Charging the 12V Battery??

There were some reports of short stand by life in hybrids using AGM. I recall a poster here with that issue in their Auris hybrid.

 

Was that a Yellow Top Optima ?
I remember several reports of them going bad.
But that does NOT mean that all brands of AGMs fail at a higher rate.
The exception often proves the rule.

 

I replaced the original battery in my Prius V with a yellow top optima in 2017. The original battery in the Prius V lasted five years, I hope the yellow top last at least that if not longer. Time will tell.

 

So today I had to call AAA to jump my 2017 prime because the battery was mostly depleted only 11 days since I last drove the car (2 trips about 8 miles each way with about 2 miles of ICE running on 1st trip since the ICE comes on even in EV-only mode when going downhill on a full traction battery charge). As far as I can tell I didn't have any lights left on. Car is kept in garage on ChargePoint level 2 charger.

Really annoying that this can happen while plugged into the house
And really surprised since it was not sitting that long. How long does it need to be driven in EV only mode to ensure the 12V stays topped up?

FWIW battery has "84 months" as part of the top label (this was a replacement battery installed by the dealer when the original battery died a few months after I got the car and they found it had dead cells). And AAA thought the 12V was a lithium ion battery (I thought it was lead acid).

 

Your OE Prime 12v battery is indeed a Lead Acid battery.

Read this if you haven't:

12v Battery Is Discharging, Re-Charge Now ! | PriusChat



Rob43

 

You might have unintentionally caused the 12v AUX battery to drain faster by keeping the car plugged into the wall. The system continues to check the charging system when the EVSE is connected using the 12v battery. See the high lighted warning from the manual. And as we found out, EVSE connection does not really charge the 12v battery. Only trickle charge to maintain during the actively charging the traction battery (so with L2 that's ~2 hours).

For my brand new 2020 PRIME LE driving 1-1.5 hours once a week is good enough to keep the 12v charged but even with this routine, after 3 weeks, the 12v battery SoC was down to 50% (12.4v). So far, only way to keep this car's 12v battery charged and healthy is to use a battery charger/maintainer if you are not driving every day.

 

That's good to know and I guess I missed that when I read the manual. Funny that I didn't have a problem a couple of summers ago when I was gone for 3 weeks.

I'll just remember to unplug it in the morning and I just ordered a jump starter so worst case if the 12V runs out I can jump it from that.

 

It's very important that you immediately put your 12v battery on an external 12v charger, you need to bring it back to a full charge.


Rob43

 

Your battery was newer then but it is now ~3 years old. This last discharge might have done it for good. Yap, portable jump starter and battery maintainer would be great accessories to add to your car.

 

Needed charging too....


Rob43

 

I

After getting it jumped I drove it for 2 trips of 20 minutes (the 1st purely in HV mode) - shouldn't that be sufficient? The manual says in reference to the low battery error to run the engine for 5 minutes.

 

Nope.


Rob43

 

It all depends on the overall health of the battery and how deeply it was discharged before jump-starting it. If it is a healthy new 12v battery that has never been deeply discharged, I think daily 5 min READY would be enough to keep the 12v battery charged. Without knowing the state of your battery, I am not sure how good the SoC is holding. Note that you do not have to run the engine to charge the 12V battery. All charge comes from the traction battery, so if you have enough SoC in the traction battery, the 12v battery will be charged by just keeping the car on READY mode without engine turning on.

If you have a voltmeter you can check the resting 12v SoC. It should be above 12v, preferably above 12.4v after 12 hours.

 

I just shared that exact thing yesterday on the Facebook Prius Prime group. I was about to go find it again for here, but you beat me to it. They really should have made that more prominent in the manual. I'll bet that a huge number of Prime owners think they are helping their 12V when they are really killing it.

 

I just thought (until yesterday) that leaving it plugged in didn't make any difference at all (having already discovered back in 2017 that it doesn't keep the 12V charged). Now I know better.

FWIW, I manage my charge schedule from the ChargePoint charger and not from the car. That's because I have time of day pricing so I only want to charge between midnight and 8am. And I discovered the one time I got home early in the morning that the car won't start charging on its own if you plug it in after the charge start time (which was set to something like 12:05am).

 

It will if you simply unplug the handle from the car and plug it back in within roughly 5 seconds of first plugging it in. Takes about two seconds and $0. I do it all the time.

 

That's good.
BUT....letting it run down in the first place causes premature aging.
For extended periods of non-use (more than a week) it is MUCH better to invest in a ~$35 automatic smart charger for the 12 V.

 

This is true. The jump starter is nice to have AND the charger. Different situations call for different tools. If you only have one of them, Murphy dictates that you'll need the other one.

 

I remember hearing 20 minutes of running to recharge the 12v after starting the car, but that probably assumed the battery was at 100% before starting the car. Modern cars put a larger parasitic load the the battery while parked than back when I heard the 20 minute bit.

In addition to battery age, I bet your daily use of the car was different back then. Regular daily drives would have had the 12v at or near 100% before parking for three weeks, and then you went back to that when you got back. The car now isn't getting driven as often. A trip every week or two isn't enough for the car to charge the battery back up to 100% after repeated periods of parasitic discharging while parked.

Part of the aging process could also mean loss of water. The 12V is just a flooded type. These will lose water over time, but yours could also have developed a leak at some point, so mare is being lost. I revived a battery near its end of life from dead simply by adding water a couple times.

A battery charger to top off the charge once in awhile won't hurt. At least a maintainer if parking the car for weeks at a time will be semi a regular occurance. These are low power chargers that apply enough current to make up for the battery's natural discharge rate. Some might bring a discharged battery back up to 100% with enough time.

Lead acids need to be kept at 100% for longest life, and the deeper the discharge they see, the shorter that life.

The low battery warning is probably at a charge level just before a low state a charge starts really reducing the battery lifespan.
Or it is just above the point where the battery wouldn't be able to start the car.
Either way, running the engine for five minutes is just enough to get the battery charge over the low warning trigger level.

My current charger says the Camry battery is at 70% to 80% when I hook it up periodically, and it can take a couple of hours on the auto setting, which starts at 12 amps.

 

Yup, rule of thumb for solar backup and gridless solar systems, which use deep discharge lead acid, is never let lead based batteries drop below 50% - ever.


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