Monitoring the 12v battery

While the car is in READY, it should be 14V.. down to 13.5V (depending on headlights, etc)

Sadly you can't monitor the battery terminal voltage when the car is off, the lighter is on a relay.. You have to put it into ACC which powers the radio, lights and some ECUs. AGM are very sensitive to the load. With the headlights, radio, and rear defrost on it will be a much lower voltage.


AGM is more complicated and depends on temperature and is slower to respond to charging and discharging.. Some graphs:
http://www.bdbatteries.com/images/lifelinechargingraph.jpg
http://www.technomadia.com/uploads//2011/10/Lifeline-Discharge3.png
http://i1112.photobucket.com/albums/k496/echomancer/33ah_battery_1a_discharge.jpg


It should be above 12.2V after sitting overnight. Ideally more like 12.6V... The real test is the voltage when in ACC with everything on and see how low the voltage goes. It really only has to be good enough to power the ECUs and allow the HV battery to turn MG1 to start the ICE. It should probably stay above 11.8V with everything on... Some will say that is a bad battery, and maybe it will have problems in the winter. But it still is enough to start the engine.

When you shut off the car, from ON to ACC, you will notice the voltage drop slowly from say.. 13V .. then 12.9V.. then 12.8V..

 

(standard 12V answer.. does not apply to Prius)
Depending on if it really cold out, but a new battery should be at 12.6 to 12.8V. What can be useful is to check what voltage it drops to when you are trying to start the car. Since the car always runs everything on the alternator, it only matters for starting. Alternator voltage (ie. 12V when the car is running will vary with rpm around 13.5V up to over 14V)

http://www.digikey.com/Web%20Export/techzone/energy-harvesting/article-2011october-battery-fuel-guages-fig1.jpg

http://1.bp.blogspot.com/-0PNKH1Rf9T4/TyiVh-sIMoI/AAAAAAAAAeM/_J_a5UQc3ug/s1600/Battery+State+_+Voltage+for+lead+acid+batteries.png

http://www.aaroncake.net/forum/uploaded/Audioguru/20085220297_lead-acid%20battery%20charging.PNG

 

Just a thought, you could try a solar battery maintainer. I do not know if it works on the AGM battery ,but I wouldn't know why not..

 

Do a net search, can't remember where I got it but there is a secret key tap sequence on the touch display monitor that will show you the voltage and diagnostics on the system.

 

Here's a link to another post with youtube video's on how to use the Prius' self diagnostic system, scroll down to the bottom of page 2 and you'll see the videos I posted last night:

Typical lifetime of 12V battery... | Page 2 | PriusChat