The 2007 Prius starts and runs fine with no problem. However, the 12v battery check with a multimeter shows 9.9v reading. Does this mean that the 12v Battery is coming to end of life and needs to be replaced?
9.9v is very low. I am surprised it will even run the pc's. The 12v is not what starts your car, it only runs the computers. I would not hesitate much longer in keeping the 12.v. H
Check the calibration of the multimeter. A new AA / C / D cell should read about 1.5 to 1.6 V. Alternatively, confirm that the reading on the Ohms range with the leads shorted is 1 Ohm or less. My guess is that the battery in the multimeter, not the car, is low.
9.9v was measured with car off. When turned on the engine starts right away and the charging reading goes to 14.+ v. Then soon turned off it goesdown to 12.48 and stays there for sometime. Later, after several hours the reading is around 10.5v. In spite of all this the car starts and runs fine with no symptoms of any weakness. When driving, the HV battery goes through full charge and discharges to low in relatively short intervals. Any comments?
Based on the above, I withdraw my theory about the multimeter being wrong and join the "change the 12V battery right away" faction. Richard
If you do replace it soon. Please take careful not of your fuel economy now and after replacement. If you can report the last few tanks and the next few tanks post replacement that would be helpful for us. Thanks!
Thanks for all the responses. Now the battery is replaced for safety and security.... One piece of info that will throw some light on the mystery: in March, the previous battery was jump started (from almost dead) after 4-months of nonoperation. It worked for all these months with no sumptoms of weakness. The only symptom is low voltage which I measurd recently in June. The only conclusion is that (1)the battery, when jump started, lost one of its cells and became a good 9v battery which shows 10.5v when fully charged, and (2)Prius seems to work well with a good 9v battery.
I am no expert, but from what I have read here on PC, 9V out of the 12 V battery is indeed very low. I am surprised to hear that the battery would actually give enough power to boot up all of the electronics in the car without some sort of error or problem ie: No ready mode, ICE wont start, etc. When looking at http://priuschat.com/threads/weird-stuff-happening-mpgs-dropping-test-the-battery.73400/, to me I find it hard to believe that the Prius would actually function normally. After reading this thread, it seems that even the Prius can be forgiving in certain instances such as yours. I understand the traction battery is what actually starts the ICE, but the computers and such is another story for 9V in my opinion. I would be interested to see what Patrick Wong and other's would say about 9V being enough to power the computers in the Prius. Thank you for your post's and your follow up, as this is very interesting to me. Ron (dorunron)
It's not as shocking (no pun intended) as it seems. Arvind's conclusion - that he has a "good" 9.9V battery, i.e., with all good cells except for one shorted - seems to make sense. If the battery were reading 9.9V because all the cells were weak, the car wouldn't start. But if the battery stays at 9.9V even with the normal starting load, it's just good design, not an anomaly. Still, the battery should be replaced. Richard
The OP was lucky to not experience a no-start condition or other driveability issue with the battery in such poor condition. Not all owners can count on being so lucky, given the many no-start stories posted here. Also, the battery voltage measurement under no-load conditions is not necessarily indicative of the battery's performance under load. For example suppose a battery measures 12.2V under no-load. If it has limited current capacity then when placed under load the battery voltage may sag down considerably, causing either no-start or the ECUs to behave unpredictably. In the OP's case if the battery maintained 9.9V or thereabouts under load and he was lucky then his Prius may have operated satisfactorily.
