Different resistances

[Jamblik]

Good day, from new me. I have 2 battery banks, Trojan, lead acid, 2500amps each, in parallel. 5000 amps total, 12 volt. Equal cables to charging equipment from banks to chargers, Victron chargers.. 2Volt cells, interconnected with copper busbars. The 1 bank gives more amps during night times and takes more amps during charging time as the other. More or less 30 / 70. I suppose that means its resistance is less than the other bank. it also has a higher SG than the other bank. 1.27 against 1.25. Is there a way that i can decrease the resistance in the other bank so that they will use more or less the same amps 50/50 ?. I thought about " breaking it in" like a new engine. Disconnecting the 1 bank and only working with the other for a couple of weeks. Putting it under heavy load without damaging it and then extending the Absorption time for a decent SG like 1.27 Any battery geniuses out there with sound advice. ? Thanks.

[SunEagle]

Hello Jamblik and welcome to Solar Panel Talk

You might be able to get both battery system to work equally the way you describe but IMO it is still a gamble. It is hard to get the internal and external resistance to be equal.

I also think you may have a charging issue since you need to have about a C/10 charge rate which is 500amps for a 5000Ah battery system. If you have less than a C/12 charge rate (or about 420amps) but at least 250amps my guess is one battery will not get the proper charge.

How many charging amps are you getting?

[Jamblik]

Hi Mr Eagle. I get more than enough amps. I have seen 350 amps charging when it was cloudy with no sun for 2 days. But most of the time they get to float around 3 to 4 pm I have 8 mppt chargers with 50 amps each, so that is more than enough. The batteries cycle at about 8 % average, 92 % of capacity left every morning. I thought of the more batteries i have, the lighter the load on them and then the longer they last.

[SunEagle]

Hi Mr Eagle. I get more than enough amps. I have seen 350 amps charging when it was cloudy with no sun for 2 days. But most of the time they get to float around 3 to 4 pm I have 8 mppt chargers with 50 amps each, so that is more than enough. The batteries cycle at about 8 % average, 92 % of capacity left every morning. I thought of the more batteries i have, the lighter the load on them and then the longer they last.

You are correct. In most cases a battery system will last longer if they are cycled less or for a smaller DOC. But getting multiple battery systems to charge and discharge equally can sometimes be difficult even with large wire jumpers that are indicated to be the exact length. Also as you have seen the internal resistance are different based on the plate size and fluid amount in each cell. Batteries from the same "batch" should be as close to equal as possible but you still take a chance.

[Jamblik]

Any idea on how to get the resistance of the 1 bank lower?
What i did not say in the original post, the 1 bank is much older than the other.

[SunEagle]

Any idea on how to get the resistance of the 1 bank lower?
What i did not say in the original post, the 1 bank is much older than the other.

Wiring two banks at different ages are a no no. The older bank will always bring down the newer one. I would say you are stuck with what you have but next time make sure all of the batteries are the same type and age.

[Jamblik]

Well i have enough equiment, so i might as well split the 2 banks, use 1 for lights and tv and the other for 2 fridges and a freezer.
I knew that it was not going to work, but i hoped someone had an answer that would work. Thanks.

[bob-n]

You have this well understood. Your solution of separating the strings for different functions is smart.

The old pack acts like it has higher resistance but there is no bad wire or place to add more copper. It's just the state of the metal plates in the cells and you can't fix that without new plates or new batteries.

We occasionally hear of someone with an idea of refreshing plates by pulsing current. A few companies claim that their chargers have cycles that will desulphate batteries, but these devices are rarely helpful. As the farmers say, you can't put the carrot back in the ground.

[Jamblik]

Hi Bob, thanks Bob. Its a pity the news is bad, but i know thats how it is. I have 2 Phoenix Victron inverters in parralel so i am going to use them seperate now. Luckily all my chargers are connected to 2 ,busbars + and -, so that can also change easily.