Charging 2005 Prius 12V Dead Battery, How to Dismiss the Alarm?

Put the fob in the slot and start the car.

 

Just pushing the unlock button on the fob has always worked for me.

 

I suggest you disconnect the 12V battery before you charge it since the battery is in such poor condition. Then you don’t have to worry about the alarm. Or just face the fact you need a new battery and buy it now.

 

This is the way. I'm going through this right now. The battery was very low -- about 6V. I started to charge it from the fuse block connection, as the voltage rose the alarm would go off but the key fob wouldn't shut the alarm off because the voltage wasn't high enough to enable all the systems. I finally accessed the rear hatch, disconnected the battery and am charging directly. When it's recovered, I'll reconnect the battery and be able to silence the alarm by the key fob or putting the car into IG-ON.

 

It's been my experience that you can bring back a relatively new battery, that hasn't been severely discharged back by charging. However, they don't seem to come all the way back. If you've got a few years on the thing, or have discharged it way low, it'll probably be time to just change it. This is the time of the year when we get a cold snap and find out which batteries need to be replaced.

Prius really don't handle a low 12V battery very well. Especially if the car is trying to turn on. It starts to power up, but gets to that point where it activates the power brakes and steering, which pulls that little battery down to the point where it shuts off the ECUs, which allows the 12V battery to recover, somewhat which starts the process all over again. Even when you're trying to shut the car off. The irony of the situation is that there's not enough 12V battery at that point to shut the car down, so it starts cycling between powering on and powering off.

I've got a 12V charger, but it's only about 6 Amps, which is not enough to power the car up, so even with the battery charger attached to the 12V connector in the fuse box, the car is trying to pull more than the charger could provide - sending it into an overload cutoff.

Since the rear deck latch is also electric, and the door lock ECU doesn't power up fast enough to unlock it before the 12V system crashes I couldn't open the rear hatch normally. At that point, I had to go all "Mission Impossible" to disconnect the 12V battery. I had to pull the back seat down, then remove the rear deck carpet and spare tire covers through the back seat. Then I crawled through the back seat to where I could reach into the hole, feel around and find the rear deck latch manual release. Finally, I could take the battery cover off and disconnect the negative battery cable off with a 10mm socket. I suppose I could have also taken the battery cover off and disconnected the battery without opening the rear hatch, but it was also dark and I couldn't see well enough to do that. That's what finally let the car turn off enough to stop the alarm.