I got a 12AH SLA battery pack that mounts in the frame under the crossbar, with my ebike kit. I had dropped the bike a few times and in a couple accidents. I decided to go to rear rack mount.

Now the case the batteries are mounted in is screwed together with allen cap head screws of the metric type. To get to those screws I had to pull the little rubber plugs out with needle nose pliers. The side that doesn't have the charge connector is the side to get at. After popping the one side off I had to get at the connections that were on the other side. They use 1/4 quick connect slip on connectors, really loose there. So I soldered all the battery connections in place on the batteries, I eliminated the 3 pin charge connector. That was my goal to get rid of that charge connector. Don't need that extra weight.<grin>

Next I bought an Anderson power pole connector kit and took off the old 3 pin connector from the charger, and installed a 2 pole connector on the charger. This connector matches perfectly with the connector that goes to the controller. So now to charge the battery I unplug the battery from the controller and plug in the charger.
The battery case is now rear rack mounted above the wheel.

I found out 2 things while doing all this:

The batteries aren't 12AH they are 14AH woo hoo.

When I did my 26 mile trip, which I can't make it all the way on electric power if the wind is strong, usually about 4 miles short of 26 miles, and the wind was 15MPH gusting to 25MPH, I made it all 26 miles and with power to spare. Initially I gained a little more speed also. Soldering those connections made a really big change in my mileage.

After making this trip twice a week for 2 months now, this was the first time I made it on electric power all the way.