Batteries. Breaking taboo?

[Chef_Tim]

Ok so here's my question. Due to a real knuckleheaded move on my part this winter, I had to replace my 2 group 24 deep cycle batts this season. Don't ask. And no I did not get 6 volt and it's too late to get 'em now. That being said, I have an extra Group 27 battery in very good shape not doing anything. Can i wire this in parallel to my group 24's that are wired in parallel to increase the bank size? my understanding (limited though it may be!) is that batts wired in series have to be same everything, but wired in parallel they can be different size (need to be same voltage though).
Any reason I should not do this?

Thanks in advance.
BTW thanks to all those who answered questions with the solar install. Followed some recommendations and it is working like a charm.

[trail-explorer]

I wouldn't recommend running batteries of unequal age, size or chemistry as one large bank. The weak one will draw down the others.

[Shadow_Catcher]

SoundGuy gave the right answer but not the reason, unmatched batteries age, capacity... will default to the weakest battery drawing the others down.

[SoundGuy]

Chef Tim wrote:
I have an extra Group 27 battery in very good shape not doing anything. Can i wire this in parallel to my group 24's that are wired in parallel to increase the bank size?

You could, but I'd be inclined to wire the batteries in with this Blue Sea m-Series 6007 Disconnect Switch and wire the two G24s as Bank 1 and the G27 as Bank 2, allowing you the choice of running the two 24s alone, the 27 alone, or all of them together, depending on the situation.

[time2roll]

Will work fine. For best results attach the main cable at each end of the stack.

[GordonThree]

Not a problem, as long as you can keep the wiring short between the batteries. if you have two g24 on the tongue and the g27 stuffed in some box eight feet away, that could present a balance issue