It bothers me no end that I cannot use my 5.4 kW Solar array for backup power without a $15,000 modification to add a transfer switch, some really expensive batteries, and associated equipment.
As an alternative, I’m looking into using my 2015 i3 (BEV) as a home backup source. I bought a 12V inverter and made some initial measurements which I’d like to share with you. My concern is I don’t know how much power I can pull from an i3 without damaging the DC to DC converter or some other component. I can not locate the specs for this converter and have seen only brief posts on using the i3 as a backup source.
Since the i3 has three 12V accessory sockets and they are protected by 20 amp fuses (I believe), a simple calculation would indicate it might be safe to pull 720 Watts over a long period (12V x 20A = 240W, 240W x 3 = 720W).
I purchased a VertaMax Pure Sine Wave 1500 Watt (3000 W surge) 12V Power Inverter from Amazon for $261. I picked this particular model because it included battery cables and a 150 amp ANL fuse.
Connecting the inverter was more difficult than I expected. After removing the two mesh covers and frunk (6 hex screws) and disconnecting the negative (black) and positive (red) terminals from the 20 amp-hour accessory battery, I couldn’t easily connect the black inverter cable to the negative terminal. After some research I determined it was safe to attach this cable to the chassis (the attachment point I used had just 0.1 ohms of resistance to the negative terminal). The red inverter cable easily attached to a Bolt on the forward part of the positive battery terminal. I don’t believe battery clamps are sufficient for a good electrical connection. Also, I wouldn’t connect a 1500W inverter without a fuse.
As a load I used a ceramic space heater that averaged a draw of 700W on its low setting. Before powering on the inverter I disconnected the front running lights (it’s easy to do, just press the tabs on the sides and pull back). The goal is to use the High Voltage battery as backup power for the house so we want to minimize the drain on the battery from the i3 accessories.
I put the i3 in READY, turned off the lights (position 0), fan, A/C and radio. The seat heaters and all other accessories were off. Note that it is necessary to exit the i3 from the passenger side and leave the key fob in the car in order to keep it in the READY state. It is also best to have the car windows rolled down.
I used a multimeter to measure the input voltage of the inverter and a watt-meter (kill-a-watt) to measure the load. The State of Charge (SOC) was displayed on the instrument cluster (press the button on the end of the left steering column stalk repeatedly until it appears). The i3 was at 100% SOC when the inverter was turned on. Ambient temperature was between 50 and 60 degrees F during the measurement period.
Minutes--SOC(%)--Load (W)-------Input (VDC)
At the end of the measurement period the inverter was turned off and the input voltage jumped to 14.39 VDC.
After removing the inverter I took the i3 for a one mile test ride. It functioned without fault but I noticed the SOC was 55% after this test drive.
If the load were a constant 700W then the energy used during the 292 minutes (4.87 hours) would be 3.4 kWh.
A 30% discharge of a 18.8 kWh HV battery should be 5.64 kWh. This discrepancy might be explained by the the losses in the inverter and the DC to DC converter as well as the power draw of the i3 electronics.
If I were to use the i3 for backup power I would probably limit it to 3 kWh or less per day in order to make it last for three days.
A few questions:
How much power can be safely pulled from the i3 without damage or degradation to the car?
How many kWh can be realized for the load if the i3 is discharged from 100 to 10% SOC.
Is the SOC indication inaccurate under this test setup?
Is there some other way to determine remaining battery capacity?
Does anyone have the specs for the DC to DC converter?
I don’t believe the EV manufactures are aware what a strong selling point they would have with EVs designed to provide backup power. At least they should provide terminals for easy connection of a 12V inverter along with info on the limits of its use. EVs already have inverters. Would it be so difficult to modify them to provide a single phase of 60 Hz power along with suitable outlets on the outside and inside of the vehicle?
MikeB, 3/2/2018, owner of 2015 BMW i3 (BEV) with parking and technology packages.