Check if the battery you wish to charge is a gel type or lead/acid. If it is a gel type, and your battery tender is not intended for gel cells (puts out more than 13.5v), I'd get the one for gel type cells. I used an automotive type charger that put out 14-15v and it damaged my gel cell.
Incidentally, I bought a float type charger at harbor freight on sale for $7.99. I checked measured it's output at about 13v, max .5a and it seems to work fine.
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