iFly GPS Forum

If I understand you correctly, the external battery and probably the internal 740 battery were both dead or close to it. So the power drawn thru the 1 amp fuse was to charge the external battery, charge the internal battery, and power the 740. I'd bet that adds up to more than an amp. Ironically, I have the exact same setup and had the same thing happen today - I forgot to plug ships' power into the external battery, so the 740 drained the external and internal battery. It shut itself down as I was landing the first leg. After fueling the plane and ourselves, I plugged ships' power into the external battery, started up, and everything charged/worked fine. The difference is, I'm pretty sure I have a 5 amp fuse in that branch.

The 740 will only sip what it needs, as will the battery, etc. An unfused circuit, or a 50amp fuse, won't hurt the devices. A fuse is mainly to protect the battery and charging circuit from a short to ground, such as the positive wire chafing against the metal frame. Typically you'd add up the needs of the devices on the circuit and add a small fudge factor to determine what size fuse to use. Just make sure the wiring is rated for the max load else you risk heating/melting the wire, and starving the devices of the power they need. In my case, a 5 amp fuse has been enough to carry the load and is safely below the wiring's max capacity.