Battery charge

Thinking of putting a extra battery on the van , have a dual charge already fitted .
Got a few 13x7x81/2" high deep discharge Pegasus 12 volt 115 ah 20 hrs , lead acid , Were used as solar panel batteries .
Also a old car battery , charged it up a month or so ago , checked voltage at12.20 , Think it will be ok as a batt for tyre pump etc . But more importantly to act as a jump battery / helper battery or is that voltage a bit low after a month in the shed . . ?? would be handy as its a varta nice compact 44 amp hour Think 360cca.But for what need .
Saves trying to squeeze in the big one .
I don't want to stress the alternator either .

Actually 12.85 according to charger after 5 mins , allowing for drop .

12.2vdc will jump start as long as it is holding it's charge.

Some peeps use an old battery as a second battery which they stopped using because it were old. Then wonder why it's not as good as eggspected when they come to use it.