Second battery likely to blame for Prius no-start

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This article was published 09/08/2019 (2488 days ago), so information in it may no longer be current.

Question: I have had my 2012 Toyota Prius for more than a year and have never had a problem. Only one time, several months ago, I had the key fob with me, sat down to start the car and nothing. I even noticed the interior light didn’t come on. I hooked the booster cables to the car and voila, everything came on and the car started. Funny thing is, I had just turned the car off 15 minutes prior to this occurrence. I have left the car for a week and came back and no trouble! Comments?

— Ed

Answer: The Toyota Prius has two batteries: the high-current, high-voltage hybrid battery and a conventional 12-volt car battery. When the car is operating, it runs off the hybrid battery, but for starting, the system relies on the 12-volt battery.

There are several reasons the vehicle may not start. It could be weak batteries in the key fob, the vehicle immobilizer (anti-theft) system has a fault, the park switch is not making contact, a malfunction of the hybrid system computer or a dead 12-volt battery. Because you found the interior lights were not working, this would indicate a fault with the 12-volt battery or its electrical connections. The 12-volt battery is not big in capacity because it is not the main operating-system battery, so it can become discharged fairly quickly if the headlights or an interior light is left on. You had only turned the vehicle off 15 minutes earlier, but if the headlights has been left on and the 12-volt battery was already low on charge, this could be long enough.

Another possible fault could be internal in the 12-volt battery itself. One of the internal battery connections between cells may be weak. Sometimes, when a battery is boosted, the connection gets hot and “welds” itself back together again so the battery operates normally. Load-testing the battery with a high-current load will usually place enough load on the battery to cause this type of fault to occur again, so you may wish to have the battery load tested at your local service shop. Since your Prius has only acted this way once, I suspect it was a discharged 12-volt battery that was the problem.

In your situation, the 12-volt battery may have been dead enough that a boost was the only way to start the vehicle, but if the battery has a little energy, yet not enough to crank the engine, there is an emergency way to start the engine: set the parking brake, turn the power switch to accessory mode and press and hold the power switch for about 15 seconds while depressing the brake pedal firmly. Even if the hybrid system can be started using the above steps, the system may be malfunctioning and should be checked by your dealer.

james.kerr@sasktel.net