Check the battery for corrosion or loose terminal connections, cracks, or a loose clamp.
If the battery is corroded, wash it off with a solution of warm water and sodium bicarbonate. Coat the outside of the terminals with grease to prevent further corrosion.
If the terminal connections are loose, tighten their clamp nuts - but do not overtighten.
Tighten the retainer just enough to securely hold the battery in place. Overtightening can damage the battery case.