Charging the 12V Battery??

Own a 2017 Prius Prime that used to see 100 miles/day but like many of you here, has gone down to 250 miles or so for past year. 12V battery is near 0 and have used a jump battery couple times to get car moving. Decided yesterday I better do a full charge and connected a Schumacher SE50 battery charger at about 2-6A charge. It’s been charging 12 hours yesterday and 6 hours today (didn’t want to leave charging overnight) but charger still says not done charging. Is this expected because the 12v has been in low charge state so long? Will it eventually finish? Or is this a sign that the 12V battery needs replacing? I didn’t think the traction battery had any role in 12V charging so have not charged it since I only have outside overnight parking. Thanks! Bob

 

Your understanding is only partially correct. The traction battery does have a role in charging the 12V, but only when the car is in READY mode. When you are charging the traction battery, it is not charging the 12V, and actually is running the 12V down a little bit because 12V power is needed to monitor the charging operation. It sounds like your 12V is beyond saving, but you could always take it to a battery shop for a definitive analysis. Take the 12V battery from the car and carry it to a battery shop for analysis and rehabilitation, if possible. If not, you've already got the 12V out of the car and you can purchase a new one and get credit for the battery core you just brought in.

 

Most smart-charger will not start charging a 12v battery unless there is still a little bit of SoC left in it. If it is drained to empty SoC as you have described, then you may have to force charge it with a high amp non-smart charger to get it started. But in my experience, when a 12v has been drained to that level, the battery is most likely beyond recovering by force charging and/or reconditioning.

 

Could be. As @FuelMiser suggests, take it to a shop to have it tested.

Partly right. While the EVSE is plugged in, the car does run down the 12V a tiny amount more than at other times while it's not charging the traction battery. While it's actively charging, the DC/DC converter feeds some extra juice to the 12V to maintain it. But not as much as the PiP does while charging the traction battery. This is easy to verify with a multi meter.

 

What the shops will likely use to test is a pro-level electronic battery tester, that measures both voltage and Cold Cranking Amps. They need to enter a few of the battery’s stays; basically type and rated CCA (or CA, or a few other systems, sim to CCA). Push a button and it tests in a split second, gives a verdict.

You can get similar prosumer level testers to DIY the test. Clore Solar BA9 is one (good) example, around $70~90 USD.

 

Just a caution about using a battery tester and interpreting the results. Depending on the battery tester, what it reports after the test may be somewhat confusing in deciding whether you will need a new battery or not.

I recently used the Ancel BA101 battery load tester to check the weak 12v battery on my son's Honda CRZ. He had experienced a dead battery issue on his car and needed to jump-start his car several times a few weeks before. For each time he had to jump-start the car, it would run fine for a few days, but it did not last long. The CRZ is a hybrid and the 12v battery is only used to start-up the computer normally similar to Prius, but unlike Prius under certain conditions when there is not enough juice in the traction battery, the 12v battery does turn the engine. The 12v battery in the car was less than 2 years old Exide Classic 151R rated 340CCA. Upon the first testing the battery when the car had a problem starting, the result was only 70CCA (3% SoH=state of health) and 10.13v. And the tester reported to "CHARGE-RETEST".



I charged it overnight and the car started fine and run. But when I retested it in the car. It now reported 295CCA (61% SoH) and 12.32V with "GOOD-RECHARGE" comment.


So, according to the tester, the battery was still good but SoC was a bit lower than it should be after a full recharge. This was after my son had to jump-start the car at least 3 times in a few weeks. Even though the battery was only two years old, the fact it was not holding the full charge after charging overnight and starting the car only once, and the fact the SoH was down to 3% and only back to 61% after charging, I decided to replace it with a new battery. BTW, this was the third 12v battery in his car in the last 5 years, including the first OEM battery lasting only a few years.

Here is the reading on the brand new Napa Power 151R 340CCA battery.

The SoH of the brand new just picked up from the store was 92% with 362CCA with 12.56v, and the tester showed "GOOD BATTERY". After installing this brand new battery in the car and testing its function by starting the car and turning on the light, etc. I tested this new battery again. Now I got this.


SoH was down to 90% with 358CCA and 12.22v, and the tester reports "GOOD-RECHARGE". This is the same message I got on the old bad battery after charging it overnight, although the CCA is substantially higher on this new battery.

To illustrate the confusing report used by this battery tester. I tested the old battery that was removed from the car and charged it overnight. And I re-tested it immediately after the charger was removed. I got this report.


So, even bad battery that does not hold the charge, the tester is reporting it to be "GOOD BATTERY". The problem is that its SoH is only 44% with 250CCA (rated 340CCA), despite the charge is at 12.79V. This old battery after sitting in a house for a few weeks drained the charge completely and was basically as dead as it was in the car when replaced. SoH 7% (100CCA) and 10.35v.

 

YES.
NO.
YES.
Actually automotive batteries are near to totally "dead" when they read 11 Volts or less.

And your new one will do that same thing, probably within a year, if you don't keep it charged somehow.

Given a new, fully charged 12 V battery, actual driving time of 1-2 hours a week will probably keep both batteries charged AND keep the oil moving too. AND use up the gas in the tank before it turns to grease.

 

In my experience a “good” battery invariably reads higher than spec.

Once they’re at or lower than spec I’m shopping.

 

Thanks to all for your replies, and my apologies for the late follow up. Real life has a way of intruding into my online time. After posting, I got out my voltmeter and measured steady 12.16 volts over 2-3 hours of charging. Evidently this is not enough for the charger to deem the battery as charged. Left it charging for couple more hours and still 12.16VDC so disconnected the charger, charged the traction battery and parked it outside. After a week, drove it for errands and started fine. But I’ve now added a calendar reminder every 2 weeks to drive the baby, and noted someone’s recommendation that the battery is now less capable due to long period of low charge state since normal voltage should be 13+. Will look for a replacement once I know I’ll be using the car more regularly and further from home (on the far side of this pandemic!). Glad to have found this community! Thanks

 

If you’re using the car infrequently, and can swing it (garage parked with 120 volt outlet): a smart charger on pretty much all the time, you’ll never have problems.

Thats my scenario, and solution. Next September my Optima Yellow Top will be 6 years installed. Happened to check it first thing this morning; car had a good run the day before. Voltage a bit low, but overall good. Will probably replace next fall anyways.

Tested Cold Cranking Amps (spec is 450):



And voltage:

 

Normal voltage is not 13+ volts. 12.16V will start the car just fine, but at 50% charge that's lower than it should be. 12.6-12.7 volts open circuit would be an approximate full charge at room temperature (2.1V per cell). It'll be a little lower with a load. Using a hydrometer to measure specific gravity will be more accurate. When I first start my Prime it feeds about 14.1V to the battery. Once the battery is charged back up, it feeds a fairly constant 13.1 volts to the battery. At rest after sitting overnight and checking it with a multimeter, I have never seen anything close to 13 volts. If that's what you're shooting for, you'll be frustrated.

 

Nope. That statement is grossly misleading.

The battery eventually will wear out.
IF you have a tender on it all the time when it is parked, even just for a day or few when it really isn't needed.......
Then that will pretty much guarantee that it will exhibit it's failure somewhere AWAY from home instead of in your garage.

It really is best to use a tender ONLY when the need is present. Like when the vehicle doesn't move for a week or more.

 

What exactly makes you think that will be any more "accurate" ??
Hint: It isn't really.
It does allow you to see if more than one cell is getting low.
Also it is not possible to measure the specific gravity with a gel or agm battery.

 

A situation that has no bearing on the lead acid battery in the Gen 4 & Prime.

 

Just because it came like that from the factory doesn't necessarily mean that will be true for it's entire life.

 

Hi Brad, I am in a similar state. My Prius Prime 2017 Adv was parked for about 5 months and now it wont start. My friend has used a small recharge tool for 5 minutes. He said the dashboard lights up but nothing is displayed on the monitor screen. Also when he pushed the START button nothing happens. How did you solve your problem?

 

It's going to take longer than 5 minutes to recharge the starter battery from dead. Even with the lower start load on a hybrid's battery.
Was the car plugged in? If so unplug it to reduce vampire losses on the 12 volt. Maybe even disconnect the 12V from the car.

 

And it might take more than a "small recharge tool"......whatever that even IS.
AND it might not recharge fully at all.

The solution probably is a new battery.
But it can be taken out, fully charged and tested if you want to take the time.
At almost 5 years old, a new battery is advisable either way.

 

When I left the car in February it had 60% of the EV battery charged. I just found that AAA don't carry the replacement battery for Prius Prime. Called one of the Toyota dealership, they were also not sure about the battery, but I will check with them on Monday again.

When you say was the car plugged in, do you mean if the EV battery was being charged by power outlet. Then the answer is 'No'. In the 5 months, EV was not charged.

The battery is still connected and the battery clippers are not taken out during the re-charge process.

 

Thank you, for now my only goal is to get this started, so that I can take it to the dealership and replace the battery. If recharging didnt work, then my plan is to tow it with AAA to Toyota dealership and get the battery replaced. I am hoping that recharging will resolve this issue.