Check the battery for corrosion or loose terminal connections, cracks, or loose clamps.
If the battery has been 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 the terminal nuts - but do not overtighten.
Tighten the retaining clip only enough to hold the battery securely in place. Overtightening may damage the battery case.