Battery bank with slightly damaged battery

Options
System
System Posts: 2,511 admin
in Solar Beginners Corner #1
Hi everyone,

I have a pretty basic question, but have done some searching and haven't found the exact situation I am in... Hoping someone can help shed some light on my situation.

I have 3 180ah gel cell sealed batteries (20 hr rate). They sat for a while while discharged (about a year). I have had a batteryminder on them for a good few weeks each. (from batteryminders.com - does full time desulfation, etc..)

One of them came back quite well. I've been testing by charging them fully and then running about a 12A load off them. I have an analog clock that stops when the inverter shuts down due to low power. The first one ran for 9.5 hours, which I was totally happy with.

The second ran for 7.5 hours, the third for 6.5 hours. I've been concentrating on the 6.5 hour battery with the batteryminder, but have yet to see the improvement that I saw on the first one.

I run a radio station in the desert off these batteries and some solar panels for a couple weeks a year. I am wondering what the best way to get the most out of my batteries would be. The options I can think of are:

a.) connect them in series. Will the battery bank perofrm:
a1.) only as much as the worst battery? (i.e. 6.5hours * 3) = 19.5?
a2.) an average of all 3, so (~7) * 3? = 21?
a3.) somewhere in between?

b.) put a battery switch between the batteries and the load and the batteries and the charge controller, charge one battery at a time, run it down, swap in a charged battery on switch position B, flip the switch, take the drained battery and charge it, and so on. 6.5 + 7.5 + 9.5 = 23.5?

Interested in other ideas.

Thing is: I can't replace my batteries, not enough cash/time/etc... so I need to make do with what I have

and

I would REALLY prefer not to have to power down to switch batteries. Ideally, I would like to avoid swapping batteries out and monitoring the state of the batteries, which would be necessary in scenario b.

Thanks for any thoughts you might have.

Take care,
Erich
· Share on Twitter

Comments

  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #2
    Options
    Re: Battery bank with slightly damaged battery

    use them for now, but start saving for new batteries for if you multiply the time by that 12a load this is giving you a rough ah capacity for each of those batteries. the best one at 9.5hrs and a 12a load is 114ah. this is about 63% of full rated capacity and generally when it's as low as 80% it's suggested to get new ones. the worst at 6.5hrs at 12a is 78ah or about 43%. they aren't long for this world.
    · Share on Twitter
  • System2
    System2 Posts: 6,290 admin
    #3
    Options
    Re: Battery bank with slightly damaged battery

    I understand, they really just have to get through the week.

    So would you say just connect them in parallel and use them as a bank? I guess I was wondering if the bank would perform at the worst battery rate (78ah * 3) or would it sort of be an average of the 3?

    Thanks for the suggestion - definitely going to look at other options for next year.

    Thanks,
    Erich
    · Share on Twitter
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #4
    Options
    Re: Battery bank with slightly damaged battery

    if you can use them as a bank then fine, it may take some stress off of it, but the worst battery will start to pull down the other batteries. no matter what way you do this they will go sooner or later and depends on your loads and charges to them and other factors as well. a simple small trickle charger (solar or otherwise and regulated) connected would have stop this from happening. if you go with the same ah ratings and number of batteries this is 180ah x 3 = 540ah total. the trickle would be 1% or 2 % of this for 5.4a-10.8a.
    · Share on Twitter
  • System2
    System2 Posts: 6,290 admin
    #5
    Options
    Re: Battery bank with slightly damaged battery

    Yeah, I know it's gonna hurt and they're not going to make it after this week of use. It was my mistake leaving them discharged, I just didn't know at the time.

    Thanks for the info!

    Cheers,
    Erich
    · Share on Twitter
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #6
    Options
    Re: Battery bank with slightly damaged battery

    don't feel too bad as most of us have learned things the hard way with batteries, including me.:cry:
    · Share on Twitter
  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #7
    Options
    Re: Battery bank with slightly damaged battery

    Erich,

    Just to be clear--in your case, connecting them in parallel is probably your best solution (assuming 3x 12 volt batteries in parallel). Yea, some will provide more power and others less.

    Putting the three in series (3x 12 volts = 36 volts) will be a nightmare. The "weakest battery or cell" will die first and start reverse charging (more damage)--while the others try to keep pumping out the current.

    But--I think you may have misstated the "series" connection (like 3x D cells in a flashlight) and you really intended to say parallel... Not many devices out there can run happily on 10.5 - 36+ VDC range.

    If your devices can run on 10.5-36+ volts--paralleling the 3 batteries will still give you the most available power and least side effects. I would not bother running one battery at a time.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
    · Share on Twitter
  • System2
    System2 Posts: 6,290 admin
    #8
    Options
    Re: Battery bank with slightly damaged battery

    You're right, I meant parallel. Everything I use runs 12v :P

    Thanks!

    -Erich
    · Share on Twitter

Categories