Join Date: Apr 2006
Thanked 183 Times in 180 Posts
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If that is a plain old fashioned "nuke 'em" charger they are meant to be manually superivsed and just used until the batteries are charged. Leaving them on for an extra three weeks or so will probably destroy the batteries in short order.
If you've priced batteries lately, the cost of buying a modern charger (shore power or solar, either way) is way less painful than the cost of new batteries.