Battery Bank capacity help please

[Hilltopgrange]

Hi Guys

              I am trying to get my head around my battery bank capacity but unfortunately the cells in my head don't work as well as the cells in the battery!.

The battery is from an electric forklift and was originally configured for 72v ie 36 cells x 2v per cell and weighs 1.3 ton. I have the id plate from the battery but it is a bit tricky to get a clear pic but the capacity reads 511 A.h. at 5 hr rate.

Forgive me if this is a stupid question does that mean you can draw 511 amps for 5 hours or 1 amp for 511 hours? Is it that simple?

I have reconfigured the battery for 48v by disconnecting  one 12v string on each side of the bank and then reconnecting it in parallel. The pic below should explain it better. The top drawing is the original 72v setup as I got it and the one below shows how I reconfigured it. I have been using it since last Christmas and it works fine but I would like to know what its capacity is now with the 2 strings in parallel any ideas?

Thanks in advance for your help.

Russell

[DanB]

"Forgive me if this is a stupid question does that mean you can draw 511 amps for 5 hours or 1 amp for 511 hours? Is it that simple?"

It's supposed to be good for 511 Amp hours if you drain it in 5 hours.  (so in theory you could draw about 102 amps for 5 hours)  It's likely rated that way because it's a forklift battery and they want it to last 5 hours for 1 shift or something (but I'm not sure).  If you drain it more slowly - say draw only 1 amp, it's capacity would be more - you'd be draining it at a much slower rate.  Most deep cycle batteries are rated for a 20 hour rate.  I'm not sure how many amp hours a battery like yours is good for at a 20 hour rate  - but it would be more, maybe 600 or so (maybe someone here has a good idea about that).

"I have reconfigured the battery for 48v by disconnecting  one 12v string on each side of the bank and then reconnecting it in parallel."

Thats not a good situation.  Think of it as 4 12V batteries now, all in series - but two of them are twice as big as the other two.  When you drain the batteries the voltage between the two 'big' strings and the two 'little strings' will be uneven.  It probably wont take too long before you find yourself cooking the 'small strings' and never getting the large ones full charged.  It might work - if you barely use your batteries at all and keep them topped off all the time, but I think it's very bad practice.  

 "The pic below should explain it better. The top drawing is the original 72v setup as I got it and the one below shows how I reconfigured it. I have been using it since last Christmas and it works fine but I would like to know what its capacity is now with the 2 strings in parallel any ideas? "

Overall you'd have to consider the capacity the same as the smallest string of 12V - so 511 amp hours.  I think you should disconnect the xtra 24 V (like I have done because I have exactly the same problem - a 72 volt battery but I can only use 24) and then give me your extra batteries so that I can make use of mine!!! ;-)

Actually - what I've been doing is trying to keep my extras charged up and in good shape while shopping for a bargain on another 24V.  I do think you're better off not to use those - you risk ruining all your batteries.

[Flux]

Dan is right, you still only have 511Ah and you will sulphate the parallel strings.

Probably your best bet is to use one string at 48v and use the other 24v for a separate system with its own 24v mill. That would keep the spares alive and they could do something useful rather than just trying to keep them good as spares. If in later life you need some spares for the 48v bank you could use some of the 24v bank and find some lesser things to keep that mill running.

You could work a mixed voltage system with your present set up by feeding an additional mill with its controller into the 24v parallel string but it would be messy and need fairly constant watching. I think it would be better to use two systems.

Flux

[Hilltopgrange]

Hi Dan and Flux, thanks for the reply and sound advice I have disconnected the 2 spare strings as you advised, I had an idea it wasnt the way to go but I am glad I asked.

    I have a spare 24v future energy turbine on another tower with spare 24v inverters and controller etc so I think the obvious solution is to use the spare battery strings with it for my lighting circuits, Kill 2 birds with one stone! thanks for the advice guys! I hope I can return the favour someday!

Russell

[cyplesma]

if he has the bank wired so it's two equal battery banks isn't that 36v?

[cyplesma]

with a 1022ah @ 5 hour?

[Flux]

"if he has the bank wired so it's two equal battery banks isn't that 36v?"

Yes.

He hasn't done that, he has it wired for 48v with 2 strings of 12v in parallel, connected in series for 24v and the remaining 24v is a single string.

36v will give double capacity but that is not possible at 48v.

flux

[cyplesma]

neber mind, now that i reread an rereviewed the battery connections I see what he did.

[cyplesma]

if you rotate the banks by say once a month reconfigure the connections so that you use last months "spares" and set aside a different pair of banks for spares, will this swapping of spares be a better way to keep them alive, or does it kill all of them quicker?

Or does it just create too much hassle to keep track of the rotation?

[Hilltopgrange]

Hi, I think that would be to much work lol. I have installed a second turbine 24v that will run on the 2 spare strings, I also have 3 spare inverters(24v) and conrtoller ect so that should sort it. Thanks for the reply.

Russell