If it's not being frequently disconnected to charge the battery, 2-pin Delphi Weatherpack connectors. They're commonly used in automotive applications, and do a decent job of being both lightweight and everything-proof.
Bullet or Anderson connectors aren't ideal here, as there's enough DC volts that corrosion is likely if things get wet.
If you need the connector to unplug every time you charge the battery, you probably want something that's a bit more robust at frequent disconnections. I've used a IP-rated Neutrik Speakon connector on barakta's trike. Other reasonable options might be the Neutrik Powercon (which is actually designed for power - I used the speakon because I needed 4 pins for the separate lighting and motor supply) or an IP-rated XLR connector.