2008 - 12V acceptable range

I assume that you were checking the voltage with the Service Menu screen. Correct?

If so, I would recommend checking the voltage at the 12V battery terminals and compare with the Service Menu reading. On my 2010 Prius, I found the Service Menu reading 0.7V less than my actual battery terminal reading.

A couple of others with the Gen II Prius reported that their Service Menu reading was pretty close to the actual battery terminal readings.

 

I believe the 2008 Prius has the "jumper terminal" in the engine compartment in the fuse box, similar to the 2010 Prius. You can measure the 12V battery voltage between the jumper terminal and the engine ground point.

I agree with the age of the technicians. I took mine in for the 5K service and all of the technicians that I saw seemed to be younger than my kids !

 

Hi Chuck,

Suggest that you use a digital multimeter, and measure voltage across the 12V battery after the car has been IG-OFF for several hours. 12.9V is 100% charged, 12.6V is 75% charged. You don't have to remove the spare tire but you do have to remove the folding hatch floor panel, the black plastic tray, and the hatch trim covering the battery in the right rear corner.

Or you can remove the lid on the main relay/fuse box, find the positive jumpstart terminal covered by a red plastic cover, and measure voltage from there to a strut nut.

A wide range of battery voltage can be expected depending upon the car's state: When READY, should be ~13.8V. When IG-ON, can drop down to 11V or lower depending upon battery condition. When ACC-ON, will drop down but not as much since the battery is under a light load.

 

You don't need to remove the spare tire, but you do need to remove the plastic cargo floor over the top of the spare tire and a plastic panel over the top of the battery in the right rear corner of the cargo area. It isn't difficult to do, no tools are required, just remove cargo mat, twist the two latches if necessary to lift the floor then lift it out. The plastic panel on the side just clips in, pull to remove.

 

Chuck,

With one press of the power button (no brake pedal) there is very little load on the 12v battery and the menu reading should be about 12.4v.

The second press (again no brake pedal) puts a small load on the 12v battery and the voltage will drop slightly but not below 12v. Adding the headlights will drop the voltage further, but my 12v battery (the original one in my 2004 at 165,000 miles) stays above 12v.

Pressing the Power button with the brake pressed puts the Prius into READY mode and turns on the constant voltage charging system (a DC-DC converter driven from the traction battery, which can supply 100 amps to any load) and my system goes to 13.8 volts on the menu.

If you are seeing 11.5v with only a light load after a long drive, you need to consider a new 12v battery. Many dealers (and some owners) have weakened the 12v battery buy letting them sit in a discharged state while on their lot. It may be difficult to get a dealer to pop for a new battery under the 36month/36kmile warranty so check out:

12 Volt (12v) Toyota Prius Auxilary Battery for 2004, 2005, 2006, 2007, 2008, 2009 and 2010 with installation kit

for a better battery (the Optima Yellow Top with an adapter kit to fit your Prius) than the original one.

JeffD

 

Get yourself a Scanguage....it's an invaluable tool!!!

 

10V can be OK for an old technology car, but Prius is demonstrably sensitive to low voltages. If your battery is less than 12.0V when unloaded it's definitely not well.

13.6V is the bottom of the charging range for a lead-acid battery. One might show that immediately after being taken off a charger, but that's not a proper measurement.

 

If you can't afford a Scanguage, get a cheap volt meter (on sale for $3 at Harbor Freight... - Harbor Freight Tools I saw one for $16 at Radio Shack; either way a good investment). Agree with the majority that testing under load (e.g. headlights on) is most important. Wife has the Prius today, so don't have a reading handy. Even if it is not covered, I would get a new battery if it reads 10.9 or lower under load. Half an hour with a dome light on and you are up the creek!