First of all, regular battery maintenance is vital for reliable starts. If there's corrosion, remove the battery for cleaning with a battery brush and a solution of warm water and baking soda. Check the battery tray and clamp bolts. Corroded parts should be primed and painted. Remove the battery for charging to avoid paint damage. Use a charger, remove cell caps, cover holes, detach the negative cable, and attach the charger. Look over the battery to prevent overheating. Check the charge with a hydrometer or voltmeter.