Extreme Voltage drop in starting battery -2006 Prius

How old is your battery? If it's over 5 years old, it may no longer hold a charge. Time to replace it

 

welcome! could it be that when you put a load on a damaged battery, you get the extreme voltage drop? or do you think something in the car is doing it?

 

Please give us numbers and provide details on ANY non-factory installed items.

EDIT: try checking voltage at the jump connection points under the hood with all doors closed and the car completely off.

 

not sure what a middle connector would be...

 

you are clearly doing it wrong. there should be no middle connector to 12v battery. there are only 2: + and -.

 

Oh boy I hope your not swapping the battery terminals....that will toast a Prius.

On the 12 VOLT BATTERY:
The positive + side of the battery under the red plastic cover is 2 wires. A thick black one and a smaller thin black wire. Both terminate into white plastic plugs that just click in and out of the assy on top of the positive post of the battery. Then the red plastic cover goes over that whole positive post assy. The thick black one goes directly into the main fuse box under the hood on the right in the black plastic box. Then inside that box that wire is bolted under the main positive + jump point stud in that box. The front convenience jump points. The thick black wire is the main 12 + to that fuse box. The smaller black wire is 12 volts to the metal box right next to the 12 volt battery. That is a bank of capacitors used as 12 volt back up power supply for the brakes.

The negative side of the battery has a short jumper cable that bolts right to the back wall of the trunk. That wire is only about a foot long.

There's really big + and - signs on the battery itself to identify the posts.

If your not nuts and have the battery hooked up correctly and its killing the battery most likely the Inverter is shorted. Very surprised its not blowing the fuses.
There's 2 in that positive assy under the red plastic cover. A 100 amp and a 5 amp.

What got you to this point anyway? Is this a salvage title or auction buy?

 

How about posting a photo - I have no idea what you are referring to.

 

I think he's referring to the harness. That harness has 3 plugs. The "middle" plug (big white one) is the main connection.

 

Are you putting in a NEW battery or just a different battery that's not new?

 

Check the hybrid battery safety plug. I experienced exactly what you describe when the interlock mechanism wasn't engaged as in step 3:



The fix was a different safety plug. Oddly enough the problem safety plug worked without issue on a different pack. I didn't troubleshoot any further.

 

OEM = Toyota? If you got a yellow top Optima, there have been reports of issues with them. Personally, I don't buy the their hype.

11.0V isn't that alarming as long as it holds that voltage. There's a pretty large demand right at power-up.

12.5V fully charged isn't great. 24 hours after fully charging a battery, it should be at 12.65V or higher. 12.5V is about 80-85% SoC, It's more likely a weak cell with sagging voltage that will drop out under load. If it's been sitting awhile since charging, it's probably fine. If you charged it recently (within the last few days), it's highly suspect.