Goop
X3.. But to make it even easier/better, grab yourself a package of assorted sized rubber grommets from Home Depot or harbor freight tools. Drill your hole, squeeze rubber grommet in hole, run wires through grommet, then goop. I've been using this method with no issues for years now...
For the actual connection to batteries, go to your local OEX, pick up some 2 pin marine connectors. You can attach these to the battery itself, the lead wires, and your charger.. makes plugging/ unplugging and charging a breeze. I coat the connections on the battery with goop as well.
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