Need a Battery charger
Look through your junk drawer for wall warts or lap top chargers.
Look for anything with 12v to 15v DC output.
Cut the low voltage connector end off and strip back the wires.
Thrift stores and good will stores are a good source.
Amazon
12v DC 5A $12.00
It even comes with an adapter to hook up wires!
https://www.amazon.com/dp/B01GEA8PQA?tag=fabo03-20
You could also get a motorcycle / ATV battery and use your existing charger to keep it charged. Then use it to test.