Q: I'm using a 400-watt power inverter in my truck. It is mounted on the floor tunnel on the passenger side. I ran the cables through the firewall from the truck battery. Now I want to have a larger one for the small chest freezer I'll have in my cargo trailer for hunting. I'm wondering if 1,000 watts is big enough, and I'll need to find a new place for the larger inverter, probably under the back seat. What are your thoughts on extending the cables to this area? How long can I run the inverter with the engine off?
_ Steve A.
A: Power inverters are very handy gadgets if they're properly chosen and installed! Connected to a robust 12-volt system, they make household 120 volts suitable for a variety of small to mid-sized appliances.
Let's get a handle on watts, and how the trade-off occurs: Volts times amps equals watts. Voltage is similar to pressure, amperage is the quantity of electrons flowing (current), and watts is the usable power that can result. Household power is 10 times the voltage of a 12-volt automotive system, so only 1/10 the current (amps) flows to get the same amount of work done. This is why 120-volt extension cords and wiring are not particularly large in diameter and can be somewhat lengthy compared to 12-volt systems. Current flow through a wire is similar to water flowing through a pipe, the larger and shorter the pipe is, the better it flows!
Your existing 400-watt inverter may cause as many as 30 amps of current to flow through its supply cables. This would best be managed with the shortest run possible of eight-gauge wire and a 40- to 50-amp fuse at the battery connection. Upgrading to a larger inverter placed at perhaps three times the distance will require larger cables. Your small chest freezer probably requires 200 watts to 300 watts to operate, so choosing a more robust inverter is a good idea, along with possibly carrying a spare unit to insure your hard-earned protein makes it home safely. I'd size the cables to the inverter rating as opposed to your expected freezer usage, as you may decide to add additional loads in the future. Two-gauge cable with a 100-amp fuse would be a good choice (AWG cable sizes are like shotgun shells; a smaller number is a larger physical size).
The 120 volt side of the system enjoys minimal current, so a typical 25- to 30-inch 14-gauge extension cord would work fine between inverter and freezer. Now, some safety concerns: Be extremely careful when routing both the 12-volt and 120-volt cables (avoid chafing with protective sleeves), and passing them through metal. I'd consider two holes in the back seat floor, one for each, with an appropriate rubber grommet. The truck frame can be used for the 12 volt ground connection; remove paint from the mating surface.
I would run the inverter only while the engine is running, and/or only briefly afterwards to avoid running down your truck battery to a no-start situation. Adding a pair of deep cycle 27 series batteries in the trailer, connected to the truck electrical system for charging (while driving) would be a good idea if you won't be running the truck frequently (disconnect from the truck while parked). The pair of batteries should power the freezer for perhaps three hours before being recharged (avoid discharging the batteries below 50 percent). During road trips 10 hours on/14 hours off works OK for keeping pre-frozen goods solid (I have done this many times with a freezer full of fish) but your fresh meat would realistically be kept just cold for your return trip, unless it's lengthy.
