checking the prius 12v battery

Everyone knows the Prius has a big nicad battery that powers its electric motor. This is called the “traction battery”. But what they might not know is that it also has a smaller, 12 volt battery, called the “auxiliary battery”. Contrary to popular belief, its the latter that is most likely to be a source of trouble for the average Prius owner.


Learned about this the hard way today. I already knew about the auxiliary battery’s existence. In fact last winter I had the exhilarating experience of jump starting my Prius when the 12 volt went dead — probably due to our leaving an interior lamp on all night.

But today was different. Today the Prius refused to start after leaving it parked for an hour down town in Raleigh, and its control panel got all whacky on us. I was able to finally get it started after a nice waitress from the restaurant we had been in gave us a jump.

A search on the internets revealed the bad news that the aux battery going bad was not uncommon in Prii only as old as ours (2006).

Of course I wanted to confirm there was actually an issue with the battery and so looked for an found instructions on how to do a diagnostic. Here’s a video:

What the video shows is a simple 5 step process: (1) turn on the car without pressing on the brake pedal; (2) hold down the “Display” button while turning the lights on and off 3 times; (3) when the diagnostic menu comes up press “Menu”; (4) hit “Display Check”; (5) press “Vehicle Signal Check”. If the value for “Battery” is anything less than “12.0V” then you’ve got a bad battery. Click the power button on again to put a load on the system to see if the voltage drops further.

As long as you’re still in diag mode you should also check to see if the charging system is working. To do that power the car on (with your foot on the brake). The voltage should go up to about “14.1 V”. If it doesn’t, then you may have a problem with the charging system.

My battery tested out to a little over 10 volts cold. After around a 20 minute drive I got this back up to 12. The next morning it showed 9 volts before my morning commute. While it shoots back up to 14 volts when the car is fully powered up in traction mode, it’s clear that the battery isn’t holding its charge well any more. As a result I’m going to have to replace it. After checking all my options I’ve decided to go with a direct replacement OEM battery (for my 2006 Prius that would be a Toyota part no. 28800-21171) from either a local dealer or online store. I’ve seen it as low as $124 online, which isn’t bad for a factory replacement (particularly one that requires special venting like this one).