12v Battery Wacky Issues

Sounds like a failed 12v battery. The 12v is a weak point on the Prius as it's only a small battery - it doesn't have to crank the engine, just boot up a computer.

The 12v also only seems to last about 5 years or 80,000 miles so if yours is an original, then it sounds like it needs replacing. Also, if your car has been on a ship for weeks or months and then on a dock for as long, then it will have flattened the 12v. Once they've gone really flat they'll never be the same again or hold charge correctly.

Just replace the 12v and all should be well. If it isn't, then you have a major problem.

Not sure where in the world you are but here in the UK an official Toyota 12v costs £95 fully fitted including tax. (I checked the other day when in for service work on mine).

 

Easy test for you. First thing in the morning put the car in Aux mode (NOT in Ready), where the warning lights all come on. Now wind all the windows down at once and immediately wind them all back up again. If they go painfully slow or stutter and/or your interior light goes very dull then your 12v is shot.

If you then put the car in Ready mode and do it again they should all be normal again.

There are many posts on this forum about failing 12volt batteries and the weird electric issues they cause when failing. The Prius 12v is a weird one in that it's quite hard to find other than via Toyota. In America there are after market options because theirs is a big market with many Prius sold. Here in the UK it is possible to find an alternative to the Toyota 12v but the cost savings make it barely worthwhile. Can your local dealer not import one? Can you not get one off ebay? Make sure there is the vent tube option as you don't want fumes coming off it into your boot.

One last thing - take care if you're jump starting the Prius. The other car shouldn't rev its engine and make sure you don't get the terminals the wrong way round - even for 1 second, as you will destroy the cars computers. An expensive mistake to make.

 

The best way to check the 12V battery is to measure voltage across the battery terminals using a digital multimeter, after waiting overnight to dissipate surface charge in the battery.

An alternate approach to measuring battery voltage is to remove the lid of the main relay/fuse box near the inverter, then find the dedicated positive jumpstart terminal which is covered by the red hinged cover, and measure voltage from that terminal to body ground. To remove the relay/fuse box lid, press hard on the latch at the front of the cover, and pivot the cover upwards. The cover can then be removed.

A new, fully-charged 12V AGM battery will measure 12.9V. If your battery measures 12.5V or better then it is in reasonable shape. If the reading is much below that then I would replace it now.

A subjective way to assess battery health is to note how quickly the power windows work when the car is in IG-ON (not READY) mode as previously suggested. Or, turn on the headlights when the car is IG-OFF and see how bright they are.

You might also replace the fob battery while you are at it, the fob uses a CR2032 battery which (at least in the US) is available for purchase at any drugstore.

 

The Prius 12V battery is AGM (absorbed glass mat) so there is very little free liquid acid within. It has a vent to allow any hydrogen gas produced during the charging process to be routed to the car's exterior.

If you buy a standard battery it will be a traditional liquid acid battery and will not have a vent fitting, hence this is a somewhat unsafe arrangement especially in the event of an accident that impacts your car's right-rear fender.

The Prius 12V battery terminal posts are very small, so please be aware of that when shopping for a new battery. Note the battery terminal polarity: when the terminals are closest to you, you want the positive terminal to be on the left.