Battery question

[thatoneguy]

I have two batteries in a 36v setup that need replacing (the other was new last year). All are group 27. I would like to move to group 31s - can I run two along with the 27, or should I swap out all 3?

[delcecchi]

Since the group 31 batteries would have more reserve capacity, it would be best to do all three.  Discharging them too far could over discharge the group 27 and ruin it.  But if you want to save money, the setup would work if you were careful to not run them down too far.

[Pat K]

Growing up on a farm with many diesel motors with multiple batteries hooked up either in series or parallel and as an adult owning several boats with multiple batteries I have always been told that any batteries connected to each other should be the same brand, age, size and have the same specs. Different batteries will work in the short term but the battery life will be much shorter because of the load the weaker batteries put on the stronger one.

The new battery you installed last year has been weakened by the 2 older batteries it was connected to and if you add 2 new batteries they will be compromised by the year old battery. If you add the group 31 batteries to the mix they will be weakened by the lower output of the group 27 battery. 

[papadarv]

Mixing batteries of a different age is an issue, size has no effect as long as they are the same age. I have a 1995 motor home use 2 deep cycle size 31 and one standard size 24 ( due to physical size holder) connected in parallel. The batteries lasted 14 years before they started to loose power or fail to hold a charge. I did however change all three when this occurred 2 years ago. I also remove them during winter and store them in my basement.

Found this link on mixing batteries supports what I have found over the past 17 years.

https://www.morganscloud.com/2011/10/03/mixing-battery-sizes/

[delcecchi]

Huge difference between batteries in series and in parallel.   Putting batteries of different capacities in series, even if all are brand new, is a bad mistake.  I will be happy to explain further if necessary. 

[thatoneguy]

Thanks, fellas - good info. Looks like I will swap out all three.