Group 23 in Parallel w/ Group 34

[Rye]

Would there be any harm in wiring a new group 24 Mk with a new group 34 Mk in series?

[Rye]

Ignore the title. I'm wiring in series to get 24v

[LROBBINS]

Mismatched batteries wired in series will have a very short life - during charging one will be over-charged and the other under-charged, a situation very bad for both.

[Rye]

Ok, thanks. I'll buy another battery

[Burgerman]

Connecting an old and a new in series = soon to throw away both of them.
Connecting a different brand or type of battery = same thing.

If connecting batteries in series they MUST be same age, brand, batch, history. As a pair. From new. You cannot successfully add one replacement to a pair unless new or almost new within a few cycles.