Charging the 12v battery

1 and 2 are close but not correct. The wheel does not have to turn nor the engine needs to run. The car just needs to be in READY. 3 is incorrect. A typical Toyota dealer answer. LoL

4 is needed if the 12v battery is weak or old and if the car is not driven (or just on READY mode) often or long enough.

 

#1 is closest to correct. The car is capable of charging the battery whenever it is in "Ready" mode. It doesn't matter what else is going on.

...as to whether that can fully charge the battery vs merely deliver a useful maintenance charge? I haven't studied the system well enough to offer any advice there.

Battery maintainers are great when you've got considerable free time and your car is going to sit a lot, but it's extra junk to manage for most drivers.

 

Being in READY is all it takes, but one hour is far too short to fully charge a depleted battery. In Gen 1 and Gen 2, it would take more like 15 or 20. Gen 3 (and c) are better than that, but I would still allow several hours if the battery was deeply discharged.

 

They are all wrong, at least as you describe the options. To keep it simple based on my 15 years of gen2 and gen3 experience, just drive the car several times a week. Keep a reasonably new 12v battery in there, maybe six years or newer.

If you leave the car for two weeks or more, consider a $35 Battery Tender Jr to keep the voltage up. Even then, I have left my hybrids at an airport for three weeks at a time with no issues.

Some people have experienced overnight 12v discharges due to lights left on or hatches not completely closed. The agm 12v batteries used in a Prius can take this once or twice a year, particularly if you notice it in a day or two because you are using the car frequently.

Having a $100 lithium jump starter for the above situation is reasonable and is easy to carry in the car (under a seat, not in the hatch that may not open). It can prevent a potentially damaging car to car jump start. Even this is optional especially if you have road service coverage with your car insurance.

Often owners forget about their 12v battery for seven or eight years and are "surprised" when it fails without warning in the grocery store parking lot. Since the small battery does not crank the engine, there is no old school slow cranking warning. When you proactively replace the battery you can shop for a quality Toyota replacement while saving a few dollars.

Finally if your 12v battery is discharging quickly with no obvious reason like lights left on, have the battery load checked. If that free service seems to pass, then have a vehicle parasitic draw test run to ensure your continuous off state current is 25ma or less.

 

5) none of the above

 

With less than 4K kms yearly, #4 works for us.

No kidding about the usage: last fill up was end of January, and gas gauge hasn’t made it to half yet.

 

Hmmm. That's not too much more than I WALK every year (about 3k km). Gas up once a week in the Gen2.

Posted via the PriusChat mobile app.

 

Amazing! And you still have chronic egr clogging issues! 30k miles a year is my average.

 

No issues lol. Just my crusade.

 

That's right till you got to that one hour part. It takes a lot more than a hour of driving to bring a dead battery back to normal. But it might charge it enough for you to start it again if you don't let it sit too long.

Now that's funny right there! I wonder where he got such a screwy idea.

Almost as dumb as #2. The things dealers say! [Edit to add: No, I think it's even dumber than #2.]

Good idea if you don't drive more than once a week and/or your trips are very short. I have almost never used the one I have but I drive 200-300 miles a week or more. And I carry a lithium jump pack which I've used three times but never needed for my own car. (Yet. My battery is five years old.)

 

In addition to the above replies ...

I believe the emphasized part is also wrong. The 12V battery charge rate should not depend on the traction battery's state of charge. At least not until the traction battery is too deleted for the inverter to work properly.

Until then, i.e. over the traction battery's full intended working range, the inverter output should be regulating to whatever 12V output is needed, independent of the input condition.

 

Thanks. I missed that it said it depended on how fully the traction battery is charged. That was another falsehood. Good catch. It will vary a little depending on the 12V SOC and that's what I read since it's what I expected.