# Powering a large electrical load with an alternator and high idle system

I have to install an inverter in a work truck that will power work lights, some power tools, and other equipment too be used while the truck is parked. I've calculated the load to be at max 750 watts, which takes into account inefficiency of the inverter. So to run the load the vehicle should supply about 62 amps to the inverter while it's running. The vehicle battery is 72 amp-hours, so I don't count on it powering the system, but since the alternator is rated at 175 amps I considered just adding a high idle controller, that will speed up the engine, hence make the alternator produce its nominal output. I've read that this still requires a secondary, isolated battery for which to power the the inverter, but I'm not sure why. It would seem that with the engine idling high, 175 amps would cover the inverter load, and charge the vehicle battery, if necessary. So is the secondary battery really necessary?

At high idle would an alternator be sufficiant to cover a large load like 62 amps without it affecting the battery? ( I'm not sure how much of the current I should expect to got back to the battery and also keep the vehicle running, so I'm not sure how much is left over to power the load. )

Also, is there any drawback to running in high idle for extended periods of time, say between 2 - 12 hours?

• your calculation of 62A (@110VAC) is not accounting for power surges. Starting a circular saw or plowing it through a 2x6 is going to produce surging power draws. I don't think I would use anything less than a 2000W half- or full-sine wave inverter and I would be more comfortable with 3000W. I've built a couple of successful isolated battery/inverter systems and never questioned the isolated second battery design beyond learning that the isolated battery acts as a capacitor between power generation and power delivery.
– user16128
Dec 7, 2019 at 1:14
• Sorry, that should be 62A (@12VDC) = 750W.
– user16128
Dec 7, 2019 at 1:21
• One issue with high idle can be engine temp. The fan is only going to do so much to cool, but YMMV.
– Paul
Dec 8, 2019 at 20:22