Author Topic: switching from 12 to 48V, battery bank configuration (Read 5864 times)

madlabs · « **on:** December 13, 2012, 12:25:54 PM »

Hi All,

I just ordered all the stuff I need to upgrade my system to 48V, plus a bunch more panels. Woohoo! But now I need to figger out what to do with my battery bank. I currently have 10 golf cart batteries in series/parallel for 1200aH @ 12V. These batteries are almost 4 years old and are showing some signs of age. While I have been very good about not discharging too deeply (very occasionally 70% but usually 80%) I haven't been very good about doing proper absorption stage charging.

Obviously, 8 of them will add up for 48V. But what to do with the other 2? Just parallel 'em in there somewhere? If I do that, what would be the best configuration?

Thanks!

Jonathan

mab · « **Reply #1 on:** December 13, 2012, 03:24:30 PM »

I would either stick with the 8 and keep the other 2 as spares, or try and get another 6 (ideally of a similar age/condition) and make another string.

trying to parallel them in with the other 8 will just cause problems IMHO.

thirteen · « **Reply #2 on:** December 13, 2012, 03:37:17 PM »

test them and use two as spares or as a set for a work light when working on the other system They would give you power for a light or tools. They might make a good set for another building just using the batteries alone and a small solar panel would keep them charged up if not used much.. Maybe a light or two ina store room or wood shed or to keep the beer cold for short perioid of time. They could be used anytime if one of the others would fail. That way they would be kept active. 13

phil b · « **Reply #3 on:** December 14, 2012, 07:59:46 PM »

I have 20 six volt golf cart batteries on a 24 volt system. There are 3 strings in series with the rest in parallel to those strings. I cannot tell any degradation from using them in parallel. I've learned the batteries that are low on water will have a few 10ths of a volt less than the others.

In your situation, I would make a 48 volt string from your best batteries and put the other two in parallel.

madlabs · « **Reply #4 on:** December 15, 2012, 09:01:50 AM »

Phil and 13,

Think what I will do is equalize the batts and then test 'em and see if there are any substantially weak ones. If there are, I will cull the two lowest from the herd and use them elsewhere. If they are all in similar condition I'll toss 'em in parallel.

If I wind up with them in parallel, what configuration (physically) would be best?

Thanks!

Jonathan

southpaw · « **Reply #5 on:** December 15, 2012, 10:02:47 AM »

Hi Madlabs
Adding the two extra batteries anywhere in your bank would be a mistake in the long run. They will throw the entire bank out of balance and reduce the life of your batteries. If battery life is not an issue then throw them in anywhere but there is no good way to add them to your system . Minute variances in the bank can have large consequences in charging and discharging rates. I found the link below very interesting and it explains how small things in a battery bank can have large consequences. Paralleling the two extra batteries anywhere in your bank is like having different size batteries in a bank it will work but not well and will be detrimental in the long run.
Take a minute to read this.
http://www.smartgauge.co.uk/batt_con.html
Southpaw

phil b · « **Reply #6 on:** December 15, 2012, 09:15:14 PM »

Southpaw that link you posted looks good for a 12 or 6 volt system. How can this be reconfigured for a 48 volt system?

southpaw · « **Reply #7 on:** December 15, 2012, 11:04:06 PM »

Phil
for your 48 volt system your 8 6v batteries would just be hooked in series. The point I was tying to make was how if you try to use your other two batteries hooked in parallel some where in your system it would throw all of the batteries out of balance and cause some of the batteries to over/under charge and you would gain no extra capacity as the batteries not hooked in parallel would limit the amount of useful power you could obtain. A chain of batteries is also only as strong as its weakest link. The article shows how things that may seem insignificant or trivial can have a drastic effect on battery life.
Southpaw

DamonHD · « **Reply #8 on:** December 16, 2012, 05:20:55 AM »

I'd say don't do it, too. I don't think it will increase usable capacity, and I think would definitely would push the system out of balance.

Find another project for those two I'd say.

Rgds

Damon

madlabs · « **Reply #9 on:** December 16, 2012, 11:08:28 AM »

Hey Phil, how long have you been using your batteries that way? Curious if the setup has survived for a while.

I know it is bad to have unbalanced banks but it's one of those things that seems like maybe the trade off can be worth it sometimes. A friend of mine runs an inverter off of a couple of his golf cart batteries all the time and has been abusing his current set of batteries for years.

Jonathan

DamonHD · « **Reply #10 on:** December 16, 2012, 11:17:39 AM »

I just can't see that you are *ever* going to get value out of a lopsided arrangement, since the other series components will prevent you from doing so.

Rgds

Damon

madlabs · « **Reply #11 on:** December 16, 2012, 11:26:34 AM »

Yeah, maybe I am grasping at straws. The thing is that the bank just meets my needs as is and with two less batteries I am worried I am going to work them too hard. After throwing down all my savings for a new system, I just don't have money for new batteries and it will be at kleast a year and likely more before I can buy more.

Jonathan

DamonHD · « **Reply #12 on:** December 16, 2012, 12:12:39 PM »

You do have my sympathies, but those kind thoughts wont butter any parsnips!

Any chance of trading/swapping them for 4 batteries of similar total Wh capacity?

Rgds

Damon

Flux · « **Reply #13 on:** December 16, 2012, 12:41:01 PM »

It was probably a mistake to upgrade to 48v without having the finance to sort out the batteries.

Like the others I think it will be a big mistake to try to use those two in parallel somewhere. I can see where you are coming from, reducing the AH may not be a good idea if you are already short on capacity. To some extent you may gain a bit more efficiency at 48v that may partly make up for the reduced capacity, but you are making a big jump downwards in capacity.

The worry is that with parallel banks you tend to get away with sickening batteries unless one dies completely ( they equalise out over time). With the series string any weakness is going to start to show quickly. I agree with what you said previously about weeding out any weak members. I can see a point in keeping the 2 spares alive by some means ( even a small solar panel) so they are available as spares but I can't see any practical way to use them in the main bank without unbalancing the rest.

The only way would be with a 12 to 48v converter and it's just not worth it for a makeshift job.

Flux

madlabs · « **Reply #14 on:** December 16, 2012, 01:32:50 PM »

Well, I'm still glad I'm making the switch, if I waited until I had the money for batteries I would have lost out on a solar incentive and a tax write off. And it would have been a while. Plus two less batteries won't kill me I guess. Try to use less at night. I hadn't thought of the positive offset of a more efficient system. It won't make up the capacity loss, but at least I can delude myself into thinking it's helping.

They are going to be better charged then in the past and that may help as well.

Another thing I might consider is taking the batteries out of my 36V golf cart and adding them to the mix. The golf batts are newer but have been beat on more. Guess I'd need to test 'em to make sure that comes out as a net gain. Then I'd only have to get 6 new batts.

However, I'm guessing that in reality I will cull the two weakest and then beat on them somewhere while I beat on the others with my new system. Milk as much as I can out of them while saving pennies for more.

Thanks for all the opinions.

Jonathan

Flux · « **Reply #15 on:** December 16, 2012, 02:00:36 PM »

Using the golf cart batteries makes sense. You are coming into the worst time of the year and if you can ease things through the bad months it will help.

As with any renewable system conserving power is the best way to go, by cutting the non essential things back a bit you will give the batteries a better chance.

You are right about making the change even though you may have to go easy for a while. When you look back you will see the advantage.

Flux

phil b · « **Reply #16 on:** December 19, 2012, 12:07:03 AM »

Jonathan I have been using my batteries in series parallel since they were new. The first month I bought 4 and seriesed them. the 3rd month I bought 4 more and put them in series. The three additional stacks were all out in parallel with the originals. So far, none have shown any weaknesses. If I had them, I;d move a seried batt to parallel. If that didn't work, I'd remove it from the bank and charge it independtly. older batteries get put in parallel.
I'd like to have an ammeter on each one just to tell if there any real differences. It's just an ongoing experiment for now.
BTW, the batteries have a 2 inch space between them and some unequal lengths of cable I'm not proud of.
To my way of thinking the dirrerent lenght will be worse than parallel by far. I just ordered a battery carrying handle last night so I can build a good battery rack. I hope to cure the different lenghts of wire and add a common bus bar with meters.

Flux · « **Reply #17 on:** December 19, 2012, 05:01:56 AM »

Phil your set up is different.

20 6v batteries for 24v works. 5 parallel strings of 4 in series would ideally need all identical batteries from new but unless there is a scarp one in the system it works ok.

The case of 10 at 48v doesn't work. 8 are ok in series but you then have 2 that don't fit and there is nowhere to parallel them without a problem.

Flux

OperaHouse · « **Reply #18 on:** December 19, 2012, 08:52:17 AM »

I'd have to think that anyone going from 12V to 48V would still have a lot of 12V stuff still around. I would have no problem doubling up the batteries on the the lower 12V. This would require an extra charge controller on the lower batteries. Wouldn't be hard with a UNO to do up something smart. Just wasting two batteries doesn't seem like a great option.

Flux · « **Reply #19 on:** December 19, 2012, 10:22:32 AM »

Yes that would be fine and I am sure there may be 12v stuff left around. It comes down really to using a separate 12v system for the spare batteries. It doesn't offer any benefit to the 48v system in terms of increase capacity or reduced cycling but may be one of the best ways of keeping them alive as spares.

Flux

madlabs · « **Reply #20 on:** December 20, 2012, 11:53:30 AM »

I do have plenty of 12V stuff left laying around. I'm leaning towards marrying the golf cart bank with the power bank and getting new batts for the cart. I just don't have a need for a secondary system at the moment and we all know that unattended batteries die quick, might as well throw 'em out. Dunno yet, although most of the new system arrived yesterday, I've got quite a while to think about it. Lots of work to do.

Thanks for the help folks, I really appreciate it.

Jonathan

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Author Topic: switching from 12 to 48V, battery bank configuration (Read 5864 times)