New OEM 12V shows 11.5V after two months?

Please see edited post #19.

 

A jump pack is for starting the car with a dead battery, emergency use only, to get you going. It's not a charger.

Get something like a Clore CTEK 4.3 charger.

 

Yes it does seem the battery is charging. But that doesn't explain why the battery is not reading 13.8v or higher when it's ready.

You should fully charge the battery and go from there. That'll take a good overnight charge. Getting battery readings right after you drive is not an accurate reading

 

You can take the battery out and charge it. Your hatch won't open without the working battery so you'll need jumper cables (connected to the charged battery) to connect to the front jump points to open the hatch. Or climb back there and release the hatch manually.

 

Stop testing and jump starting from the battery in the hatch use the much safer jump point under the hood.
It’s in the black plastic box under the hood the positive jump lug is in that box under a protective plastic red cover. Toyota put that jump point there to keep you out of the hatch. Jumping starting that 12 volt battery in the trunk is how so many Prius get damaged with reverse polarity jump starting.

It does not matter what the voltage level reads at the battery posts it only matters what it reads at the front jump point where the main battery 12 volt cable is bolted too. By then it has gone through many connections and fuses and a long dc lead.

Your battery has become sulphated and seems to not hold a charge well, buy a battery charger that has reconditioning mode that will help this issue.
I bought the CTEK MUS 4.3 that Mendel recommends on amazon and it works very well. It has reconditioning mode which takes quite a while
To accomplish.

 

Yes.

If the battery is Toyota, it likely has only embossed plus and minus indicators on top of case. I'd suggest to color them with something like a silver Sharpie pen.

 

Don’t worry what the voltage is when you just shut the car off. If it is charging it will have a surface charge. Check it after it has sat for some time, like when you get off work or in the morning before you go. Those are more accurate readings.

 

Sulphated? The battery is only a couple of months old and it seem he uses it several times a week for work. It would be unusual for a battery to be sulfated under those conditions.

 

Or you could have taken the battery out of the car and took it to where you bought it so they could test it. And since it’s only a couple of months old if it were bad they would have given you a new one for free. Just sayin.

 

That's a 7 amp (max) charger. You may be ok, but there's a caution in second gen Owner's Manual, page 302:



The CTEK 4.3 is 4.3 amps, and the CTEK 3300 is 3.3 amps. 4.3 is close enough to get away with it, 3.3 is safer.

All that said, the 7 amp version may be ok, but overkill. More info on CTEK's:

All Products– smartercharger.com

 

Did he not supply you with the receipt for the battery? Typically you need it to claim a warranty. Everyone knows this and your mobile mechanic should have.

 

Good stuff. I have one of each, a 3300 and a 4.3. I use the latter more now, but the 3300 has been simple and bullet-proof. A lot cheaper too.

 

I have one of those chargers too but don't remember which model, they are high quality chargers.
Once you get your battery charged overnight take it somewhere and have it tested. A battery has to be fully charged to get a good test.
They are always willing to sell you a new battery. If it is bad you should contact the mobile mechanic and ask about warranty.
After the battery is eliminated you will have to go after the charging system or a parasitic draw test. The Prius doesn't seem to have very many parasitic draw problems.