2005 Prius - 12v Battery Issue

two things:

how often are you driving the car and what distances?

there is a draw on the battery when the car is off. if it is abnormally high, it can drain the battery prematurely

 

I suggest you test the new battery and see if it’s any good.

 

And test the charging voltage with the car in the READY mode too.

 

A couple things.

The MAINT REQ indicator light on a Gen 2 has ONLY 2 meanings and they are both related to oil changes. If you start the car and it blinks for several seconds and then stays off, it means you've reached 4500 miles since the last time it was reset. If it blinks for several seconds and then stays solid, it means you've hit 5000 miles. The recommended oil change interval for your Gen 2 is 5000 miles. When the oil is changed, the "timer" is reset manually using buttons on the dash. The proper button sequence is in the owners manual or can be found online.

I would not invest ANY money into those codes until you resolve the 12v battery situation, because it's likely they'll go away. Start with ensuring the 12v battery is fully charged. It's recommended to not charge this AGM style battery at more than a 4 amp rate. It will take ~11-12 hours to fully charge at this rate since they are typically ~46AH . Higher rates risk damaging the battery. I've tried it and the warning was accurate. The Prius is VERY poor at self-charging a weak/discharged battery, so "I put the charger on for a while and it started up" may be ok for that single time, but even after a 15 minute drive, it will likely still be too dead to start the car again if you shut it off. You would need to drive for hours to get it at enough charge to reliably start the car again. Keep in mind, every one always says the 12v battery is only needed to power up the computers, etc. Unfortunately, most people also forget that the 12v battery is usually going to initially get hammered by the brake actuator motor prior to being put into READY, and that is a significant electrical load, which will immediately kill a weak battery, preventing it from having a high enough voltage to close the main relays for READY.

After turning the car off (and make sure you actually turn it completely off, because I have seen cars left on (but not in READY mode) overnight, draining the 12v battery). This is most often done by new owners. The car does not have an alternator and uses an auxiliary circuit inside the inverter to charge the 12v battery. This aux circuit in the inverter keeps the 12v system in your car at about 14v, just like an old style alternator, but only when in READY mode. When not in READY mode, only the 12v battery is used, and it's a relatively small battery so it doesn't take long to drain it.

 

that is confusing.

idk if you have to leave the fob in the slot to keep it in, you might have to push the power button to power down whether the fob is in the slot or not.

 

that means it is charging correctly. the voltage with the car off looks okay, but should be higher for a new battery. my 4 year old battery measures 12.78v in the morning after the car has been off about 16 hours

 

give eric becky a call in madison @ericbecky

 

i don't think the cca would affect the voltage, but i've never had a meter like that. charge it up overnight and read it again after 4 hours or more.

not many scanners work well with prius, idk if those codes mean anything or not. you might need a coolant control valve, but it wouldn't affect the battery

 

Two notes:
That second reading while "ready" is the inverter charging voltage......which is good.....but
it has nothing to do with the battery itself.

That first reading of 12.53 with no load is borderline.
IF it is an AGM battery, the no-load reading should be at least 12.7 or 12.8.
This probably indicates that it wasn't fully charged when new.......and/or that
you don't drive much.

I suggest that it needs to be fully charged, allowed to rest for a few hours
and then tested again.

Where did you get the new battery ??

 

napa, post #1