Power problems

I recently got interested in ebikes/mopeds. after some searching I decided that until I learned more about the subject, I am not going to spend thousands up front. the lead acid batteries that came with it are junk. here is a solution that I think can work. The moped I bought is 36 volt 1000 watt motor with a 750 watt controller . three lead acid batteries. 12 volt 10 WH . I want get either a 36 volt generater or power supply. Also I thought about a power bank. I could get a emergency power bank like they use to start vehicle and strip it down of all the stuff I don"t need.....
All I really need is to be able to charge and store 36 volts ?? watts. Also and this is what I worry about getting right.. I need it to switch from batteries to alt. power source automatically so Can charge the batteries. just like a car...
I know this seems like a lot of problems and expense but it is worth it and as soon as I get it worked out and tested. I will move everything over to a adult trike.... that will be great to have one that is unlimited to miles...
So my questions are would it be better to use a power supply, 36 volt generator or to use a power bank.. something like this might help..
truecharge2 battery chargers...

I also thought about using a gasoline generator and inverter but I really don't want gas motor.. Any Ideals or advise?