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.
 
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Any Questions?
Grandpa Chas S.
Great graphic, Chas, very easy for me to understand. It answers some important concerns, so, Thanks!
What is the make and model number (and price) of each charger and the proper relay?
Are there any extra considerations to make when scaling up to the apparently common 48/60/72-volt systems?
Would there be any important changes if someone was to make a similar set-up for a LiFePO4 pack?
Here's a question... I think using a relay adds a bit of complexity to the design, though granted it would make the system more convenient to use. Clearly you could just stop somewhere and reel out the cord for the charger setup and plug in and walk away.
The question is.. it's only slightly harder to use a set of connectors instead of the relay. It would change the work required from just plug in, to a) connect the connectors, b) plug in. What is the tradeoff you see that makes using the relay better?
- David Herron, The Long Tail Pipe, davidherron.com, 7gen.com, What is Reiki
Two very simple reasons.
1. The batteries in the Trike are inside a closed compartment under a false floor in a box on the back if the Trike. With only a power chord to connect I don't have to unload the back of the Trike, storage box, to get to the compartment cover to open it and connect the connectors.
2. I'm lazy!
Grandpa Chas S.
I'd add two more reasons:
3. You can recharge in wet weather and only one plug gets wet.
4. You can use a relay with an "OFF" function (turns one pole off when power is supplied to the relay) to build in a safety mechanism stopping the motor from running when the bike is still plugged in.
Mik
This information may be used entirely at your own risk.
There is always a way if there is no other way!
Dont the tender chargers disconnect when turned off so you don't need relays. I thought they were designed to remain connected to the batteries they are dedicated to. The battery chargers are plugged in when the ship is in port or the generator is on.
Eric Fisher
The Battery Tenders do shut off but they do not have an automatic disconnect, that is why I use a 120-volt relay. When you connect the power plug (120-volts) to the wall socket the relay closes and makes the physical connection of the Batteries and Battery Tenders. Without this relay the Battery Tenders are always connected and if something shorts for what ever reason then my batteries are toast. I use a cheap $4.oo relay to protect a $95.oo battery, well worth the investment.
Grandpa Chas S.