If it’s shifting gears N-1-2-3-4-5 then I doubt it’s an electrical power issue preventing it from shifting into R. That said it’s never a bad idea to make sure the battery is fully charged.
I’d trace your reverse lockout lever cable and make sure everything is connected and tight when it is pulled. You can adjust that lever completely out of existence anyway (search bar in the upper right can dig up threads on it “reverse mod”). The ECU won’t let it shift from R-1 or 1-R without hitting zero MPH anyway from the multiple people on here that have tested it.