2003 Prius that wasn't driven for over a year- what to do with the battery?

Without question, the 12V is shot. The 12V is what starts up the computers. Don't even bother trying to start the car without replacing it.

The hybrid battery starts the engine. It may still be good. Even though NiMH cells will self discharge over time, they are not excessively damaged. If the car starts and the engine runs, let it run long enough to put a good charge into the hybrid battery. If the engine does not start, then the hybrid battery may need to be charged or replaced.

 

I agree that the 12V battery needs to be replaced, as a starting point.

After one year of rest, don't be surprised if the traction battery is also dead. The battery cannot be easily charged and replacement is typically not DIY unless you know how to safely deal with high voltages. That will cost ~$3K to replace at your local Toyota dealer.

 

So what you are saying that if you left a perfectly fine Prius parked for some time, the 12v and traction battery will "self-destruct"?

What is the average time span one should leave a Prius without starting before this happens?

 

Not just Prius, this happens to 12V batteries in any vehicle.

 

If the Prius traction battery was good when the car was parked you probably only need to replace the 12V battery. There is no draw from the traction battery when car sits. I had a Prius that sat for almost 2 years, 12V battery was shot but once replaced car started and ran fine.

 

This is not typical. NiMH will self discharge eventually to the point where there is not sufficient EMF to spin the ICE fast enough for ignition. Further, the longer the ICE sits, the more likely that one or more exhaust valves will be stuck open and cause misfiring if not outright failure to run.

1 month is about the max that you can expect a 12V to survive. 6 months is about the max that a hybrid can sit without running the risk of problems. I am sure that people can come in and say they let their car sit for 1, 2 or 3 years and it started right up with a fresh 12V. That is not the point. The point is that the risk of a problem, whether hybrid battery, ICE rust, or mouse chewage, begins to increase at the 6 month mark and letting the car sit longer than that is a poor practice.

 

This is especially true with regards to Classic, which is the model that the OP asked about. The batteries are now 10-12 years old, near end of life under the best of circumstances; and without frequent charging are likely to die.

 

As the 12 V battery discharges which in an NHW11 takes a few weeks to a month or so, the electrolyte changes from sulfuric acid to water. Then the cold freezes the water. In the old flooded-cell, 12 V batteries the frozen water would crack the case. At a minimum, it crushes/destroys the lead sulfate leading to shorted cells. If fully charged, the 12 V battery is fairly immune to the cold as the freezing point of sulfuric acid is pretty low.

The NiMH battery also self-discharges but they hold a minimum charge for a lot longer.

It makes sense to replace the existing 12V battery with one that is fully charged and then try to start the car. If it runs, let it warm-up until it stops by itself. If you get get codes or errors but it runs, I would probably let it run for a minute; turn it off; disconnect the 12 V battery for a minute to clear the codes; reconnect the 12V, and; try to restart. If it works, let it run until it shuts down by itself. If you get another code, you're going to need a scanner to read the codes and/or get it to a Toyota service center for diagnosis.

GOOD LUCK!
Bob Wilson

 

I paid for an hour on toyota-tech.eu, and one of the things I found was the procedures for storing vehicles. The minus side of the battery should be disconnected, and periodically the battery should be checked and charged if needed. I don't remember the interval, either one month or three months. Hybrids should also be put in READY
for 30 minutes every month. I don't know why.

 

The reason why is to charge the traction battery.

 

Likely the 12 volt is dead, and that's all. But as far as I know, there's no harm in just trying to start it, you might get lucky.

If you have access to a digital multimeter: a healthy 12 volt should be around 12.6 volts, or better. 12.5 or less if it's dead or near-dead.

It might also be possible to revise the battery with a charger. A long charging session with a lower amperage, "intelligent" charger is best bet, say something around 3 amps.

"Intelligent" chargers can be left on indefinitely, they typically have several steps in the charging process, and taper off to a very low amperage charge at the end, just enough to maintain the battery's charge. They sometimes have rudimentary lights to let you know if the battery needs charging, is fully charged, or is a lost cause.

 

Ah, I was under the impression that just putting it in READY would not run the combustion engine.

 

Putting the car in Ready mode means it is ready to drive. Normally after 10 seconds or so the engine will fire up, warm itself up and then switch off again. In Ready mode the 12v will be charged also as required.

Your misconception is common though - many a Prius meets its maker when a garage not familiar with them changes the oil. They wrongly assume that because the engine isn't running the car is off. They then drain oil from engine, car detects drop in engine temperature and fires up engine with no oil in it; bang, clatter, clatter, bang. New engine required.

 

The Classic (at least the North America version) engine will run for several minutes upon cold startup if left READY. The point of this is to heat up the catalytic converter. Then, if you leave the car READY for a total of 30 min., you will find that the gasoline engine will periodically turn on as needed to charge the traction battery.

If you follow this procedure, you should turn off the cabin heating/ventilation system so that the engine ECU does not run the engine just to keep the cabin heater core warm, and also to eliminate an unnecessary electrical system drain (for example, the cabin heating system relies upon the traditional heater core heated by engine coolant as well as two auxiliary electric heater elements.)