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TomChum
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# Posted: 15 Aug 2011 13:00 - Edited by: TomChum
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I have two (1 year old) 125Ah 6v batteries in series to make a 125AH 12v system.
What are the considerations..... if I add 2 new batteries to this system? I would keep the sets in their original 12v pairs.
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MtnDon
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# Posted: 15 Aug 2011 14:21
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Up to a year old is generally recognized as the limit for adding new to old. However, I have a couple of 18 month older than the rest in my battery bank of 12 units and I can not see any difference. Most are now 24 months old with the two being 42 months. The day may come when the older pair begin to show deterioration; we'll wait and see.
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TragicPhilosophy
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# Posted: 15 Aug 2011 23:24
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Wow, where do you keep a battery bank that large? Do they tolerate sub-zero temperatures in the winter or do you have to heat them?
Sorry, renting a cabin in AK, coming up on my first winter 'up north'.
Previous experience was a sailboat in Juneau, didn't have to worry about freezing cause batteries were below waterline.
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TomChum
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# Posted: 15 Aug 2011 23:40 - Edited by: TomChum
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Quoting: MtnDon The day may come when the older pair begin to show deterioration; we'll wait and see.
This is a certainty.
The concern that I have heard is related to the older pair degrading the performance of the newer bank (Even while the older pair are still servicable).
My batteries had light usage during this one year, Probably less than 10 cycles. It's nothing like running a forklift all day for a year. Maybe this is a question for the mfr.
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MtnDon
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# Posted: 16 Aug 2011 00:31
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Quoting: TragicPhilosophy Wow, where do you keep a battery bank that large? Do they tolerate sub-zero temperatures in the winter or do you have to heat them?
They are in an enclosure dedicated to the batteries only, located on the end of the cabin.
One reason I have 12 is because of the cold winter temperatures. Batteries lose capacity as the temperature falls. As long as the charge is maintained there is no danger of them freezing though. Batteries actually have their lifespan increased by the cold (and reduced by high temperatures).
The charge controller runs the show automatically when we are absent. In November I just make certain the fluid levels are up and run a full charge cycle then an equalization cycle. The system (Outback hardware) runs itself. Water usage is minimal because the system has no real power draw, only the slight self discharge. We normally snowshoe in twice a month December through April or May and its waiting for us.
TomChum, you are right, theory says that the old ones can drag down the performance of the newer ones. We'll see how much eventually. I keep a log book, each battery has a number and each cell specific gravity is recorded every three months or thereabouts. The logs from the charge controller are also saved for future comparison.
It is my personal belief that more damage is done more frequently by more folks who have undersized their battery bank and are continuously drawing down the state of charge to 50% and even lower. We seldom draw our bank down more than 25%, more often the daily draw down is in the neighborhood of 12 to 15%.
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TomChum
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# Posted: 16 Aug 2011 00:40 - Edited by: TomChum
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Yes Don that poses an interesting conundrum. Should I get 2 more batteries 2x $150 (6yrs lifespan) or another 135w solar panel $365 (~25 year lifespan)? Seems like I should get more batteries.
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TragicPhilosophy
Member
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# Posted: 19 Aug 2011 02:20
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Thanks for your response MtnDon.
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MtnDon
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# Posted: 19 Aug 2011 10:15 - Edited by: MtnDon
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TC, on more batteries vs another module
If the issue is that the present batteries do not provide sufficient capacity doesn't what you need depend on whether or not the batteries are being discharged during daylight or nighttime. If during the day another module could help recharge, but if the batteries are discharged in the evening then more storage, more batteries are needed.
Then the question is do the existing modules have sufficient capacity to recharge the increased number of batteries? And does the charge controller have sufficient reserve capacity to carry the increased power from more modules?
Not to imply you did anything wrong, but many folks run into this same set of questions after they put their first system into use. That's why I am a big advocate of careful generous planning the system and building in at least 20% more capacity than first thought needed. And that is after considering to make allowances for cold weather degradation of the performance, the self discharge loss, the loss due to inefficiency in changing electrical energy to chemical energy (charging the battery) and then the loss when the chemical energy is converted to electrical again. Then on the opposite end of cold, hot weather reduces the efficiency of the PV module.
EG. Batteries are rated at 77 F At 50 F they lode 12% of their rating. At 30 F they lose 22%, At -20 F the loss in capacity is 55%
The total loss in taking power from the module and charging the battery and then out of the battery again can range from 10 to 20%.
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