Charging the Plug-in Prius with Solar Power




When specifying the solar installation at the house in Arizona with the installer, one of the requirements was to be able to run essential equipment during a grid failure.  I suggested using five 4kW inverters instead of the somewhat less expensive option of a single 4kW and two 7kW units.  That was because the "Sunny Boy" inverters he recommended each have what they call a "secure power supply" capable of supplying 1500W of off-grid power.  For a modest cost increment, I would have five "secure power supplies" instead of just three, and a full 7.5kW to keep my ice cream from melting.  Just as important, the 1500W specification implied that I could charge my Plug-in Prius from any one of the inverters simply by plugging the Prius charging cable into its outlet. 

Whoopsie.  I was very distressed to discover that, for whatever reason, the "secure power supply" didn't quite meet spec.  I would plug in the Prius, the current would ramp up, the inverter would cut out, and I would spend a few minutes staring incredulously at my trusty Kill-A-Watt wondering what was wrong while the process automatically repeated. 

Making a very long, tedious, well-documented series of measurements and volumes of correspondence with the inverter manufacturer, SMA-America into this one-paragraph note, I ended up agreeing with myself and vehemently disagreeing with the manufacturer who claimed that the Sunny Boy was just fine and I must be wrong.

Unfortunately, ge tting the manufacturer to do something about the power shortfall was (and may continue to be) a difficult task.  Invoking the "life is too short" clause, I decided to do something about it myself.  I hypothesized that if I could somehow reduce the Prius charging power, I would be able to use the inverter as intended.  While I haven't found any detailed information on the Prius charging system, I guessed that charging it at 100V would reduce the power demand even as it increased the charge time by an amount equivalent to the percentage voltage reduction.  (This is a logical but not foregone conclusion; some power supplies take more current as the voltage input decreases,  Fortunately, the Prius isn't one of them.)

To reduce the input voltage, I went to the local Home Depot and picked up a landscape lighting "power pack."  Although it was much too fancy for this application, it allowed one to bypass the photocell and digital timer and still produce 12VAC at 200 watts.  By connecting this in a series "bucking" arrangement with the AC line, I was able to reduce the power demand from the Prius charger to 1200W and 10.78A which the inverter was able to handle.

The graphics below the solar panel outputs before and during the Prius charging. 

Solar panels during normal grid-tie operation

Solar panels delivering charging power to Plug-in Prius

Normal grid-tie operation - Panels producing maximum power on this sunny mid-November day.

One solar inverter disconnected from the Grid and the "Secure Power Supply" switched on.  Prius is now charging from the inverter AC outlet.  Readouts below show power being delivered to the Plug-in Prius charging cable.

"Kill-A Watt" showing 112.2 VAC

"Kill A Watt" showing 10.78 amos

"Kill A Watt" showing 1202 watts

"Kill A Watt" showing .99 power factor

Inverter voltage 112.2 volts Inverter current 10.78 amps Inverter power 1202 Watts Load power factor .99

Solar panels delivering end-of-charge power to Plug-in Prius

Solar panels after charge is complete

Prius almost completely charged - charging power decreases.

Prius now fully charged, only remaining current drain is from parasitic load of 12V transformer.

I made an interesting if minor discovery.  Right before the Prius completes its charging, the power decreases by almost half.  Presumably this facilitates measuring the charge state of the Prius lithium battery pack and may "top off" the charge.  I've never seen it discussed in the Prius information I've found on the internet.

This is the temporary test setup I used to confirm that I could charge the Prius.  The block at the bottom left is the "power pack" that delivers 12VAC at 200 W.  It's connected in series "bucking" with the Prius charging cable at the right.