Parasitic drain - is 55 milliamps ok?

what has the charge started at in the evening, and what is it in the morning? have you checked it with the neg cable disconnected?

 

55 milliamperes is pretty high; on our 3rd gen I saw 17~20, with sporadic spikes to around 40 (which might be due to the security warning light in dash??). Have you got anything extra plugged in?

Not sure what you mean by "volts, pressing power once". If I was wanting to check the battery volts, I'd pop the hood the night before (not absolutely necessary, but gives you a completely comatose car for the reading), then in the morning measure the voltage with a mutlimeter, at the jump-point in the under-the-hood fuse box: meter positive lead on jump-point, and negative lead touching solid engine or body metal.

12.00 volts measured by that method, I'd consider the battery gone. 12.7~12.8 volts would be a 100% battery; down around 12.4~12.5 is getting iffy.

 

Assuming that you are testing draw at the 12V, have you used a tool to "close" the tailgate latch?

Do so and close all doors. Let the car sit 20 minutes then check the reading. 50mA is "OK" for a luxury car with a big battery. A Gen2 is usually 15-20mA.

If there's any aftermarket stuff (stereo or remote start-alarm) then all bets are off.

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If you pop the hood in the morning, just the action of getting in the car to do that uses some juice. Less if you go in through passenger door, reach across. This’ll drop the battery voltage, a minor amount though.

but if you pop the hood the night before (assuming you have secure enough parking situation) , by morning voltage will have stabilized. At least that’s @bisco’s theory.

yeah using the dash display to read voltage, it’s dropped, by turning the car on; you don’t want that.

 

Those rows are, top to bottom: ACC, IG-ON, and READY. There is a current draw from the battery in the first two cases (assuming all the doors are closed, that is ~2.3A for ACC, anywhere from 9A to well over 20A in IG-ON, depending on what is running,). That current draw will drop the voltage of the battery some, and it will recover when the load is removed. In READY everything you see is just the inverter output voltage.

Your battery has quite a low SOC but the voltage drop from ACC to READY is much less than the one on my screwed up 12V which has only about 12.5 Ah of capacity instead of the rated 46Ah. I believe some early Prius gen II years might also have had a 36Ah battery, or thereabouts. So my guess is that your battery is low, but its capacity and internal resistance may still be pretty good.

In any case, I suggest the OP charge the battery at not more than 4A until it is at 100% SOC. Wait overnight, and measure the voltage under the hood as Mendel Leisk suggested. Then again using ACC. Do that for a few days and see if it will hold charge.

To see roughly what the battery capacity is, charge the battery to 100% SOC and let it sit overnight. Measure the voltage under the hood with the car off. Then go to IG-ON, all doors closed, all lights off, AC, defroster, and radio off, cabin blower set to 3rd highest position. Wait 15 minutes. If your car is like mine that will pull very close to 5Ah out of the battery (20A load for 1/4 of an hour). Turn the car off, pop the hood, exit the car closing all the doors, and wait 20 minutes. Measure the voltage again. Look up the two voltages on an AGM SOC chart and subtract. Let's say that you see .9 - .6 to give .3 difference, then the capacity of the battery is 5Ah/0.3 = 15Ah. On a perfectly healthy battery it should only drop around 0.11 SOC.

I don't recommend taking the Prius battery to an auto supplier for a conventional load test. Apparently these batteries can have terribly low capacity and still pass those tests. Mine did. I estimate it would have to lose about another 2/3 to 3/4 of its remaining capacity to fail, and I don't really want to be driving around with a 12V that weak.

 

A lot of battery retailers (and the the dealership, though they'll charge for the test) have electronic load testers. You can pick up a DIY level tester of that ilk for $50~60 USD. Solar BA9 is the current itteration of the the BA5 I'm using, works well. You enter your battery type, CCA spec, and run the test. It tells you the as-tested CCA, and gives a verdict, either pass, pass-but-recharge or fail.

 

Agree with everyone above, you need a full charge, then see what it has dropped to overnight.
But if 55 is way too high, you’re gonna have to chase the fuses regardless

 

Get the smartkey more than 10 feet away- or turn the system off. If the system is turned on and the key is in proximity , the car will be "awake" to some extent.

Might have to "close" the door latches if you want to check draw with the door open.

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You charged it beforehand too? Yeah that's low. Not crazy low, but I'd start shopping, and have a jump pack in the car.

FYI:

I've also got an Optima Yellow Top, installed September of 2015. Within a year or two of installing both my wife and I were retired, and our use of the car (2010 Prius) dropped off. Then with the advent of COVID, doubly so. Currently we're lucky to do 3K kms yearly, driving as little as once a week.

In that first year or two, we would be going for drives, "just to keep the battery healthy", and then I started thinking that's a little absurd. So I installed a quick-connect harness that came with my CTEK 4.3 charger, installed the charger on the wall adjacent to the car's engine bay, and just started plugging it in, any time the car was sitting idle. So pretty much round the clock.

 

Next time I probably won’t go with Optima Yellow Top; I’ve heard it’s higher CCA comes at expense of Amp Hours.

 

If the battery was fully charged the night before then that is terrible, it implies a voltage drop from 100% SOC to 12.38 V which is about 75% SOC.

https://learnmetrics.com/agm-battery-voltage-charts-12v-24v-deep-cycle-battery-voltage/

With just the 55 mA load, for 10(?) hours, which is just just 0.55A. It is a 46Ah battery, so it should have taken about 11Ah to drop that much.

On the other hand, if you did not charge the battery the day before it could be fine. 75% SOC might just reflect insufficient driving time. The battery in this car never charges at more than 4A, so it takes a lot more driving to fully charge than does a typical car battery. Unfortunately, as has been explained elsewhere on this forum, by I don't recall who, this isn't a deep discharge battery, so all the time it spends significantly below a full SOC damages it, slowly but surely. This is one reason some people are using deep discharge sealed batteries in their Gen 2's. Despite the potential safety hazard from having that sort of battery in the cabin.

 

For batteries generally, and for a given physical size, higher CCA tends to come at the expense of deep-discharge durability. I thought, at least as of a few years ago, Optima offered a choice of high CCA or deep-cycle capability---the latter being more appropriate for Prius usage.

 

i wouldn't buy a battery until you find the problem, might be a waste of money

 

I use Foxsur bat chargers. If you're interested I can give you the model numbers I like. The most important thing I've found is to charge the bat when it's disconnected. Of course this requires a secure place to park because you have to wedge the back hatch open or access the car with the key and then crawl back and work on the bat.