Broken down on highway

"My battery test showed 12.5v.and 12.0 with ac on low."

1) If that is with the car running you have a problem. Should be 14.2 volts
2) Napa says battery is the correct battery for a Prius, but this is the first I've heard of it.

 

I suggest you check the battery voltage at the rear of the car with a DVM. It is possible the main battery cable from the rear to the front of the car is corroded through, "I know a long shot" but this cable is aluminium. I have come across 3 Prius so far of 2004/5 vintage with this problem. A jumper cable from the 12 volt battery to the jump point will prove if there is problem or not.

John (Britprius)

 

I suggest that you connect a digital multimeter from the positive dedicated jumpstart terminal in the main relay/fuse box next to the inverter, to body ground. Place the meter so that you can see it when you are in the driver's seat.

Look at the voltage when the car is IG-OFF. What is the reading?

Then try to start the car and see what happens to the voltage as you go through that process.

If you see a problem with the voltage, then connect the meter directly to the 12V battery and have a helper start the Prius while you watch the meter. See if the meter behaves any differently vs. when the meter is at the front of the car.

If yes, then look for a loose battery cable or as Britprius suggested, perhaps the battery cable has a fault.

 

I would double check the voltage in the "Ready" mode as well. Assuming that is significantly higher than what you are getting with the vehicle off, you need a battery.

 

Yes. After you jumpstart the car, the voltage should be ~13.8V which shows the DC/DC converter within the inverter is functioning.

This is not a bad idea but you may want to disconnect the negative 12V battery cable where it bolts to the body, and wrap the loose end in insulating tape. The point of doing that is to prevent the defective 12V battery from absorbing a substantial amount of power from the car's electrical system.

I suggest you fully-charge the new 12V battery before you install it. Other posters have recently reported their newly-purchased Optima battery was received in a partially-charged state. A new fully-charged AGM battery should measure ~12.9V.

 

12.7V is not bad. If you don't have a battery charger you could leave the car READY overnight (make sure there is sufficient fuel in the fuel tank) which will help to fully-charge the 12V battery.

 

The difference between 12.9V and 12.7V is approximately 25% of battery capacity. If you aren't concerned about that, then no worries.

 

It doesn't really matter, at least assuming that you're going to be driving the car a "normal" amount over the next week or so. Essentially in about 2 weeks time, the state of your battery would be virtually indistinguishable whether or not you had "topped off" before installing.

 

FWIW, a new fully charged AGM type battery that goes in a Prius will measure 13.1 volts. Optima Jim who post's here has said that. I also read 13.1 on a new battery that I bought. 12.7 vs 13.1 is really quite a bit when you consider that these batteries are usually considered discharged when they get down to just under 12.0 and dead at 10.5 volts.

I would have to agree with Patrick. If you want full longevity of your new battery, it is best if you fully charge before you install it. For some reason it seems that the smart chargers do a better job of completely charging the battery rather than the charging system on the Prius.

If you have a smart charger, you can hook it up via the jump post under the hood. Quite a few of us here on PC do that to keep our 12 volts fully charged.