Author Topic: Adding parallel battery banks  (Read 2290 times)

Zytygyr

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Adding parallel battery banks
« on: September 26, 2015, 01:23:53 PM »
Hi

I currently run 2 banks of 4x 12V 150Ah batteries in parallel for backup power on my solar setup, creating 300Ah at 48V. I have been offered another 8 batteries to add to my existing bank. I am very keen (and in desperate need of) more backup power and the final 600Ah bank would be very welcome.

I have however recently come across a few websites saying that 4 battery banks should not be run in parallel as it creates a "weak" bank which "feeds / steals" off the others and ultimately fails prematurely. Has anyone heard of or had any experience with this? And if so could you please offer more information? (how inefficient would the bank be, and how long would it last?)

Thanks in advance for any assistance.

The quote from the website reads as follows:

Quote
Please note that it is highly inadvisable to connect more than 3 batteries in parallel due to the inefficiency and losses that occur i.e. When a battery bank is connected in 3 or more parallel strings, one of the strings will be the easiest path for the charge current thus becoming the strong string. The other two strings will compete for second place and the looser will be the lazy string. The lazy bank will tend to lose its equalization, resulting in accelerated failure of any weak cells. Weak cells will be difficult to detect for at least a while, because they will "steal" from the surrounding cells, and the system will suffer as a whole and will cost you more in the long run. 
 
Eventually the lazy string will prematurely develop bad cells and require the whole string to be pulled out of the bank. No replacement will be possible at this time because the whole bank is now more than a year old. Now the next lazy string will begin to develop bad cells. All you can do is either try to make due with the decreased capacity or replace the whole bank with large series connected battery sized for the job, which is, by the way, what you should have purchased in the first place, and avoided all this trouble. 

The goal is to maintain all of the cells at an equal state of charge. Cells that tend to receive less charge are likely to fail prematurely. This can take years off of the effective life of the battery bank. A fraction of an ohm of added resistance in one battery string can reduce the life of the entire string.

Free Energy Freak

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Re: Adding parallel battery banks
« Reply #1 on: November 20, 2015, 05:30:24 AM »
I currently run 2 banks of 4x 12V 150Ah batteries in parallel for backup power on my solar setup, creating 300Ah at 48V. I have been offered another 8 batteries to add to my existing bank. I am very keen (and in desperate need of) more backup power and the final 600Ah bank would be very welcome.

I have however recently come across a few websites saying that 4 battery banks should not be run in parallel as it creates a "weak" bank which "feeds / steals" off the others and ultimately fails prematurely. Has anyone heard of or had any experience with this? And if so could you please offer more information? (how inefficient would the bank be, and how long would it last?)

For some time now, I recall receiving similar advice. No more than three strings in parallel, typically. Two, at most, seems to be preferable. Four, I think - and you would be asking for problems.
Paul