12 volt battery does not seem to be charging correctly.

The system works completely differently from what we're used to with a "normal" belt alternator/voltage regulator type system.

Did the previous owner replace it with the proper battery? I assume so, if it was warranty.

I haven't been able to understand it - my battery is just over 5yrs old. From what I gather, it doesn't insist on getting to max charge and keeping it there - as it doesn't do much - well, compared to a car with a starter motor.

Basically - it's got a brain of its own - and it's something us mere mortals maybe won't ever understand: These diagrams are from a 2016 TOYOTA document - totally different from anything I've seen before, and I won't try to second guess what's going on.

 

The "electric motors" only charge the high voltage battery. Then the high voltage battery charges the 12V battery, but only when the car is in READY mode. If you want to monitor the 12V "charging" voltage in real time, put one of these in your front 12V outlet. The outlet is OFF until you press the Power button. Pressing Power without the Brake pedal once puts you in ACC mode, and you'll read around 12V. Pressing Power again without the Brake pedal puts you in ON mode and the voltage reading will drop slightly as more systems are powered. Pressing the Brake pedal and Power again will put the car in READY mode, and the voltage should now indicate over 14V, which signals the high voltage battery is charging the 12V battery. If you keep the device in the outlet while driving, you'll see the reading drop below 13V periodically as the smart charging circuit modulates the charging voltage. It will return above 14V when the circuit detects the 12V needs more charge.

 

Instead of charge duration, consider getting it tested with an electronic load tester. Dealerships have them, but likely won’t test for free. Automotive retailers selling batteries often have the testers, and likely WILL test for free. You can also DIY test, with something like Solar BA9 (retails around $60~80 in the States?).

 

First, you need to test the "float" voltage of your tender and the charging voltage while the car is running.
If both of those looks good.......13.2 to 13.8......and too high is often just as bad as too low, then you need to get a thorough test on the battery. It might be going bad. More likely if you don't recognize the brand name.

 

Good advice.
BUT.....one should not trust a single test.
It needs to be repeated 3 times in a row with about a minute in between each test.
That is if it tests "good" the first time.

 

It still could have failed.

 

What @alanclarkeau said. The Gen 4 doesn't try to maintain a 100% charge on the aux battery. Mine is the original in a 2017. I don't obsess over it. But on the times I've checked it, it was never at 100%. It's never been a problem. I rarely drive more than an hour at a stretch; usually about 20 minutes or so.

I do have a little readout plugged into the 12V outlet. It shows 14.1 volts right after I start the car. As the battery's SOC increases, the voltage on the display drops, indicating that the DC/DC converter is reducing its charge rate.

 

I had to replace mine at just a little over three years old. I have a 2016 4 model.

 

Same here. Had mine replaced a few months ago at around 3 to 3.5 years old. It got completely depleted twice over the past year due to lockdowns and not being used for a couple of weeks at a time and then only used for a few minutes, again and again. After a cold spell (-16C / 3F), at the start of the year, it would deplete in only a couple of days to the point of being unable to unlock the doors or go into Ready mode.

To my surprise, it was replaced free, under warranty, after a compulsory overnight discharge test.

In my experience, every lead acid battery I've ever owned has shown significant performance degradation after around 3 years. By that I mean that I notice it... Dimmer than normal interior light, dimming lights while starting, slow starter motor, etc. I've always had to replace the battery, in every car, at some point between 3 and 4 years, with the exception of an AGM style battery that lasted an extra year.

The Prius 12V battery is tiny, because it doesn't need to be able to turn a starter motor. However, that does mean that things like the alarm will drain it more quickly when compared to other vehicles. I suspect lockdown hurt a lot of Prius batteries, more than other cars, because they were more likely to run low during that period. Discharging a lead acid battery too low takes months of its life every time it happens.

SM-T813 ?

 

It very much depends on the car I think. I had a Mitsubishi Starwagon (Delica) with the battery under the driver's seat near the engine - and it was always hot there - and I didn't get much over 2 yrs. In contrast, the VW Aircooled Microbus - the intake air for the cooling and induction flowed over the battery - and they lasted a long time - I had well over 6 years from one. And the VOLVO 240 series, the battery had a big intake vent just in front of the battery - I had mine for 4 years, and the battery wasn't new when I got it.

 

Thanks alanclarkeau! Is that from New Car Features, or could you give a reference to the document?

Do you know of anywhere that Toyota details how (ie the algorithms by which) specific 'battery condition' and 'electric load' parameters are used to calculate the voltage requested from the DC-DC converter?

 

I have an '05 with 230k miles. I've put 5 yellow tops in her.

Via Prius Chat for Android

 

It's only headed by "Features" - but the name of the file is NCF... I'm not sure where you'd get a hold of it.

I "acquired" my copy - but it's ¼gb in size.