Onan NHC Series Operator's/Service Manual And Parts Catalog page 295

If one or more cells are low on water, add distilled
water and recharge.
Keep the battery case clean and dry. An accumulation
of moisture will lead to a more rapid discharge and
battery failure.
Keep the battery terminals clean and tight. After
making connections, coat the terminals with a light
application of petroleum jelly or grease to retard
corrosion.
Poor contact at the battery cable connections is often a source of
trouble. Make sure battery cables are in good condition and that
contacting- surfaces are clean and tightly connected. Oo not
reverse battery leads. Use recommended battery tools when
disconnecting leads to avoid mechanical battery damage.
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