Dude, don’t fret, your gem will love the extra voltage. The salesbag actually did you a favor with their frack up. More range and potentially more speed. All you need to do is mount he extra battery, install some extra battery cables to hook it up and depending how you connect it, possibly an 84 → 72v shunt on pin 1 of the controller. If that’s all you do, you will get more range and better acceleration. If you want more speed, you need a programmer or get a magic magnet from @Inwo, who usually also has the shunts if you need them.
You can charge the extra battery with any basic 12V portable charger that is compatible with gel batteries. .
Oh, and check your DQ to see that is is set for the proper profile for the six (6) 12V gels. Wrong profile = premature battery death.
The links to how to do it:
Rodney’s original thread:
The thread where Dave and I went a bit over the top, but found a way to do it without the shunt and without having to disconnect the 7th battery to charge as we isolated it on the other side of the contactor from the main pack.
I used lifepo4 pack for #7 in that experiment, but others have been successful with the same wiring arrangement and traditional batteries