In some cases there are several cells in a 12v battery, almost like your car has 6 cells @ 2v each = 12v. Those cells are in series (internally) giving the 12v.
If you were using stock replacement batteries that have a connector that can't be reversed or mis-connected, your voltage would be correct and the amps would only give a longer use time. If you mis connected only "one" of those 12v batteries, it would be outputting 24v! Many people I know will take apart a sealed battery and get individual cells and make their own 12, 16, 24 or what ever voltage is required. I did this with my grandsons small electric Jeep. The battery was shot and hard to find new, so I made him one, but instead of 12v, I put in a couple extra cells and boosted it to 16v. He loves it. It really flys. LOL