Version: 02.12, 01/20/11 Edits and technical details.
Version: 02.11, 12/17/06 Inrush thermistor testing.
Author: Bob Wilson, 9011 Randall Rd, Huntsville, AL
The computer controlled engine and battery system provides a unique
opportunity to make a Prius into an intelligent, gas powered, UPS.
Testing revealed this can provide ~1 kW of 110 VAC, enough to power
a gas furnace.
Theory of Operation
The following circuit sketch shows how we will get 110 VAC from the Prius:
The main advantage is the engine runs ONLY when the main traction battery
voltage falls below a threshold.
Then the engine starts, recharges the traction battery and shuts off again.
For example, the following chart shows typical charge-discharge cycling of
the Prius in operation:
Using the "Toyota Prius Electrical Wiring Diagram 2003 Model," an
analysis of the circuit fuses reveals:
- HVC inverter B+ output goes to F11, F10 in "FUSIBLE LINK BLOCK NO. 1" - the
total fuse protection sums up to 592.5 A. (pp. 50-51)
Remember that fuses protect wiring, not specify the expected load which
will be lower.
- 100 amp fusible link connect to F12, F13 - the total fuse protection
for these circuits adds up to 262.5 A. (pp. 50-51)
- 120 amp fusible link protects the battery and probably the cable
that runs from the engine compartment to the trunk. (pp. 50-51)
BTW, there is a fuse in the inverter:
XPower 1000 Inverter
A review of the XPower 1000 manual reveals their recommended maximum
battery fuse is "150 ADC" and a minimum cable of "No. 2 AWG" for less
than 5 ft.
They also recommended minimizing inductance of the power cable 'loop'.
The inverter to frame ground needs to be "No. 8 AWG" but that also means
an "earth ground" needs to attach to the car during operation.
Finally, they also report it can drive a 1/2 HP motor and our gas
furnace uses a 1/3 HP motor.
BTW, there are
1 wK inverters that
have built in voltage and current indicators and
an optional remote control capability.
But our inverter was left over from an earlier
emergency power project.
The challenge is to find:
- mounting space
- remote control, inverter
- mount inverter
- connect inverter
- run remote control ON/OFF
- install AC outlet
The ideal location for a 12 V. inverter is adjacent to the auxiliary battery.
This can be reached on the left hand side of the trunk by removing the
This photo shows the power cables for the inverter from an earlier
One problem is the ground cable was connected to the battery terminal
which risked overloading that relatively small wire with inverter power
requirements, 90 A.
In our implementation, both the battery ground and the inverter
ground are terminated in the battery strap bolt.
We had to Dremil the washer to make it work but it is a cleaner
Already having a 1 kW inverter, it lacked a remote ON/OFF switch.
This was solved by mounting a 5 VDC, 1 A, double-throw relay and a mini-audio connector.
The circuit uses two of the three mini-plug conductors,
not the ground, to operate the 5 VDC relay.
Both of the 1 A. relay poles are wired in parallel with the manual switch.
Ordinarily the manual switch is left OFF and the inverter is operated via the
remote switch which gets power from the cigarette lighter via an
under the carpet jumper:
The cigarette lighter circuit is relay controlled by the ignition
that insures the inverter cannot be left on without the
ignition key on and drain down the 12 V. battery.
To operate the inverter on JUST the battery, it has to be turned on
in the trunk (NOT RECOMMENDED!)
We are reworking the cabin outlet mount to make a better mount at the
base of the armrest.
We've made a clay mold of the cover and will reform a utility
cover that will be mechanically more secure.
A plastic protector covers the battery B+
so a notch has to be nibbled out.
One tricky part was unbolting the 13 mm. bolt that
connects the battery clamp to the fuse and fusible link holder.
This bolt was attached quite firmly so expect to use both a socket and
vice-grip to loosen it.
The inverter vendor recommends minimizing battery cable inductance
so the toroidal cores will be removed.
Furthermore, the B+ cable and AC power lines will be fitted into
flex-duct to avoid chafing.
All exposed wires were wrapped in
plastic electrical tape to prevent accidental shorts.
Other lesson's learned:
- remove the battery to speed recovery of stuff that falls into the battery well.
- remove ground first, then B+ when taking out battery.
- attach B+ and then ground when installing the battery.
- a longer cable to the B+ makes installation easy.
- the ground cable can be shorter since it is more exposed.
The mounting panel is first modeled in cardboard.
The cardboard template is taped to the 2 ft. x 2 ft., 1/4 inch board and
any rough or interference parts are trimmed out.
The plywood panel is the mount for the inverter.
One valued suggestion from the Yahoo Prius Technical group was to
make it a hinged panel.
This really worked out well.
BTW, we used an adhesive to hold the inverter to the plywood
panel so the four bolts are just clamping the inverter while the adhesive sets.
Once complete, the unit normally is closed and used to provide power for
laptops or other low-power applications.
But during high-power operation, the panel is lowered to maximize cooling
This is what it looks like buttoned up:
For now, we're using bungee cords to hold it closed.
There is more work to be done:
running the remote ON/OFF control line, AC power,
and installing the AC outlet.
But the real question is, how well does it work?
We setup of a three-level, space heater as a load, used a clamp Hall-effect
current sensor, and tested the system.
All currents were measured on the battery cable that comes from the
engine compartment and voltages at the inverter terminals.
The Prius was started with the parking brake set to defeat
the day-light running lights.
The results were:
- 13.94 V., 1.1 A. with inverter OFF
- 13.94 V., 1.7 A. with inverter ON and no load
- 13.83 V., 50.3 A. with heater on lowest setting (~ 700 W)
- 13.75 V., 71-74 A. with heater on medium setting (~ 1 kW)
In this mode, the inverter ran at normal temperature in
the 50 degree (F) evening.
The system was stable and ran without a problem or attention.
This is the mode used for the fuel-burn testing.
- 11.48 V., 89-90 A. with heater on high setting (~ 1 kW)
In this mode, the inverter ran very warm and would shut itself
off every 5 minutes or so with an alarm.
More telling is the Prius dropped the battery voltage, further
limiting the output power to ~1 kW.
There is no advantage to trying to draw more than 1 kW as the
voltage will drop to keep it at 1 kW total.
| Start Time | End Time | Hrs | Load | US Gal | Load | Gas | Effic |
| | | | kW | | kWH | kWH | |
| 11/19/05 17:00 | 11/20/05 09:30 | 16.5 | 1 | 4.215 | 16.5 | 151.74 | 10.9% |
| 11/25/05 22:00 | 11/26/05 10:40 | 12.7 | 0 | 0.811 | 0.0 | 29.20 | 0.0% |
* 0.25 gal/hour fuel burn with 1 kW load
* 0.06 gal/hour fuel burn w/o load
* includes warm-up fuel burn
Even with the day-light drive lights out, the car continued to operate other
systems such as the displays, the trunk light and by accident, the cabin fan
To get a handle on the 'overhead', we repeated the test with just the car running,
not the inverter.
The 'idle only' test revealed a substantially lower fuel burn per hour (temperatures were similar), 20% of the loaded value.
In a second test, we verified the MPG display does update the MPG while
the car is parked and idling.
Using 'stealth mode', we returned home showing 99.9 MPG
but the next morning it was down to 1 or 2 MPG.
As pointed out before, the Prius operate the engine long enough to put a charge
on battery and then shuts down:
Under light or no loads, the engine charges the main traction
battery at 5 A.
The normal vehicle overhead discharges the battery at ~1.7 A until the state of
charge (SOC) reaches the threshold that requires recharging.
Under a full, 1 KW load, the traction battery discharge increases to 5 A so the engine cycles
on and off at roughly a 50:50 duty cycle.
When the engine is running, the power needed for the inverter comes
directly from the generator which is why we only see the traction battery
charging and discharging current in the graph.
To compare the Prius to standard, gas-only, powered generators:
Using data from a Great Northern Tools catalog of portable generators
listing half-power and full-power
fuel consumption rates, we see the Prius inverter data point easily follows the
half-power curve of these commercial generators.
One reason the Prius is not more efficient is the vehicle overhead that
is in the 350-500 W range depending upon outside air temperature and
But consider this, to generate 1 kW of output power, the Prius overhead of 0.5 kW has to be generated too.
If we can increase the Prius AC power output so the overhead is a smaller
percentage, the efficiency could easily exceed the power of a standalone
Altneratively, there may be efficiencies if we can map the overhead power
drains and see if they can be minimized when in Park. For example, the power steering, power brake and air bag systems along with daylight running lights
might be powered off when in Park or a 'special' park.
One interesting observation is the calculation of traction battery
SOC versus voltage.
Understand that SOC is a synthetic number calculated by the
battery controller using unpublished, Toyota algorthms.
What is interesting is the battery hysterisis that shows how charging
SOC lags the rate of traction battery charge voltage then lags
again on discharge at a different rate.
Battery charge-discharge is not 100% efficient and the area
inside the curve maps out this small but measurable loss.
Operating a furnace, especially in anticipation of an 'ice storm'
emergency needs to be carefully tested against these risks:
- Inverter power vs. furnace load - the inverter must have enough
power to handle not only stable operation but also startup loads.
- Furnace control - the modified sine-wave of low cost inverters
has a voltage profile that can cause unpredictable behavior for digital
- Furnace load P factor - the blower motor looks like an inductive
load and this can limit the power available for lights and other loads.
- Extension cable loss - the high frequencies of the modified sine wave
can pass through the capacitance of the extension cords and with
I(2)R losses, reduce voltage at the motor
- Motor runs too slow - if the blower motor does not have enough power,
it can run too slow to dissipate the heat and burn up.
We ran a 7 hour test after instrumenting the gas furnace with
a motor case temperature probe.
We also measured the VAC and current:
121.4(VAC) 6.6(A) 81(F) - house line power (801 W.) at furnace
103.1(VAC) 6.4(A) 85(F) - Prius inverter power at furnace
5.09 (G) = 191 (M) / 37.5 (MPG) - ending fuel burn at 23:40
3.96 (G) = 191 (M) / 48.2 (MPG) - starting fuel burn at 16:35
1.13 (G) fuel burn / 7.08 hr -> 0.16 gal/hr.
* We drove a mile to refill the tank with 4.7 gal., not the calculated 5.09 gal.
* The lower fuel burn reflected not only the lower load but higher
efficiencies from not running the inverter and Prius 12 V. system near
their peak power setting.
Our furnace has a 1/3 Hp. motor rated at just under 600 W. but the
measured utility load was 800 W.
The higher load may be due to bearing lubrication and back-pressure from an
aging gas furnace.
Unloaded, the inverter puts out 120 VAC using a modified sine wave
and was rated to run a 1/2 Hp. motor.
However, we observed a significant voltage drop, ~17 VAC, during the furnace
We brought the inverter power into the house using a 25 ft. contractor grade
and a 50 ft. outdoor extension to the furnace.
The contractor cord has heavier gage wire and provides three outlets into
the primary living room.
We measured a 3 VAC voltage drop using utiliity power through both cables
to the furnace.
This suggests the bulk of the voltage loss was capacitance through
Another problem, not unexpected, was the digital thermostat ran the gas
burner constantly while on modified sine-wave inverter power.
Once the furnace went back to utility power, the thermostat returned to
We will install a mechanical thermostat for power outage use.
Inrush Thermistor Testing
One problem under load is the effects of inrush current spiking the inverter.
Modern, switching power supplies in TVs and laptops often have a full-wave,
bridge feeding a substantial capacitor.
Although the average power usage is small, the initial capacitor charge
can overload one or two cycles that an inverter detects and protects itself
by shutting down briefly.
After the timeout, the inverter comes on again, sees the capacitor inrush and the
whole system slowly oscillates . . . with all of the loads.
To mitigate the effects, certainly with the laptop,
an NF08AA0330M (33 ohm/2.2W) thermistor was wired in series in a
The inrush current sees a room temperature, high resistance thermistor,
so it can not overload the inverter.
But the current that gets through warms up the thermistor to ~150 F and
its resistance goes down allowing full power, current flow.
Testing with the laptop revealed an order of magnitude reduction of inrush
current at a power loss of 0.42W from the power needed to keep the
By unplugging and plugging in while the thermister is hot,
we see a less attenuated, inrush current spike:
This plot comes from a 0.01 ohm resistor connecting the second power
receptical and wired to a 600 ohm, audio isolation transformer.
The transformer secondary is connected to the audio input of a Mac and
AudaCity used to record the signal.
There was no attempt made to calibrate the signal levels but rather,
just show the relative signal level . . . what inrush looks like.
The inrush is consistent with a one-cycle, charge of a capacitor although
the subsequent increase in current is consistent with an attempt to
'soft-start' the power supply load.
A cold, higher resistance thermistor extends the initial capacitor charge so
several power cycles are needed to fully charge the capacitor.
Several samples are displayed to try and capture a worst case event:
The sampling rate was fast enough to capture individual cycles but slow enough
that several seconds are captured.
The thermistor slows the capacitor charging so it takes more than one cycle
to fully charge the capacitor.
Then a couple of seconds later, the power supply begins drawing power at
much lower currents to operate the 45 W laptop.
It is the sudden, unlimited charging that triggers the overload protection
built into the inverter.
Our goal was to operate the gas furnace and testing revealed out unit would be
safe and stable.
We do not anticipate further changes until the gas furnace and air conditioner
are replaced with more modern and efficient units.
We would also like to thank contributors from the Yahoo Group,
"Prius_Tehncial_Stuff" for their valuable suggestions.