Dead Egg

you need a new 12 volt battery

 

What he said.

 

Maybe something else going on

 

I read this more like a bad connection at the newly-replaced battery. Recheck terminal tightness.

Remember: pry open the clamps a little with the tip of a flat screwdriver. Put the clamp over the post, and tap the clamp down to a wider part of the post with the back of the screwdriver. Then tighten the clamp.

 

Might be some phantom load too, running down the batteries.

 

Pull off both battery cables, clean the posts and connectors, and put them back on.

Check the main ground (short cable from minus post to car body near taillight.)

With all doors closed and the battery exposed, wait 10 minutes and measure the battery voltage. Come back in a couple of hours and measure it again. It should not have fallen much (assuming the ambient temperature didn't fall a lot during this period).

If you have a DC clamp ammeter you can check for parasitic loads. Again, be sure all doors (including the hatch) are closed. Also wait 10 minutes or so after the car is turned off, because it does all sorts of odd tasks for a while and those use current.

 

While even "new" batteries sometimes fail...........
They often come almost totally discharged......or purpose.....and MUST be fully charged before using.

But your symptoms really sounds like a bad main cable connection.
The negative cable connection to the frame or body is often problematic.

 

You can check amp draw with a regular multimeter, less convenient, but maybe more accurately: disconnect the 12 volt neg cable, splice in the (turned off) multimeter in series, at the end of long jumper wires that run out of the hatch. Close the hatch gently on the wires. Wait half an hour, with fobs well away from the car. Then, set the multimeter first to amps (just in case there's a massive current flow), then milliamps.

What I've seen doing this, and I think it's normal, is milliamps around 20, with periodic spikes (every 5 seonds or so) to around 40. I'd speculate the spikes are due to the security icon blinking on the dash, but they didn't seem to really sync with it, so not sure.

It's good practice, but I've lost count of the batteries I've installed in friends/relations vehicles, stuck by the roadside situations with a battery straight from the store, that did just fine.

 

Why is it good for batteries to come discharged, I thought lead acid liked staying charged?

 

Well, yes, but unless it is a pretty hefty multimeter there could be current spikes which exceed the meter's rating. For instance, a lot of people use Harbor Freight Centech multimeters - because they used to give them out for free with a coupon, and even now they are just a few dollars. The highest range on those is 10A, and that is much less than the cranking current on a regular ICE vehicle. I know the Prius doesn't have a starter, but I don't know that it doesn't ever get above 10A. If it gets out of range it either blows a fuse or blows up the meter forever. I bet less than 1% of users of these meters have a replacement fuse. In fact, I have some of those meters (opens one up), yup, the fuse is soldered to the board. Not so convenient to replace.

Conversely, the clamp on DC ammeter will not be damaged by too high a current. Well, maybe if it was 10K amps or something like that, which makes too large a magnetic field, but not 100A.