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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?

Honda EM3500SX Owner's Manual, page 52 Starter solenoid

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    It'd work, but even though the other terminals on the solenoid are protected, there's a lot of bare metal around, and the clamps typical of those jumper boxes won't grip that terminal too well. If your generator is like mine, the engine can jump around a bit when starting, so there's a risk of contact with grounded metal. I'd suggest attaching an extension cable to the terminal you can clamp more solidly to the far end. – Phil G Jan 13 '20 at 23:05
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    Is the starter controlled with a button or something? Some of those jumper boxes need to be connected to a battery with some voltage for them to work, so you need to make sure it will output 12v without being connected to anything. – JPhi1618 Jan 13 '20 at 23:05
  • You're right, @JPhi1618. I hooked the jumper leads to a voltmeter, and it didn't even jump when I turned the box on. I might have to get me a car battery instead... – Paul Price Jan 14 '20 at 0:01
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    Or a lawnmower battery. A used $20 car battery might suffice. Or a used golf cart battery, it's not like the load is huge. – Harper - Reinstate Monica Jan 14 '20 at 0:26
  • @Harper-ReinstateMonica - That depends on the voltage required. Golf cart batteries as typically 6V deep cycle so inappropriate for 12V CCA applications; I'm not too sure about lawnmower batteries. – user109695 Jan 16 '20 at 0:53
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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.

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  • I appreciate the thoughts, but I'm not too worried about charging the battery from the generator. I just want to get the generator started when we're desperate. If I have to purchase a battery that I can only use once for this purpose, then so be it. – Paul Price Jan 14 '20 at 0:05
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    It really depends on the type of "portable car starting box". If they are nickel-cadmium, those do have memory effects you must periodically clear with a deep discharge. But a lead-acid battery should never, ever be discharged unnecessarily; it is always damaging to it, especially a starting battery. – Harper - Reinstate Monica Jan 14 '20 at 0:26
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Since the "portable car starting box" apparently doesn't deliver anything unless it's attached to a battery, I'm going to get a dedicated 12V battery for this. It can live on a trickle charger until I need to start the generator.

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    For battery start, that's the way to go. Nice and simple and will work reliably. That said, you might want to see if your wife can pull-start the generator. My experience is that well-maintained Honda engines are really easy to start, and pull-starts don't really require much effort. It'd be good for her to at least learn the non-battery technique and verify she can do it, just in case the battery craps out at just the wrong time (e.g trickle-charger stops working and no one notices for months, or maybe the starter motor or solenoid fail, or whatever...lots of things to go wrong there). – Peter Duniho Jan 16 '20 at 0:10
  • Got a 35 AH wheelchair battery ($85 at a local battery specialist; can prolly get cheaper elsewhere), some jumper cables ($12 online) and a trickle charger ($12 online). That does the trick! – Paul Price Jan 20 '20 at 20:14

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