Emergency power from hybrid car

The aim here is to describe a simple system to use a hybrid car to provide some backup power to your house during power outages. This is a very simple system that only cost $70 and requires no changes to the vehicle, no wiring, no tools, and about 1 minute to hookup. The system only provides 500 watts of 115 Volt AC power, but this is enough to keep most fridges (see update below) running, provide a little LED light, power a WIFI router, charge phones and a laptop, and run a modest size TV -- maybe all at once. So, a lot better than nothing in a power outage.

Most hybrid cars have a large, high voltage traction battery that drives the wheels, but most also have a small 12 volt lead-acid battery that runs all of the cars 12 volt accessories. The 12 volt battery is automatically kept charged from the traction battery, and when the traction battery gets low enough, the internal combustion engine is automatically turned on to charge it. So, the car sitting in your driveway will automatically provide power to your house as long as there is gas in the gas tank. This system connects a 500 watt inverter to the 12 volt battery and uses the 500 watts to power a few key loads in your house via an extension cord.

From other articles on this, it appears that a typical hybrid can support up to 1000 watt inverters on cars like the Prius or RAV4 Hybrid, but I decided to use the smaller 500 watt inverter to stay well within the vehicles max charge rate to the 12 volt battery, and to be able to use just simple alligator clamps (that come with the inverter) to make the connection -- no custom cables or disconnects to make up. This alligator clamp connection means that from a warranty standpoint, there is nothing for Toyota to see. But, if you need the full 1000 watts, its not that hard to make the cables and get them hooked up to the battery.

I ran this setup on my 2020 RAV4 Hybrid for 2 hours with various loads and measured various voltages, currents, and temperatures and did not see any sign of problems (see below), but if you decide to do this, its at your own risk -- be careful!

Setting up The System

Inverter hooked up to the hybrids 12 volt accessory battery. Just takes a minute to pull off the battery
cover, hook up the alligator clamps, and plug in the extension cord.

I'm going to describe the setup for my RAV4 Hybrid -- things will be a bit different for other hybrids.

Go back out to the car and make sure that everything looks OK and that none of the connections are heating up. In my case, when running right at 500 watts, the wires from inverter to battery warm up to the point where its a bit uncomfortable to hold on to them at length (about 140F). That's probably about as warm as you want any connection to get.

At least for the first time you use this setup, check how things are going in the car frequently. Make sure that nothing bad is happening and that the car is staying in "Ready" mode where it will continue to charge the 12 volt battery as needed.

On the 2020 RAV4 Hybrid it is necessary to lock the doors using the lock switch on the armrest in order to keep the car in "Ready" for more than one hour. If you don't, the car will drop out of Ready after an hour, and the inverter will drain the 12 volt battery down until the inverter low battery voltage limit is triggered -- this might be low enough that the car won't start - not sure.

- If you use an extension cord to get the power from car to house, be sure its rated for at least 500 watts (4.3 amps).

- Get a KiloWatt meter and use it to monitor the amount of power you are using -- that is, plug KiloWatt into extension cord, then a good power strip into the KiloWatt, then your loads into the power strip. The KiloWatt is cheap and useful for many things.

- Make sure the alligator clips are firmly connected to the 12 volt battery, and after the system has been running some loads for several minutes, touch all the wires and connections to make sure they are not getting hot.

- On the first hookup, check how things are going in the car frequently for the first few hours just to make sure nothing is going amiss.

Testing

I did a two hour test with various loads including a full hour at 500 watts. Measured battery voltage, charging current, inverter input current, inverter output power, and inverter output voltage -- see table below.

"Ready" status - whether the car was in "Ready" mode (ie hybrid system active) - car must stay in ready mode in order to charge the 12 volt battery.

Ibat (amps) - the current coming INTO the 12 volt battery from the car charging system

Vinverter (volts AC) - the output voltage of the inverter (should be 110 to 120 VAC)

Inverter Out Watts (watts) - the power that the inverter is outputting to the house loads in watts (should not exceed 500 watts)

2:16 - With car in Ready mode and inverter off, car is supplying 1 amp to the 12 volt battery to satisfy loads in the car (this is with everything turned off in the car that I could turn off). Its good that this is low in that not a lot of power is wasted keeping the car loads satisfied.

2:20 - Turned inverter switch on, but no load on inverter. Inverter draws 0.75 amps of idle, no-load current. This standby load the inverter imposes when doing nothing drops to 0.47 amps in a later step.

2:33 - With a load of 165 watts on the inverter (two light bulbs): Charge current to 12 volt battery is 16.1 amps and 12.3 amps are going to the inverter from the battery.

2:42 - The IC engine comes on to charge the traction battery. Current from traction battery to 12 volt battery goes up to 21.5 amps. The IC engine runs for about 4 minutes and then turns off.

3:06 - The IC engine starts again and runs for about 2 minutes. Load on inverter is increased to 405 watts (bigger lamps).

3:16 - The car drops out of Ready mode. This is a feature of the 2020 RAV4 which drops it out of Ready mode after one hour. This can be defeated by locking the doors with the Lock Switch. Its important to know how this works on your hybrid -- if the car drops out of Ready, then your 12 volt battery will be drained down to nothing by the inverter. A hybrid with a dead 12 volt battery might have to be jumpered to start it.

The car was out of Ready mode for a few minutes, and when it was put back into Ready, the charging current to the 12 volt battery went up to 77 amps for a short while to take the 12 volt battery up to charged -- this was the highest current of the test. The current dropped down to 64 amps after a few seconds. Most sources say that the current that can be supplied to the 12 volt battery by the DC to DC converter is at least 100 amps, and it never got close to this during this test.

3:17 to 4:17 - Running with a 500 watt load on the inverter. Mostly about 45 amps into the 12 volt battery from the vehicle and about 40 amps from the battery into the inverter. Everything seemed to be running fine. This appears to be less than half of the 100 amp maximum charge rate from the traction batteries to the 12 volt battery.

4:17 -- Car dropped out of Ready again because I had not locked the doors using the Lock Switch. End of test.

When running the maximum 500 watt load, the wires from the inverter to the 12 volt battery got warm enough so you could just keep your hands on them permanently. On the IR picture this is 140F. This is well below the insulation temperature rating for most wire, and is likely OK. It seems just a bit warm to me, and I may replace these wires with the next gauge up -- I'll just reuse the alligator clamps, which seem to do a good job.

The alligator clamp connections to the 12 volt battery do not heat up much at all, and seem to be working fine. I was a bit concerned about this, and I think its important when you hook up the alligator clamps to make sure they are making good contact and check them after a while to make sure they are not heating up.

The inverter body was just barely warm to the touch even after running on maximum load for an hour, so that's good.

I used a 12 gauge 100 ft extension cord between the inverter and the loads and with the 500 watt load, the voltage at the load was right at 115 VAC, so that's good. The calculated voltage drop with 500 watts over a 100 ft extension cord is 1.37 volts, so, likely OK. You can use this voltage drop calculator to check your situation.

With a 500 watt load, the IC engine appears to turn on for 2 to 4 minutes about every half hour to keep the 12 volt battery charged. Don't know how fast this will use gasoline, but I suspect that a full tank will last for multiple days.

Thermal image of inverter cable connections to the 12 volt battery with maximum 500 watt load.
Max wire temperature is 139F and alligator clamp temperature is 119F.

There was a question raised on whether this small inverter would be capable of handling the startup surge for a typical refrigerator. To test this, I hooked up our 24 cubic foot LG energy start fridge to the RAV4 and inverter for a bit over an hour.

There were several startups during the hour and the inverter did not have any problems with the startup current. For the last half hour I added a 100 watt lamp to the load and again there was no problem starting up the fridge with the extra 100 watt load. So, I'm pretty confident that with my fridge I can run the fridge along with some other small loads like a small TV, LED lamp, phone charger, ...

Monitoring the power with a KiloWatt meter, the fridge load varied from a low of 5 watts when the fridge cooling unit was off to a max of 108 watts when the cooling unit was running. The power used during startup was different than I expected. From the idle power of 5 watts, the power slowly ramped up to the 108 watt running wattage. There was no real surge that could be seen on the KiloWatt, but the KiloWatt may be too slow to catch a fast surge.

During this test, the inverter was not even warm to the touch and the wires from inverter to battery that got up to 140F with the steady 500 watt load were just barely warm to the touch.

The fridge has a tag on the front that reads "Inverter Linear". I guess its possible that this technology reduces the startup surge? So, your experience may vary. Please let me know if you try this and have a problem with your fridge at startup.

Simple Emergency Power From Hybrid Cars

Setting up The System

Testing