The one time I used a brand name Battery Tender on my car when it was in winter storage, in the spring I discovered the thing had boiled the battery dry and I had to replace t. Granted, the vast majority of people who use them don't have any problems, but based on my experience those kinds of chargers can malfunction.
I went back to my old method of simply disconnecting the battery when the car was in storage, and had no more problems. If they are in good condition and fully charged to start with, batteries should be able to sit for six months or more and still be nearly fully charged when you put them back in service.
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