Third day with new car, dead 12V battery, second day in a row :(

Hey guys, noticed you're all still circlejerking over your batteries. Tough times.

Just wanted to let you all know I left my car parked for 2 weeks (was out of the country) and came back to the battery being just fine. I left my key fob in a faraday box I bought off amazon and I disconnected the circuit breaker that feeds my aftermarket amplifier and DSP. Other than that I didn't do anything special with the car (aside from the fact that I keep the fuses to DCM and OBD2 port pulled for security purposes).

I drove it for about 15 minutes to the next little city nearby and after about 10 minutes my cig port accessory showed the 12v voltage had gone from 14v (charging from HV) to 12.7v (it occasionally stops charging from HV to check the 12v level I assume). So there was no massive drain or anything.

This leads me to one conclusion: skill issue. I look forward to the next 20 pages of speculation on this topic.

 

Your conclusion of it being a “skill issue” is not quite correct, and in fact, you are lacking some skill as well, having the car for only a few months and not knowing how the complex 12-V charging system in Gen 4/Gen 5 Prius/Prius Prime really works.

The 12-V charging system is indeed fine. However, you may want to invest in a Noco Genius 1 and occasionally fully charge your battery every few months and also use it as a maintainer when the car is not driven for a long time.

Last year, my almost-three-year-old OEM battery withstood more than six weeks of standby time. This year, being another-year-older, it drained in 32 days or less of standby time. As your battery gets older and also depending on its current SOC, which can vary, your battery could well drain in two weeks if not less. Using a battery charger/maintainer as I described above will minimize issues. You might find out over time that your battery is not actually fully charged, and this could get worse as it ages, which not only increases the chances of battery problems but also degrades your battery faster.

Regarding a Faraday's cage for the smart key—according to my battery monitor, the smart key causes the 12-V battery to be drained only when you are within a few feet of the doors. Therefore, you don't need a Faraday's cage for the smart key for the purpose of reducing the parasitic drain.

 

Battery University, the go to source for lead acid battery technology. Valuable source of info.

 

Your response just shows you guys love to reread your own posts over and over to yourselves because you love the sound of your own internal voice.

I don't need to invest in a charger because I don't have your drain problem. And if I'm leaving for more than 2 weeks the smart move is to disconnect the negative line for that time.

Faraday box was less about battery but more about preventing theft via cloning.

For those of you with battery drain issues you need to all invest in a multimeter and start narrowing down where your issue stems from. I'm willing to bet that in these many pages no one has been checking fuses and isolating where the drain is coming from. This forum loves to yap and not produce results. All of you would be amazing middle management workers.

You all buy new batteries, you buy chargers/tenders, you yap and speculate, but nary a person to be found who is willing to buy a $15 multimeter and start checking fuses to see which one is carrying a higher load than normal when the car is idle. I'm bored to death of seeing this thread and other 12v issue threads come up because I'm genuinely curious what the issue is (despite the fact that I'm not afflicted by it) but none of you will take action.

 

I do tend to agree - when the car doesn't have a fault, and is working as intended, you shouldn't need to worry.

Left mine for 11 days at an airport car park without only a slight inkling of concern. On return, it was fine. No hint of the battery being low - opening the powered hatch before READY seemed as responsive as ever.

With my G2 I would have taken the precaution to turn off the smart key with the button under the wheel. I realised afterward there was actually a smart key disable setting in the G5 multimedia screen. Only thing I did do was switch the "ACC customise" setting back to on, because I knew that was also a potential power saver from the pre-delivery checklist.

But I'd never dream of making a routine about topping up the 12V battery for normal at-least-a-few-times-a-week use. That shouldn't be necessary. It would indicate a fault. (Except maybe in a particularly prolonged cold snap during a Finnish winter).

 

We do have all kinds of multimeters, oscilloscopes, and even battery monitors. And several posters here have been observing the charging behavior by using battery monitors.

The issue is not necessarily the battery drain. In fact, all batteries self-drain even if you disconnect them, which could be very fast when the battery ages, like a full self-discharge within days for an aged battery.

If you read the thread, the Gen 4/Gen 5 12-V charging system is designed to prioritize fuel efficiency over keeping the battery SOC at a high level. You only had the car for a few months and don't know how the charging system works and where your SOC will reside in a year from now, which could be permanently well below 100%, and that's why you might want to invest in a $30 battery maintainer, as these batteries don't like lower SOCs.

And, in fact, chances are that you don't even know where your SOC is residing right now, and it could be around 70% (~ 12.29 V open-circuit). Perhaps take that multimeter of yours and measure and report your open-circuit voltage (basically with the car off and battery rested, no need to disconnect it) tomorrow morning. Chances are that your $15 multimeter—unlike my multimeter and battery monitor—is not calibrated to 0.01 V in the 20-V range, but it is better than nothing.

 

Who the hell cares about charging behavior of the 12v? Your 12v shouldn't be so low that a 15 minute drive doesn't top it off.

I run a 1600RMS amplifier and rock out while driving, and even with that my 12v and the HV charging to 12v handles it all just fine. Whatever the car's 12v charging priority is, it's not of any concern since it keeps up with a much higher load than what the stock vehicle will pull.

You're damn right I don't. Because it's irrelevant data.

Either you have a trash battery that needs to be replaced. Or you have a parasitic drain that's eating your 12v.

This is a really easy solve. You replace the battery under warranty and if the drain issue persists you start checking fuses while the car is in deep sleep (remember, you can't bring the fob near or open doors, you gotta trick it by flipping the door latch before it goes into deep sleep and leaving the door ajar so you can get in and out).

But you guys are so weird for obsessing over what charge level the 12v is at. Either there's a parasitic drain and that's a problem and that needs to be addressed or there's not.

You can't claim that the HV charging of the 12v is the culprit otherwise we'd see a LOT more people with the issue. This is a small percentage of owners. Prime owners primarily. Find your parasitic drain. You guys will do everything except the ONE thing to actually diagnose the issue. Actually you guys might be qualified to work at a Toyota dealership given my experience with Toyota techs. Go apply now.

 

The great majority of reports for a depleted 12 volt auxiliary battery are with the Prime model. It appears you own a non Prime which may explain your experience differing from Prime owners. There are multiple threads on Prius Chat of reports a dead 12 volt auxiliary battery. I experienced this back in August. I have tested for parasitic drain on my 2024 prime and paid to get access to the Toyota TIS for information on the charging system and the fault isolation procedure for a depleted aux battery. FYI, Toyota parasitic drain or dark current as they describe it, allowable is 120 milliamps or less. Anything above 120 requires investigation, i.e., fuse removals, etc. Using an amp clamp on the negative lead, my car current drain is in the 80 milliamp range.

I have a battery monitor that has given me insight to how the system works along with the data I learned from the Toyota TIS. My longest drive typically is in the 20 minute range and the charging voltage typically starts at 14.1 volts and then drops to 12.8 volts within a few minutes. The battery was tested by the Toyota dealer and confirmed good. I have a load style battery tester along with an electronic tester and confirmed the battery is healthy. That said, a charging voltage of 12.8 volts probably will not keep my battery at SOC and SOH that is optimum. With winter in the future, it will be interesting to see how the charging system and battery perform.

The point in a forum is to share experience and knowledge. I have benefited from the posting of forum members. I don't take anything as fact from what is posted but compare the experience to mine. Great that your experience to date has been as described but possibly related to being a non Prime model.. My experience was no battery issue for 6 months until one morning with a dead battery. Fortunately, I own a jump pack to revive the car. This is what pushed me to learn more about the car charging system and aux battery design.

Cheers

 

And that makes sense as there's known potential fault which there is a TSB for (in the US at least). Step 1 is clearly to apply the TSB. Only then if it still occurs with that fix does it make sense to trying to dig into possible alternative explanations.

 

Parasitic drain is not the only reason why Gen 4/Gen 5 Prius/Prius Prime batteries are failing. And if you look at the threads here, many non-Prime Priuses have had problems as well. I don't have any evidence of a parasitic drain in my Gen 4 Prius Prime. In fact, I haven't had any battery issues, other than a depletion in a four-week standby in a four-year-old battery.

There is a known issue both in Gen 4 and Gen 5 that leaving the lithium-ion-battery charger cable plugged in drains the battery, and there will be not be a fix for it—TSB or not. Prius Prime owners should know not to leave their cars plugged in for longer than overnight. In places like Canada, where keeping it plugged in is desired to heat the battery, they should also plug in a battery maintainer on the 12-V battery.

I think these cars are OK. However, @Roy Peterson nailed it when he said that their charging systems are not designed to keep the SOC and SOH at the optimum level. That's why everyone would benefit from an occasional use of a battery maintainer, such as every few months. You could get a ten-year life out of your battery instead of three or four if you do that. If you also use a battery monitor, then you would know your SOC and could hook up your battery maintainer only when it is needed—in fact when the SOC is very low and when it is really needed.

 

I mean, soft_r is right, but I've given up arguing this issue. I just scroll down the long battery posts without reading them just so that they go away on my unread posts list.

It seems completely apparent that the problem is a transient issue with the Prime software that doesn't always let the car go into or stay in deep sleep. Maybe the TSB fixed it completely, maybe it only fixed part of it. IDK. I don't have a Prime to test it. (funnily enough, Gokhan doesn't have a gen5 of any sort to test with, though he certainly seems convinced of his own assumptions, but I digress)

The problem is made worse by a slightly undersized battery that speced for the gen5.

I suspect what's happening is the car doesn't maintain the battery at a full state of charge at all times, but always keeps it in a "safe" zone. So at the end of a drive, the battery is at a random state of charge within that safe zone.

But then comes the unplanned for parasitic drain.

If the battery started toward the high end of the safe range, the parasitic draw isn't enough to deplete the battery out of the safe range, and the car starts normally. But if the battery started toward the bottom of the safe range, the draw pulls the battery down into unsafe and damages it.

Replacing the battery helps the issue because the replacement battery is significantly larger in capacity than the original. That means the probability of the battery being in a danger zone at the end of a drive has been reduced. But it doesn't necessarily solve the underlying problem.

Likewise, using a battery tender will keep the battery healthy, but all it's doing is treating the symptoms and not the underlying cause.

But there's no point arguing with people who have made up their minds, so I just ignore these threads(I think there's three of them active right now?!?) and go on with my life(unless I'm really bored like today - lol).

 

Gen 4 and Gen 5 have the same GS Yuasa 45-Ah flooded-cell battery and apparently identical charging systems. Both have experienced similar issues; so, they are not new to Gen 5. However, they were new to Gen 4. Gen 2 and Gen 3 had smaller batteries, but they were AGM.

Again, these batteries and charging systems are mostly OK. We are only helping people to get the best out of their batteries and avoid possible issues.

I don't read every thread here, which is actually a gross understatement, and no one should read the threads they are not interested in.