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

[Starlight Solar Power Systems]

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