in parallel and then connected pairs in series. Since you really have series pairs in parallel, my answer is different:

The battery which shows 5 volts probably has an internal short circuit of one of the cells. That is causing the voltage of that series pair to be low and hog the charging current, boiling the good battery.
The solution remains the same: construct as many pairs as you can using only good batteries and put those in parallel while you limp along.

And for your new batteries, look at getting 2 volt batteries of the total amp-hour rating you need and putting them in series.
Avoiding having batteries in parallel will make their life and yours easier. There are also better battery form factors available now than L16.

BTW, you must have been doing something right to get 7 years out of them!

[Sunking]

Thanks for the info. I will remove the low batteries and reconfigure the good batteries into two series pairs.

You do not have any good batteries to work with.

3 Battery rules to live by:

1. Never ever mix old batteries (older than 6 months) with new batteries. If you do, the new batteries will be old batteries and need replaced very soon. Reference Battery Bible
2. Refer to rule # 1.
3. If you have trouble reading the Battery Bible, refer to rule #1.

[RoyceC]

THanks for the input on running batteries in parallel. I did not know that. Yeah, they are 350 AH. A major upgrade is in the works sometime in the next year or two. Since the money comes from grants and such, and involves large bureaucracies, I don't know when the money will be available. Meanwhile, I need to do something now - whatever is cheapest. When we upgrade our system, I plan to go to 48-volts and get good batteries configured in the most efficient way. Your response was very helpful. Our current inverter is an old 12-volt, 2000w Trace that still works fine. So, for now, I have to configure what batteries I have to produce 12 volts. I know it is kind of a miracle that the batteries I have are even working at all.

Well you just learned an expensive lesson the hard way. Never put secondary batteries in parallel. If you do it will destroy them in short time. At 7 years you are already 2 years over due for replacement. Assuming these are 350 AH L16 means you have a 3 x 350 AH = 1050 AH battery @ 12 volts. Bottom line it is time to replace the batteries, yours are toast.

If you insist on a very inefficient 12 volt toy you need to replace the batteries with 1050 AH batteries like Rolls S-1380 model which is a 2 volt 1050 AH battery and you would need 6 of them for a 12 volt system. But if you are smart you will move up to a 24 or 48 volt system and use 500 or 250 AH cells respectively to voltage.

Have a great day and hope that helps.

[RoyceC]

I see. Well, hmmmm. Maybe we can use candles and oil lamps until the funding comes through! We want to do it right when we upgrade so thanks for your help.
Royce

You do not have any good batteries to work with.

3 Battery rules to live by:

1. Never ever mix old batteries (older than 6 months) with new batteries. If you do, the new batteries will be old batteries and need replaced very soon. Reference Battery Bible
2. Refer to rule # 1.
3. If you have trouble reading the Battery Bible, refer to rule #1.

[Sunking]

Since the money comes from grants and such, and involves large bureaucracies, I don't know when the money will be available. Meanwhile, I need to do something now - whatever is cheapest.

OK then your best option is to remove 2 or 4 bad battery pairs and use them for a boat anchor, and get by on 2 or 4 tired batteries until the money comes. If you mix old and new batteries whatever you spend on new batteries will be wasted and will find yourself searching for even more higher funds again reel soon.

Trust me when the lights go out, they will find the funds. Just like when your car runs out of gas on the highway and your wallet is empty, you will find a means to money. Or you are unemployed, cannot find a job, and your 99 weeks of unemployment checks run out. You will find a job on week 100. All it takes is motivation.

[Mike90250]

to quote a TV series: "Winter is Coming". shorter solar days, longer nights to drain the batteries. You are in deep do-do.

[Beanyboy57]

Battery Bible.

You do not have any good batteries to work with.

3 Battery rules to live by:

1. Never ever mix old batteries (older than 6 months) with new batteries. If you do, the new batteries will be old batteries and need replaced very soon. Reference Battery Bible
2. Refer to rule # 1.
3. If you have trouble reading the Battery Bible, refer to rule #1.

Thanks for sharing the link.