[RE-wrenches] Mixing battery sizes in the same bank

[Starlight Solar Power Systems]

Hi Allan,

Yep…At first my thought was with the batteries in parallel, they contribute equally to the load. Upon further thinking, this could not be true because the voltage must stay the same across all terminals during discharge. That can only mean the larger battery provides more AH than the smaller at any depth of discharge. It must also be true that DoD percentage of each battery is identical at any point in cycle.

On charging, the reverse must be true again because of terminal voltage. It’s obvious that this should never be done in series strings. So….why have I always believed, preached and insisted “never mix battery sizes in the same bank”? 

Battery wrenches, has anyone ever done or seen this done? (Allan, I’m not even going to ask about your customer with five 6 volt batteries in a 24 volt bank)

Larry Crutcher
Starlight Solar Power Systems


On Nov 20, 2015, at 4:19 PM, Allan Sindelar <allan at sindelarsolar.com> wrote:

Larry,
Nobody has responded to your question: "How can this possibly be OK for the 75AH battery? And why would a battery manufacturer say this?" and like you, it goes against everything that I have assumed. But to answer that first question: when I look at, I wonder why it wouldn't work. With AGM batteries you can tolerate both high and low charging C/ rates and long absorption times. So it makes sense that as long as the voltage remains the limiting factor and the array is large enough to allow routine long absorption periods, it oughtta work just fine. 

Your thoughts?
Allan

Allan Sindelar
 <mailto:allan at sindelarsolar.com>allan at sindelarsolar.com <mailto:allan at sindelarsolar.com>
NABCEP Certified PV Installation Professional
NABCEP Certified Technical Sales Professional
New Mexico EE98J Journeyman Electrician
Founder (Retired), Positive Energy, Inc.
505 780-2738 cell

 
On 11/18/2015 5:09 PM, Starlight Solar Power Systems wrote:
> Just when I thought I was becoming a battery guru, this came up: A customer asked me if he could purchase two different size 12 volt Lifeline (Concorde) AGM batteries and connect them in parallel. Of course I told him this was a bad idea and all his parallel batteries should be same make, age, size. A few days later he called me back and said he talked to Lifeline battery and they told him it was fine to mix any size of their batteries together. Knowing that people sometimes hear wrong, I called Lifeline to inquire. The answer, "yes, you can mix any of our batteries in parallel. We have tested that and Concorde tested it and it’s no problem”. 
> 
> Stunned, I asked him if this was in writing anywhere and he said no but I could test this myself. So battery wrenches, what will happen if I have an 8D at 255AH paralleled to a group 24 at 75 AH and discharge this to 80% DoD? How can this possibly be OK for the 75AH battery? And why would a battery manufacturer say this?
> 
> Larry

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