Help! 2006 Prius won't start, 12 volt battery and fuses good

"I measured the voltage of the jump terminal inside the fuse box and it was reading at 3.9 to 4 volts.".

Can't speak for Gen II, but on my Gen III 2010 I read Battery Voltage >12VDC from the jump terminal under hood to ground. You need to get the engine light fault code.

 

I assume that you are measuring voltage from the jump terminal to unpainted metal, such as one of the nuts on the strut tower. If this assumption is not correct, please do the voltage measurement again.

If you still measure such low voltage at the jump terminal, then check the tightness of the battery cables, especially the negative cable where it bolts to the body. Make sure that there is no corrosion or physical damage to the cabling.

 

I always hesitate to tell Prius folks how to T/S because most don't there way around a wrench. But...given the info you gave that I quoted...I believe Patrick is pointing you in the right direction. If the weak link is "after" the battery, a jump won't fix anything.

 

When I measure the voltage at the jump points under the hood of our 2007, I will often get voltages much less than 12V, depending on how forcefully I stab the grounded bolt with my probe. After 10 years, that exposed metal seems to get a pretty good layer of crud on it. Are you getting a consistent 3.9 to 4V when you measure there, or does it jump around a lot?

 

It sounds like that battery may be a little weak. I have a jump kit that instructs me to leave it connected to the battery for 3 minutes before trying to start the car, supposedly to allow time to transfer the charge to the battery. It sounds like your device may actually be drawing charge from the battery. Does it have lights showing that everything is hooked up correctly? Also, to properly measure the voltage on a battery, it is recommended that it be given a time interval with no load to allow what I believe is called the "surface charge" to dissipate (I believe the electrons need time to migrate to the negative terminal, from a layman's point of view). It's true voltage may be something greater than 10.88V.

 

This indicates that there is an electrical load active in the car that is pulling the battery voltage down to 4.5.

That suggests there are two problems.

A healthy, fully-charged auto battery will not be pulled down to 4.5 volts by any reasonable load (in a Prius, any load heavy enough to do that would probably also be making smoke, which would make it easy to find, at least).

So it sounds like that battery has developed a bit of excessive internal resistance.

If you know somebody with a clamp-on DC ammeter (not as common or cheap as the AC kind), it would be interesting to clamp over that cable while it is pulling the battery down to 4.5, and see what size current is doing that.

One potentially reckless thing to try would just be to swap in a well-charged, known-good 12 volt battery.

One of two things may happen.

1. The load that was pulling the old battery down turns out to be some normal load in the car (the brake pump, for example), and the good battery just runs it successfully until it shuts off, and all is good.

2. The load that was pulling the old battery down turns out to be an electrical fault somewhere, that the old battery could not drive hard enough to make smoke, but the good battery can, which would make it easy to find, at least.

Now you know why I said "potentially reckless."

If it were me, I would try to get that amp reading with the old battery, first.

-Chap

 

You likely have a bad main battery connection somewhere.

This most often occurs at the body/frame end of the negative cable but can occur right at the battery even if those connections look clean and tight.

First, measure on the cable end connectors at the battery posts (not the actual battery posts).
If that is OK, then attach your negative meter lead to the negative battery post and measure to the frame.

 

If the OP really meant the meter probes were directly on the battery terminals here (which seems to be the most literal reading of the sentence), then I don't think I can join in attributing excess resistance to any other connections in the car, but only to the condition of the battery itself.

Or, if the probes were touching the cable clamps attached to the battery terminals, there could be an issue of bad contact between the clamps and the terminals, but not elsewhere.

(I actually saw that once in a friend's car; it seemed fine, everything powered up, etc., until you tried to start it; then everything went dark, and stayed dark for five or ten minutes, then lit back up again. It was like there was some crazy self-reset circuit breaker in there for the whole car. It was really just one battery clamp making such poor contact that the starter current would heat it up and expand it, not enough to see, just enough to break the circuit, until it cooled off and got tighter again.)

But in this case, assuming the OP really meant the meter probes were on the battery terminals themselves and not the clamps, even the clamp connections can't be implicated.

-Chap

 

usually a bad connection or ground. but in your case, i think chap is right. either the battery is defective, or you have a major load on the system, probably caused by a short somewhere.
how many miles on her?

 

Yeah, if voltage at the battery was 12.93, and at the jump point around 4, wouldn't that point to bad connections, not an ailing battery? You could check the battery condition fairly conclusively with an electronic load tester, most battery retailers have them now I think. A DIY version is Solar BA5.

 

At 11.5 volts it really IS near to 0% charged, and in that case a lower reading well away from the battery might not be unusual.
I'm wondering why YOU had a battery measurement that was so much higher ??

 

And you bought that story ??
You need a new battery.

 

Assessing the battery by seeing how it does on the charger, is not that professional. Most retailers, and dealerships for usre have a pro-quality electronic load tester for checking a battery's health. Basically the big brother of a DIY level Solar BA5. Google that for more info.