12-volt Auxiliary Battery Going the Way of the Dinosaurs?

Hey Zed,
At this point, its too early to tell. I wish you had a scanner and could read the codes. The readings you posted aren't terrible. If I were in your shoes, I would head to the local Auto Parts Store and purchase a battery charger/maintainer, hook it up until the battery is fully charged, disconnect and reconnect the negative lead on the battery and see if your lights come back.

The fact that your car drove fine with no noticeable decline in performance is a good sign.

 

In a way, I'd say it doesn't matter from the standpoint that if the aux battery is 4 years old? Probably a real good idea to change it anyway. Even if it is premature to actual failure, that battery can't have too much more life left in it.

So I'd say, your suspicion is highly justified, and the only real way to know, is to replace the aux battery and see if it solves your problems.

If it doesn't? The money spent on the aux battery wasn't wasted, you were probably close to needing it one way or another.

 

Its unclear that the 12V battery is the problem.


What is the voltage across the 12V battery, when the Prius is READY?

 

I tend to agree that at this point, the 12v battery cannot be pinpointed as the problem. More testing is def necessary and yes, good call Patrick to measure the voltage when in READY.

 

This is somewhat low. It should be around 13.8V. Are you using a decent digital multimeter for your voltage measurements?

If you should find that the various warning lights come on again, do another voltage measurement when the Prius is READY. Maybe the DC/DC converter is starting to have problems.

Also check the inverter coolant reservoir for fluid turbulence. If you don't see that, then you know the coolant pump has failed which is causing the DC/DC converter to overheat.

 

Zedhomme, in your first post you said the voltage at the jump point was 12.5 volts, but the voltage at the battery was 12.2 volts.
Unless you were in ready mode with the battery being charged, "that I doubt because the voltage would be at leased 1.3 volts higher" you cannot have a higher voltage at the jump point than at the battery.
Basically the further from the battery the more the voltage will drop when not on charge from the DC to DC converter. When on charge from the converter the reverse will apply, the jump point will be at a higher voltage than the battery.

John (Britprius)

 

Would not hurt, might help.

 

Hi Zedhomme. The problem is that you appear to have run the 12V tests almost immediately after shutting the prius off. This will tend to give false (overly optimistic) readings. I note in particular that your "jump terminal" reading was higher than the actual "battery terminal" reading. As Britprius pointed out, this is not really possible (when measured simultaneously), so I suspect that the battery voltage was rapidly falling and you measured the jump terminal first.

To properly test your battery you simply need to do it after the car has been sitting overnight. Do it in the morning *before* the prius has been started (or even made ready) at all that day. I suspect you'll then get readings below 11.0 volts in diagnostic mode (acc mode) and even lower in "ignition on" mode (where you hit the power button a second time without the brake pedal). That for example would be a conclusive diagnostic that the 12V battery is gone.

BTW. Your 13.9V charging voltage is good. So I don't think you need to worry about anything other than the 12v battery itself at this stage. There are some specific conditions where the charging voltage goes down to around 13.4V also, so don't be too alarmed if you also see that happen (particularly during a longer drive).

 

If the 12 volt battery has a shorted cell this will put a heavy load on the charging system. The charging system is supplied by the HV battery, so this will be discharged more quickly in that situation.

John (Britprius)

 

It could be extra loading on the DC-DC converter or it could just be random and you're looking more for signs of trouble atm.

Don't worry about other things at this point, just test your 12V battery *properly*. The test is very simple and seems to be extremely reliable at picking dodgy batteries. It just needs to be done correctly, and that means testing the *resting* battery voltages and not those of a battery that is straight off the charger (or same thing, immediately after being driving).

 

Look at the warranty/maintenance book provided with your car.