Unknown, only sometimes battery drain

should be very close, maybe a tenth or two lower due to cable loss.

have you considered an inexpensive battery minder?

 

In theory yes, in practice usually, but mine for some reason was reading a constant small offset for a while - then it stopped. Never figured out why.

If the 12V battery got down to 5V it is almost certainly trashed. The capacity is supposed to be 46Ah. In this post

IV discharge curves for a working Prius 12V battery? | PriusChat

I described a way to determine the battery's actual capacity using only the car itself and an AGM SOC table.

Typical battery testers won't tell you how much capacity the battery actually has. Instead they put a load on the battery for a not very long period and verify that the voltage never drops below 10V (or something like that, it is temperature dependent.) The battery in our 2007 only has about 12.5 Ah capacity but it passed easily on two battery testers.

Leaving a Prius door open, if literally everything else is off, uses about 500 mA (0.5 A). So leaving it open for an hour is 0.5 Ah. For a healthy battery that is a negligible amount of capacity used.

As has been discussed here in other threads, the problem you are running into is that since you are not driving the vehicle much the 12V battery gets discharged to some extent, and since it is not a deep discharge battery, that damages it. So it loses capacity, loses voltage even faster for the same load, which damages it even more. And pretty soon the 12V is toast. I was at a dealer's service department recently and they said that during the pandemic, when nobody was driving, they saw a very high rate of 12V battery failures in Prius. The solution to this is to keep the car on a smart charger when it isn't being driven, so that it never gets a chance to discharge much. Not terribly convenient if you don't keep the car in a garage though. Others have replaced their AGM batteries with deep discharge scooter batteries. The probably solves the failing 12V battery, but at the cost of some risk - those batteries are not really supposed to be used in a small closed space, so there is a slight possibility of gases from the battery causing injury. There is an LFP battery available commercially which should last longer than the AGM:

https://www.ohmmu.com/product-page/12v-lithium-battery-for-toyota-prius

but it is very pricey.

 

Those trips are all pretty short, the 5 mile ones, or the one with many stops over 15 miles, may not be enough to end up with a higher SOC than when you started. That said, what you described isn't enough to kill a new good battery. It could be that the battery they sold you was defective. I have had that happen before with a Bosch from Pep Boys. It worked for a week and then just dropped dead. (Different type of car.)

The voltage on the battery immediately after driving is always high like that. It falls over a few hours and stabilizes at the voltage corresponding to the actual SOC. The rest is "surface charge". On cars where you can turn on the headlights without turning on anything else, leaving the headlights on for a short time will remove the surface charge. On cars that need to be turned on to some extent that isn't as clear a measurement since it applies other loads. The ACC mode draws 2.3A plus or minus a half amp or so. So if you measure the voltage using the MFD in that mode it will be lower than if the car was just off. IG-ON is even worse, as that draws much more current. See the last paragraph of:

What causes the voltage drop between the 12V and the jump post? | PriusChat

The easiest way to get an accurate no load voltage (and so SOC) measurement is at the jump point in the fuse box after the car has sat long enough for the surface charge to dissipate.

 

Also, something to keep in mind for a garage kept Gen 2 is the SKS system. For simplicity.....The car is always trying to "listen" for the Key Fob. If you hang your fob within range of the car while it's in the garage, the car will be constantly "talking" with the fob, draining the battery much more quickly than if you keep the fob out of range of the car.

You can disable the SKS system using the button under the steering wheel labelled "KEY". This will allow a 12v battery to last significantly longer, as it stops the "listening" and "talking" from occuring.

 

If the "new" battery was discharged to 5v in a short time, there is a serious off state current drain.

The first thing to do is a parasitic draw test. Measure ma in series with a meter. It needs ten or fifteen minutes to get down to 15-25 ma. Most likely its much higher. Then the real skill is needed.

 

You have a brake booster system failing. Much more serious and expensive; but still a common problem. By itself it won't cause the battery problem unless you already have a second low capacity battery.

Capacity in a battery is much more important than voltage. People talk about voltage because it's easy. A nearly free harbor freight voltmeter or even the mfd gives you the voltage. The problem is the battery will get recharged quickly within minutes by the high power hybrid system while it's running. You then have a "surface charge". Meaning the voltage looks good, but there is very little capacity.

Instead of holding 40 amp hours worth of power, your battery might have 4 amp hours. That low power capacity means it will drain off quickly. The fact that you find it low a day or two later is all you need to know using a simple voltage check.

Do the parasitic draw test. A low cost meter can do the test. It just takes a little knowledge. The real skill is finding the draw after you determine it exists. This is an area of few experts.

There are slow techniques that usually work. Removing fuses one by one while watching the draw is one way, using a sensitive millivolt test across operating fuses is another way. The multiple fusible link module is hard wired limiting fuse pull results. Then studying the drawings allows further isolation since many loads are often on a given protection device. A truly intermittent parasitic draw needs a logging device that provides graphical trends overight.

The engineer on Pine Hollow Diagnostics uses a Wi-Fi enabled parasitic draw monitor that logs on a phone app. He also demonstrates removal of fuses and millivolts techniques.

The first thing for the third time is to determine what level of parasitic draw you have. All modern vehicles have some draw, it's just a question of how much.

 

They have not been free for a long time, but yes any voltmeter will do.

The MFD never gives the open circuit 12V measurement. The lowest current mode still pulls 2.3A plus or minus about half an amp out of the battery, which is enough of a load (~.05C) to drop the voltage on the battery a little. The value the MFD gives in that state is the same as you would measure on the battery directly in that state with a voltmeter, but it is lower than the open circuit voltage.