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I'd like to install a semi-permanent 500 watt inverter connected to the car battery for greater power delivery. The battery is in the passenger area, so there's no need to run wiring through the firewall. I plan to connect the inverter's ground to the body and the positive directly to the battery with a fuse inline. Here are my questions:

  • I'm concerned about leaving the inverter on when the car is parked and draining the battery. How hard would it be to wire the inverter so that it is only enabled when the engine is running, or at least when the accessories are on?
  • Should the inline fuse rating match the inverter rating, or are there other factors to consider?
  • What type of connector should I use to allow quick (safe) disconnect of the inverter leads so I don't have to take the ring terminals off the battery.
  • How can I find out the car's alternator rating to ensure I don't overload it?

Thanks.

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  • What size inverter? The lighter socket probably addresses almost all you needs, at least what we can tell from the given information.
    – SteveRacer
    Commented May 30, 2016 at 20:46
  • I forgot to mention that. It's a 500 watt inverter. It works through the lighter socket but can't draw nearly its full capacity. I think the lighter has a 15 amp fuse, which works out to a max of 180 watts.
    – SArcher
    Commented May 30, 2016 at 21:29
  • 6
    use relay to power the inverter. that way when you shut the key off the inverter shuts off. also look up your cars alternator on a parts website they typically list the alternator ratings.
    – Ben
    Commented May 30, 2016 at 21:46

2 Answers 2

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With inverters you also have to take into consideration the efficiency of the inverter itself when calculating power requirement. For example: 12V outlet is rated at about 10A or 120W. Inverter is 87% efficient.

120W * .87 = 104.4 W =Max size of inverter to use is 100W

You can use a battery isolator -check at places that sell equipment for RV's. A battery isolator is a device with 2 diodes inside. The center connection is for the output of the alternator, the one on the left goes to the car's starting battery, the one on the right goes to your inverter

battery. Since diodes conduct in one direction both batteries don't discharge and only function when charging.

Assuming that your inverter is 500W @ 87% = 500/.87 = 574W used at 12V @ 48A

The alternator rating should be listed in owner's manual or written on the back of your alternators casing somewhere.

The bonus feature of a battery isolator is in winter if you a little juice to crank over- just use a single battery cable short the right and left terminal and jump start your vehicle.

Check out these links: https://www.emarineinc.com/categories/Battery-Isolators

http://www.bluewatermarinesvc.com/html/bat_isolator.html

No Switches or relays required...

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I'm concerned about leaving the inverter on when the car is parked and draining the battery. How hard would it be to wire the inverter so that it is only enabled when the engine is running, or at least when the accessories are on?

For a 500 W inverter you should use a relay rated at 50A minimum (see resident_heretic's calculations). The schematic should be pretty simple:

enter image description here

Should the inline fuse rating match the inverter rating, or are there other factors to consider?

The fuse rating should be lower than that of the relay.

What type of connector should I use to allow quick (safe) disconnect of the inverter leads so I don't have to take the ring terminals off the battery.

Welding tools use these:

enter image description here

They come in different sizes. The smallest will do just fine for 50-100A.

How can I find out the car's alternator rating to ensure I don't overload it?

Typical alternators are in the 100-150A range.

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