Why my 2007 Prius 12V battery won't charge

Did you try running the 12V check diagnostic screen on the display with the new battery? It will tell you what the battery voltage on the car, while in accessory mode or idling.

Sometimes batteries are faulty off the rack.

 

Not that easy with a hybrid.

FIRST.....you need to check the voltage at the battery while the car is "running" (in ready mode).
If that comes up at 13.5 V or more, your charging system is probably OK.

THEN......you need to check the voltage on the new battery with the car OFF.
If it is less than 12.6, then your new battery might just need to be charged up good once with a manual charger.
Many new batteries come from the factory needing an initial charge.

It is also possible that your new battery is defective too.

 

25 miles can be done in less than a half hour. 12v Prius batteries take 6-8 hours to fully charge. Also 12v should be tested after a full night's rest.

 

Of course it SHOULD HAVE......but that doesn't mean that it IS.
Use your voltmeter to test it.

 

NOT a smart move. 12.6 is fine.
Guessing and taking a shotgun approach to fixing things often turns out BAD.

 

I'm curious what modifications you had to do to install that battery. Certainly it didn't have the correct posts? Can you post a photo of the top of your battery?

Measure the battery voltage at the center of the actual battery terminals posts. If the measurement changes to about 14v when the car is in ready mode, then the car is doing it's job trying to charge the battery. You could easily just have some type of parasitic drain on the battery that is causing your problem overnight.

 

If dc dc converter is not working, you won't see 14v when the car is on

 

What JC said. If you go back to the "vehicle signal check" screen on the MFD when the car is READY, what voltage is shown? (I think 13.8-ish is what I'd expect in a Gen 2.)

 

Was that 12.3 read while the READY light was showing on the dash?

 

There is an ACC, and an ON, and a READY. You can use the MFD signal check screen in any of the three modes. I am not surprised by a low voltage in ACC (to be honest, 12.3 is a very good voltage in ACC when measured at the MFD, because the MFD generally reads a good fraction of a volt low relative to the battery itself). I am not surprised by a further 0.3 volt drop going from ACC to ON.

However, from ON to READY, it should rise to about 13.8. Do you have a measurement that you are certain was taken when the READY light on the dash was illuminated?

 

If the battery was being charged as you took that measurement, 13.2V is pretty low. The charger should reach a voltage of around 14.4V when charging, until the battery is fully charged. At that point the voltage might drop down to 13.2V to maintain battery charge without overcharging.

 

What exactly is it that "showed you 14 V" ?

If you are not measuring that with an external voltmeter applied directly to the battery posts.......you can't trust that the voltage is actually getting TO the battery. Loose or bad connections might be involved here.

In addition to not charging correctly, something might be draining it overnight.

And there is always an outside chance that your new battery is bad too.

 

It's ok, I suggested checking the MFD voltage in READY condition. It does contribute to diagnosis.

If the MFD had continued to show 12-ish during READY, that would have pointed to a DC/DC converter problem (or a wiring issue early in the tree, upstream of the MFD). Now we know the converter is converting and a reasonable voltage is getting at least as far as the MFD. We didn't know that before.

It would be a reasonable next step to carry a voltmeter to the battery posts in back and measure there under the same condition. If the converter is working and the MFD is showing 14 and the battery posts are still showing 12-ish at the same time, then we know there's a problem on the way to the battery, past the sections we know to be working.

That'd be more than we'd know if the OP hadn't checked at the MFD.