Battery drained w/ hatchback open

If your battery is from 2008, it died from natural causes

 

If you can charge it up to about 12.9 volts and it can hold a charge, you can try and continue to use it. If it can't hold a charge, you would then need to replace it.

We recommend replacing batteries when they are 5 years old, you are at 7 -8 years now. So it's a good time to do it if you want to avoid future problems.

 

Yes a multimeter will work. You can use the jump starting post or directly at the battery, both will work.

I think the Toyota battery is better than Optima, it'll cost a little more but it's well worth it. Yours lasted 7-8 years!

 

I know with our 3'rd gen, when hatch is open the "footwell" lights come on, small lights under the dash illuminating the floor. So even if the hatch area light is switched off there's some drain. Not sure if there's sim on 2'nd gen.

 

Maybe the "door ajar" light did that old battery in?

My original 12v battery happened to be a flooded lead-acid type, but I don't believe that all are. If it is the flooded type (filled with free liquid), chances are it's low on water. I was able to keep mine going a little longer by (with the battery out of the car) removing the clear, plastic access panel under the "maintenance free" sticker on the top, removing the little rubber cell caps and adding distilled water until the level just covers the plate in each cell. If yours is not a flooded type, do not add water. At this age though, you'll probably just want to replace it.

I find a better measure of the battery than the open circuit voltage is how well the accessories work (power windows, interior lights) when the car is not in "ready" mode. A weak battery can still supply a reasonable, open circuit voltage but fall flat when experiencing load.

I'm happy with the Optima YellowTop replacement, and I got to avoid the dealership.

 

I've found a Solar BA5 tester good for check battery health. I'd sum it up as a smart load tester, let's you know the more than just the current voltage, will check Cold Cranking Amps, or Cranking Amps. You need to connect up directly to the battery, and set a few parameters re battery type.

 

You can also use the MFD in Maintenance Mode to check the voltage,

 

Since you have a multimeter and you know how to disconnect the 12V battery, disconnect the negative cable at the body and put the meter in series between the cable and body ground. Set the meter to read DC current.

You will probably find that the Prius will draw 1.5A or so even with the interior cabin lights off. This is because, with the hatch open, some of the ECUs are not able to sleep.

When the Prius is closed and locked, the quiescent current draw will be more like 0.02A after the electronics settle down.

Suppose you left the car for 14 hours with the hatch open. 14 hours * 1.5A = 21Ah. The Prius 12V battery when new and fully-charged has a capacity of ~40Ah. Your battery, being 7-8 years old, probably has a capacity of half that amount or less. So there is no big surprise that the battery was totally dead when you came back to the car this morning.

 

I don't know about the 2nd generation, but my 3rd generation draws about 0.7A from the battery with the hatch or any other door open, with all interior lights off, including the door-bottom lights (which I disconnected).