Leaving a Toyota Prius Undriven

Leaving a conventional car undriven for up to a couple of months is usually not a matter of much concern.  The worst that can happen is that you come back to a flat battery and have to jump start the car.  You need to be a bit more careful with the Prius, for reasons I'll explain below.  I have personally left my Prius undriven for two weeks and it started right up.  You could therefore consider two weeks or less to be safe and not worth any precautions at all.  Others have left their cars for a little longer, also with no problems.

For longer periods, say over the winter or during an extended absence, as with any car you should either know exactly what you are doing or seek professional help.  For example, it is not a good idea to leave gasoline in a car for many months as it changes chemically and can leave deposits in the fuel system.  You should remove the fuel before storage or treat the fuel and leave the tank is almost empty.

Auxiliary Battery Discharge

The first precaution to take with the Prius is to prevent the auxiliary battery from discharging.  Unlike in a conventional car, where the 12 volt battery has to supply power to the starter motor, the Prius 12 volt battery has no large power demands and is therefore small and light duty.  Its capacity is 28 ampere hours.  It can be drained in quite a short time by leaving the interior light on, doors ajar or the cabin fans running when the car is not turned on (Yahoo! group toyota-prius post 19028).  It can also be discharged even if all lights and accessories are turned off.  The current from the auxiliary battery has been measured and recorded in toyota-prius post 33179.  I reproduce the data here:

Right after doors are locked

0.54 amps

After 20 seconds from doors locked

0.075 amps

After an additional minute and indefinitely thereafter

Headlight switch ON

0.047 amps

Headlight switch OFF

0.036 amps

Obviously, if you're leaving the car for a while, you want to make sure the headlight switch is OFF.  Leaving the switch ON and letting the car turn the lights off itself would be OK for a week or two. 0.036 amps will use up the 28 ampere hours in the battery in 28 / 0.036 = 778 hours, which is 32 days.  So, less than a month should be safe, but longer is not.

If you are not going to drive the car for a while, here are some methods for preventing the auxiliary battery from discharging:

If you don't take these precautions, the worse that can happen is still a discharged battery.  You can jump start the Prius in a similar way to other cars (although jump starting another car from a Prius is not recommended).  There is no need to run the engine on the other car because of the low power demand.  You can also jump start from another battery.  See page 167 of the 2001 Owner's Manual for details.  Light duty booster cables will work as well as heavy duty jumper cables.  The only thing to be aware of is that each time you discharge a lead-acid battery completely, you shorten its life.  So, you don't want to let this happen too often.

High Voltage Battery Discharge

The second concern is discharge of the high voltage battery.  This will not happen in such a short time as discharge of the auxiliary battery, but if it does happen, it can be more serious trouble.  If the state of charge drops below the programmed lower limit for normal operation, the car will not start.  It can be persuaded to start by a Toyota technician with the appropriate equipment.  He or she may make a house call or you will have to have the car towed to a service facility.  By plugging in a tester and pressing some buttons, the car is assured that it is in good hands and will start using whatever power remains in the battery.  It will then charge itself back up.  Should the charge fall so low that there is not enough to start the car, you will definitely need a tow.  The service facility may be able to recharge the battery in the car or may have to remove it to be recharged or even replaced.

The high voltage battery is completely disconnected when the car is turned off at the ignition.  No current is drawn from the battery.  Unfortunately, nickel metal hydride (NiMH) batteries have a property called "self-discharge" by which they lose charge even when no power is drawn from them.  A figure of 2% per day is often quoted at room temperature for household NiMH batteries, but this may not be correct for the Prius batteries.

Toyota's recommendation, which appeared on their Web site in the FAQ section, is to have the hybrid system started every two months and to let it run for 30 minutes.  Of course, you'll have to reconnect the auxiliary battery if you've chosen to disconnect it.  You can be more confident that this will work at low temperatures, for example in the winter, since the self-discharge rate is reduced. You might be more cautious at high temperatures, when self discharge increases.  I would recommend making sure the battery icon shows no less that three-quarters before you leave the car.  If it doesn't, go for a drive and stay out of EV mode.  [Note to self - Search Yahoo! groups for actual experiences. 16094]

Last edited June 21, 2002.  All material Copyright © 2001, 2002 Graham Davies.  No liability accepted.