My EZ-3, a Delta Trike, uses the following setup to charge the batteries. The charges add less than 3 pounds (1.3 Kilos) total weight to the Trike. They are small and only put out .8 amps but that is enough to charge the batteries overnight. The relay coil is 120 VAC so when the Trike is unplugged the relay disconnects the charges from the batteries. With this setup I only need to carry a power cord and I can hit the batteries with a charge anytime I happen on a 120 VAC outlet. You would be surprised how many outside outlets you can find. Most parks have them in the pavilions.
Grandpa Chas S.