Greetings, I have a 2004 Prius with the battery that sits in the back. I've never had a problem with this car for the last 10 years. Consistent services seem to help. To be honest I'm not sure if the 12v battery was ever replaced during routine maintenance. If the 100k mile service does not replace the battery then it's 10 years old. (It's got about 120k now) I few days ago I left the dome light on and woke this morning to find the Prius dead. I used my other car to jump start and drove it around for 30 min or so to charge up the system. My question is; Should this 12v battery be replaced now? Does this battery affect the entire system to include the main battery system? I appreciate any feedback anyone might have about this. Thanks, -Matt
If it is an OEM battery it will probably need to be replaced, especially after being completely discharged. If it is the original or or the send one you've gotten good service from it. At this point it could leave you stranded. And repeatedly jumping increases the risk of reverse connection and blowing out very expensive stuff. A weak 12V does effect the entire system in that the HV battery continually tries to charge the 12V and the engine runs more often to keep the HV battery up. All this decreases gas mileage while increasing stress on the system. But you might want to run the onboard 12V battery check after it has set up for many hours, like overnight. Then report all three values back here. Weird stuff happening? MPGs dropping? Test The Battery | PriusChat
If you were to run diagnostics on it right now I think it would show a very borderline status. Driving around for 30 minutes is not going to remedy the state of a near-drained battery. I'd suggest for starters get an intelligent charger, something that does rudimentary display of status, not too high amperage (check the manual for limits) and capable of progressive charging, ie: starts out with desulphating cycle, then bulk charging, then taper off to maintenance. It'll likely take 1 or more overnight sessions before the charger is "happy" with it. And then maybe you'll get a bit more life out of it. I wouldn't go into winter with it as-is. And if you determine it's been in there say more than 5 years, yeah I'd replace it, soon.
Hi Matt, The 12V battery impacts the entire electrical system. If the ECUs do not have a good source of power upon startup, that can cause many issues which have been discussed on PC many times. If you do not like the idea of being stuck somewhere due to a dead 12V battery you could replace it now. Or you could try charging the battery. As previously stated, 30 min is insufficient time. You need to leave the Prius READY overnight (16 hours or more) to have a reasonable chance of recharging the battery. Or you could hook up a battery charger for that length of time. Note the battery has a warning label which advises against using a charging current in excess of 4A.
