We recently acquired a 20 year old Honda EM3500SX generator for use with a manual transfer switch when the utility power goes out. My wife would like to be able to operate the generator when I'm away, but pulling the starter might be beyond her abilities. The owner's manual (see below) mentions an optional battery kit, the installation of which involves connecting a battery ("rated 12V-18-35AH or more") to an existing starter motor. I have a portable car starting box, basically a big battery with jumper cables, which matches the voltage. Can I simply connect the red (positive) jumper cable to the bare screw (with red wire attached) on the starter solenoid (see photo, below), the black (negative) jumper cable to the frame, and have that function as a temporary starter battery?
Not all of those "portable car starting box" products are suitable for being permanently connected to a system with a trickle charger, which is what you would have to do. The batteries will cease to accept a charge if not sufficiently discharged periodically. So you need a system that can be on a trickle charger all of the time to be ready for action when called upon.
Normally on a piece of equipment that is used on a regular basis, like a lawn mower or tractor, the starting battery is re-charged by the alternator once the engine is running. But on a backup generator you don't run the engine often enough (you SHOULD test it monthly, but hardly anyone does...). Hence the need for the trickle charger to keep the battery ready to go. That then also adds the necessity for periodic battery maintenance as well, so keep that in mind. These are the reasons why a lot of backup generators are pull-start; all you need to start them is some Wheaties.