A standard Harley tender is fused for 7.5a. I'm not sure why yours is only fused for 3a, as a high end trickle charger with a quick charge or "start" mode will push 6a down the line. The fuse size is dictated by wire size. Wire size is dictated by the draw of the accessory at the end of the wire. If you take the accessory outlet and plug it into the tender wire, and whatever you plug into it draws more than 3a, you will blow the fuse. Otherwise, you will be fine. What you can't do is put a bigger fuse in there, if the wires can't handle the draw they will melt or burn. Take the 3a tender wire and hook it to your battery. You can plug the accessory outlet into that, as long as whatever you plug in doesn't draw more than 3a. Cellphone charger or GPS will be fine, and a regular tender only pushes 1 or 2 amps. Just don't use a quick charger on there unless you upgrade the wire.