Mixing different battery sizes in a bank

icarus · September 2007

I'm not sure if this should be in a new thread, but I cannot find a relevant thread.

One of my L-16 developed a terminal bad cell the other day for no know reason. Went from fine to dead in two weeks. I pulled the battery our of the bank.

My question is this,,,I know that you are not supposed to mix sizes and ages of batteries in a bank. Please tell me if you think this is a stupid idea.

My bank now consists of 3 L-16 that are about ten years old, but seem to be performing will, dead cell issues notwithstanding. I have installed a pair of 10 year old T-105 in the string as 1 6 volt. It seems that the ages are about the same. The amp hour capacity of the T 106 is about 1/2 that of the L-16 so the T-106's should be about the same.

I figure I have nothing to loose in that the T 106's were just sitting here, and since I don't want to put a new L-16 in the band with 10 year old ones.

I realize I am on borrowed time on the string, but if I can pull a couple more years out of them I will be happy. We have very small useage ~35 amp/hours/day, and they fully charge most days. I never run them down below 80% ever.

Once again, thanks in advance for all the good information from you all,

Icarus

BB. · September 2007

Re: Mixing different battery sizes in a bank

Well, if you have nothing to lose... Why not.

If they are the same manufacturer, or you have checked the original manuals and found that they have pretty much the same charging voltage requirements (less than -0.1 volt difference)--then you could probably put the two 6 volt batteries (in series as 12 VDC) in parallel with the rest of the 12 volt string.

If, however, this, for example, a 24 VDC bank and you are replacing one of the 12 volt cell sets with this second set--you probably run a risk of the two different battery cell sizes becoming unbalanced--such that a normal equalization charge may not work...

If the batteries become "unbalanced" (don't gas at roughly the same time, or show equal specific gravity)--you would then need to charge the "low" cells with a separate 120 VAC to 6/12 VDC battery charger to equalize the low cells.

As long as you carefully monitor the water and relative charges--it probably can't do more damage than requiring you to replace all of the batteries anyway...

My $0.02 worth...

-Bill

niel · September 2007

Re: Mixing different battery sizes in a bank

bb is right as this could set up an imbalance. if you watch it carefully and even charge them seperately if imbalance occurs you should be ok for awhile. but as you said you are on borrowed time and if 1 cell died then could others be far off? you just don't know so figure on starting to save for the new batteries now so that if they don't last as long as you thought or wished that it won't be an all at once deal for you. what's worse is that batteries are higher priced now and who knows if they won't continue to go up?:grr

icarus · September 2007

Re: Mixing different battery sizes in a bank

Thanks for the information folks,,,

I will continue to monitor voltage/SG. The battery cell that failed was the second in the string. The first failed a year ago and I reduced the string from 6 to four. I figure if I can have one fail next year, I will then be down to 2 in the string, then I will be forced to buy 4 new ones. I"m saving my pennys now.

I'll keep you all apprised.

Icarus

BB. · September 2007

Re: Mixing different battery sizes in a bank

Yep, as you loose capacity in your bank due to failed cells, you are cycling the older (remaining) batteries more--which will probably cause them to fail even quicker...

-Bill

crewzer · September 2007

Re: Mixing different battery sizes in a bank

Icarus,

Just to clarify: The two T-105's you're using as a substitute for a single L-16 are wired in parallel, yes?

Regards,
Jim / crewzer

icarus · September 2007

Re: Mixing different battery sizes in a bank

Crewzer,

Yes in parallel, as a big 6 volt.

As to additional cycling due to the smaller string. My original bank was the pair of t-106's I used them for ten years and they are still going strong. Some one gave me the 4 L-16's a year or so ago when one battery in his string failed. He replaced the entire string (6 batteries) He cycled them -20% for about 6 or 7 years, with a load of ~80amp/hours/day. I now load and cycle them about 12-15 amp/hours day. In both installations they get fully charged almost every day. The 15 amp/hours day we use is less than 2% of capacity. The almost never get below 12.6 vdc after being used for the evening and then allowed to rest overnight. I probably have doubled my average daily load over the years, but the 2 L-16 are more than 4 times as big as the 2 T-106's.

As I said before, I know I will need new ones sooner than later, but every month that goes by is a month that money stays in my pocket.

Once again, thanks for all the good information,

Icarus

Solar Guppy · September 2007

Re: Mixing different battery sizes in a bank

Also, every month that goes by, the battery prices go up more!

Money in your "pocket" is losing value compared to anything that contains metals ( copper , Lead ect )

If you wait to long, expect to pay 2X today price, and I'm talking a year or two not another decade

icarus · September 2007

Re: Mixing different battery sizes in a bank

Point taken, except,,,

Following that logic I should buy 50 years worth of all kinds of hardware now to preclude future price increases. (God knows I won't live that long!) Having a battery string that is performing perfectly (current issues allowed), to toss them away goes against my grain.

There is an argument to be made that much of the commodity prices for copper, lead etc, has been and is due to a combination of speculation in the markets and the (previously) strong U.S. housing market. Now that the market is softening it is possible that these prices could come down.

I am currently building a small building. I had the lumber priced out a year ago. The price is down almost 30% since last fall. Sheet goods even more. If I had followed the buy now 'cause the price does nothing but go up, I would have lost the gamble.

What will the future hold? Who knows. I try to take care of my stuff and make it last. Amortizing the cost of the batteries over one additional year makes them even cheaper. (Since I paid nothing for them, I'm out nothing in any case).

Icarus

Solar Guppy · September 2007

Re: Mixing different battery sizes in a bank

Lumber and lead are different , lumber is local and dependant on housing, lead is not. My thoughts are unless it comes from US soil, it will go up as the US dollar continues its decline to other world currencys.

Also its not looking good as mining is not keeping up with demand, and that demand is NOT US based

http://www.moneyweek.com/file/30285/can-the-price-of-lead-continue-to-defy-gravity-this-year.html

The US peso has only one way to go, there is zero economic outcome that can support the US debt and speeding. When the dollar has a 80's sytle peso or 20's German Mark type correction is anyones guess but I have not seen a single way that it won't happen as some point in the future.

icarus · September 2007

Re: Mixing different battery sizes in a bank

I agree that the market is world wide, especially China, and as a result the demand is high. Having said that the while China drive the demand on the manufacturing side, it is the US consumer that drive demand on the ultimate consumption side.

As an asside, lumber is very much NOT local. It is a commodity just like oil, wheat or lead. The US market drives the price continent wide, even world wide. Just ask any number of closed lumber mill workers in Canada why they can't sell product. (Granted the high Canadian/low US Dollar exacerbates the problem, and is indeed coulpled to it!)

Anyway, I live in the sticks and at some level it doesn't affect me,,,until it does.

Icarus

Mixing different battery sizes in a bank

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