Design challenge for a Prius - Page 1
Forum section: electronics / general

author: Connecteur
author_rank: Frequent Contributor
timestamp: March 29, 2025, 03:46:17 pm
content: The Prius hybrid has an annoying feature. When the 12V battery falls below (I think, 11.5V) the car is effectively dead, and will not start. The only options are a boost or a dealer service call, even though the main hybrid battery may have plenty of charge in it.What should happen is this: Pressing the start button should activate a controller which reads the charge level of both batteries, and if necessary, it should activate the DC to DC converter, delivering normal voltage to the 12 V system, thus allowing the car to start.It would have to read the charge level of the 220 V hybrid battery to guard against allowing its charge level to fall below its lower threshold, before activating the DC-DC converter. If the 12V battery is completely dead, the controller could draw its small power requirement from either an on-board battery, or through a switch-mode power supply, drawing power from the hybrid battery.It seems to me that someone who could design such a controller would have a very lucrative financial opportunity. There are millions of Prius hybrids out there with this problem. Parasitic drain is unavoidable and unpredictable with these cars, and I for one would welcome such a solution.

author: uer166
author_rank: Super Contributor
timestamp: March 29, 2025, 03:56:32 pm
content: I can't speak for Prius, but will speculate why that might be from Tesla knowledge: some cars precharge the HV DC bus from the 12V battery before closing the contactors. It can be done via the DCDC simply by making it bidirectional and boosting 12V to 200-400V or whatever the HV bus is.If that is the case then it's a hardware limitation, not a controls one. Pure speculation though..

author: Connecteur
author_rank: Frequent Contributor
timestamp: March 29, 2025, 04:07:01 pm
content: Quote from: uer166 on March 29, 2025, 03:56:32 pmI've owned several Priuses, and having a dead 12V battery is a constant worry. Many people seem to think that charging the main hybrid battery should also keep the 12V battery charged also, but that's not true for most models.I think Toyota could have easily solved this problem, but it was probably a marketing decision not to do so. A service call to the dealer might be the preferred option (says the conspiracy-minded me).

author: tszaboo
author_rank: Super Contributor
timestamp: March 29, 2025, 04:17:02 pm
content: Quote from: Connecteur on March 29, 2025, 04:07:01 pmIs it? I had dead battery 3 times, 2 times because I left the lights on for sure, and once probably also because I left the lights on.I wouldn't say it's a constant worry. Also you can just start the car even from a 12V power tool battery if needed.And when I had a service call to Toyota, to boost the car (it was in an underground garage with no people around) they just left without letting me pay for it.

author: Connecteur
author_rank: Frequent Contributor
timestamp: March 29, 2025, 04:23:53 pm
content: Quote from: tszaboo on March 29, 2025, 04:17:02 pmMy Priuses never got a dead battery from leaving the lights on, because they all had a timer on the headlights. The problem is parasitic drain, which I have measured and the only solution I have found is to use a battery maintainer. Of course that's not an option when I'm not near an outlet.My newest Prius does software updates over the air, and I have experienced a dead battery in as little as 2 hours when parked. I suspect it has something to do with poor cell reception in certain areas.

author: tszaboo
author_rank: Super Contributor
timestamp: March 29, 2025, 04:24:42 pm
content: Quote from: Connecteur on March 29, 2025, 04:23:53 pmIt was the internal lights.You also have to replace the 12V battery same as any other ICE car, ~6 years or so, whatever the service manual said.

author: globoy
author_rank: Frequent Contributor
timestamp: March 29, 2025, 04:26:19 pm
content: It's probably going to be pretty hard to get access to the traction battery for a 3rd party add-on since for safety's sake the battery is disconnected outside its protective enclosure until the contactor is energized. Energizing the contactor is a pretty involved process by the main ECU doing such things as precharging the inverter capacitor before switching on the HV.I know some people handle this worry by getting a 12V battery emergency start kit and keep that in the car.I have an EV that has a feature where it will energize the HV system and recharge the 12V battery when it is too low. The disappointing thing about that is the feature kicks in at a voltage level below which the car has started to shed loads on the 12V system such as proximity unlock and remote (cellular) access. It's a bit of a bummer in the winter, when you'd like to turn on the heat before going out to the car (and of course the 12V battery is performing its worst in the cold). This coming winter I plan to add a small MPPT charger and 20-30W solar panel (as a sunshade) to see if I can improve that situation.

author: Connecteur
author_rank: Frequent Contributor
timestamp: March 29, 2025, 04:29:37 pm
content: Quote from: tszaboo on March 29, 2025, 04:24:42 pmIt was not. I checked. And they are on timers also

author: Connecteur
author_rank: Frequent Contributor
timestamp: March 29, 2025, 04:33:06 pm
content: Quote from: globoy on March 29, 2025, 04:26:19 pmI realize that special instructions will be required for safe installation, but there are already mod kits that connect to the HV battery, and have been done so safely.

author: globoy
author_rank: Frequent Contributor
timestamp: March 29, 2025, 04:36:51 pm
content: Quote from: Connecteur on March 29, 2025, 04:33:06 pmInteresting. Can you link to one?

author: Connecteur
author_rank: Frequent Contributor
timestamp: March 29, 2025, 04:45:50 pm
content: Quote from: globoy on March 29, 2025, 04:36:51 pmI know of one that converts the power of the hybrid battery to standby power using a large inverter.I can't recall the name.

author: edpalmer42
author_rank: Super Contributor
timestamp: March 29, 2025, 04:53:42 pm
content: I have a Prius Prime. It seems that many versions and ages of the Prius family have a weak 12V system. The 2023 and newer models might be worse than older models.When you push the power button, the 12V battery is used to boot up all the control modules throughout the car. They perform various 'pre-flight checks' and then, if, and only if, all those checks passed, the hybrid system (i.e. the high voltage batteries, main inverter and dc-dc converter) is brought online. Your idea would require bypassing most or all of those safety checks and would result in many vehicle fires.It might be possible to reduce or eliminate this issue if Toyota optimized their 12V charging algorithm and bootup procedure, but they don't seem interested in that. They certainly aren't interested in improving the performance of older cars. Only new models would receive those improvements.P.S. Wouldn't this topic fit better in the Power/Renewable Energy/EV's area?

author: Connecteur
author_rank: Frequent Contributor
timestamp: March 29, 2025, 05:08:26 pm
content: When I took delivery of my 2012 Prius, I was disappointed to get only about 5 minutes on ACC mode. I took it to the dealer, and they said they were surprised that the 12V battery tested bad. I got a new battery, but I found that when the car was parked for more than a week, the 12V battery would be dead. I think a fairly high parasitic drain was the reason. A battery disconnect was not an option for me because it's a pain to reset everything.The car had been in the dealer showroom for several months, with the hybrid battery charger plugged in. I think they assumed it would keep the 12V battery charged, but it did not. Repeated use by customers and sales staff required it to be boosted many times, causing it to fail. That's why I took delivery of a new car with a bad 12V battery.

author: tszaboo
author_rank: Super Contributor
timestamp: March 29, 2025, 05:14:15 pm
content: Quote from: Connecteur on March 29, 2025, 04:29:37 pmHow can you tell, how I drained the battery of my car? Were you there?

author: Connecteur
author_rank: Frequent Contributor
timestamp: March 29, 2025, 06:56:33 pm
content: Quote from: tszaboo on March 29, 2025, 05:14:15 pmI thought you were explaining why mine did.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 29, 2025, 07:55:20 pm
content: Quote from: Connecteur on March 29, 2025, 04:23:53 pmI won't question that. However, are you certain that(A) it isn't cause by some aftermarket mod? Some added device? Poorly designed alarm systems, car radios, radar detectors etc. are classic examples with decades of legacy in killing batteries(B) the car isn't somehow faulty, i.e. some internal module draining more than it should?

author: TimFox
author_rank: Super Contributor
timestamp: March 29, 2025, 07:58:05 pm
content: Quote from: Siwastaja on March 29, 2025, 07:55:20 pmHow high a parasitic drain did you measure?

author: Connecteur
author_rank: Frequent Contributor
timestamp: March 29, 2025, 08:03:21 pm
content: Quote from: TimFox on March 29, 2025, 07:58:05 pm330 mA at one point, but it's all over the map.

author: Connecteur
author_rank: Frequent Contributor
timestamp: March 29, 2025, 08:05:02 pm
content: Quote from: Siwastaja on March 29, 2025, 07:55:20 pmI've spent considerable time on this problem. I believe it's the module that does the live updates.

author: TimFox
author_rank: Super Contributor
timestamp: March 29, 2025, 08:10:46 pm
content: Quote from: Connecteur on March 29, 2025, 08:03:21 pmThat's huge!

author: bson
author_rank: Unknown
timestamp: March 29, 2025, 08:35:56 pm
content: A battery switch mounted under a wheel panel or with a remote sounds like an easier solution... after parking and locking, when walking away, disconnect the battery. When returning, reconnect it and unlock and get in as usual. You could probably just buy an off-the-shelf kill switch.

author: globoy
author_rank: Frequent Contributor
timestamp: March 29, 2025, 08:41:37 pm
content: Quote from: TimFox on March 29, 2025, 08:10:46 pmYup. This is the known problem with the 12V battery in EVs (not only an issue for the Prius). These cars always have a cell modem used for remote access and for the vehicle to telemetry info back to the manufacturer. They also often have other convenience features such as proximity keyfob detection and security cameras (e.g. Tesla) that can take power. In fact IIRC a Tesla can put up to a 50W load on its 12V battery just sitting there (it also wakes the HV system frequently to recharge this battery). It's my experience that this is all managed by some reasonably heavy on-board computer (e.g. not a deeply embedded controller) that takes far more current than you'd expect. They clearly do power management (mine will start shedding loads as the 12V battery voltage falls) but most manufacturers warn against leaving the vehicle just sitting too long. Then over time, of course, the 12V battery itself fails.

author: Gyro
author_rank: Super Contributor
timestamp: March 29, 2025, 09:09:48 pm
content: My Kia Niro hybrid has a '12V Battery Reset' button for this. The need for manual operation takes out a lot of the safely related issues associated with automatic operation. I've only ever had to use it once, tailgate not fully shut.

author: Connecteur
author_rank: Frequent Contributor
timestamp: March 29, 2025, 09:26:09 pm
content: It's not a large battery. Only 38 Ah.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 30, 2025, 07:34:23 am
content: 330mA is not even close to normal. The car either has some aftermarket mod installed in it, or the car is broken. Nothing new needs to be engineered; just the car fixed.

author: andy3055
author_rank: Super Contributor
timestamp: March 30, 2025, 08:58:57 am
content: I have a 2007 Prius. Around 2014 or so, I accidentally left the light in the trunk ON and it died. I mistook it to be a bad battery and replaced it myself. No problems at all. I coguld leave the car for 3 weeks without using and the battery would still be ok. This 2nd battery died last year and I had to call AAA to get a battery in a hurry. The goon that came, somehow loosened the ground connection from the body and the next day the dashboard was blank! It would still start and run. But could not turn off the engine! I panicked and took it to the dealer and g v hthey found the loose connection. Cost me $ 226 or so! I wish I didn't use AAA. It had the same problem once more and I found that one of the harnesses near the battery had a loose or dirty connection. Just scraped all the contacts and put it back. Now for many months, it starts and runs with no issues. If the dashboard goes dark, it could also be a faulty board in there due to heat and cold we subject the car to,h in normal usage. If that happens, you could be without a car for up to 3 to 4 weeks. These boards for the older cars are not produced anymore. The dealer has to send it to a private party for repair and input the current milage to the replacement. All that takes weeks. What happens mostly is that the electrolytic caps on the board dries out and need to get replaced. The milage and any other info is on a chip. Once the board is touched by the dealer, you cannot get the car back until it is fixed because of legal issues.

author: Connecteur
author_rank: Frequent Contributor
timestamp: March 30, 2025, 12:38:44 pm
content: Quote from: Siwastaja on March 30, 2025, 07:34:23 amI have addressed this issue with the dealer many times, and they insist nothing is wrong. If you keep coming back and insisting on the same thing, you get put on their naughty list.

author: Connecteur
author_rank: Frequent Contributor
timestamp: March 30, 2025, 12:42:44 pm
content: There's no point in trying to get this fixed, I just use a battery maintainer. I went 3 days without the battery draining, so I left it over night just to see what would happen. The battery went dead in 19 hours since I drove it. Unpredictable battery drain is just a feature of that car.

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 30, 2025, 01:24:47 pm
content: Quote from: Connecteur on March 30, 2025, 12:42:44 pmThat looks pretty bad.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 30, 2025, 01:28:40 pm
content: Excellent troll thread again

author: nctnico
author_rank: Super Contributor
timestamp: March 30, 2025, 02:13:39 pm
content: Quote from: tszaboo on March 29, 2025, 04:24:42 pmThat is the best course of action. I make it a point to fit the biggest, highest capacity (quality brand) batteries in my cars. As a result I never had issues with batteries once I replaced them. Some garages fit small & cheap crap batteries to make the garage look cheap but in the end you'll suffer.

author: madires
author_rank: Super Contributor
timestamp: March 30, 2025, 02:47:03 pm
content: Quote from: Connecteur on March 29, 2025, 08:03:21 pmThe recommended way for measuring the quiescent current is to park the car and wait about half an hour for the control units to power down and enter sleep mode. The quiescent current should be < 50 mA. Some cars have a slightly higher current, but should therefore also have a larger 12V battery. With a 38 Ah battery I would expect a low quiescent current.And yes, 12V batteries in EVs die early. Their lifespan is roughly halfed compared to a battery in a car with combustion engine. It's the most common cause for EV breakdowns.

author: tszaboo
author_rank: Super Contributor
timestamp: March 30, 2025, 03:56:31 pm
content: Quote from: Connecteur on March 30, 2025, 12:42:44 pmIf you think that the car battery could die in less than 24 hours, and that's somehow normal, there is nothing we can design or say to correct the problem because it's not with the car.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 30, 2025, 04:09:09 pm
content: Maybe replace with rechargeable alkaline battery? I'm sure that will work well

author: Connecteur
author_rank: Frequent Contributor
timestamp: March 30, 2025, 04:17:10 pm
content: Quote from: tszaboo on March 30, 2025, 03:56:31 pmYou apparently got sidetracked by my story about parasitic drain, which is unpredictable and highly variable. Probably due to live updates. That's not a problem I can solve.But that wasn't the point of my original post. I'm looking for a way to draw some power from the hybrid battery, to activate the DC-DC converter to power the 12V system. I want a controller which would start the car when the 12V battery is dead. This would solve the problem of having to boost the 12V battery every time it goes dead.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: March 30, 2025, 07:23:31 pm
content: I was intending to ask a question about the RAV4 hybrid, but will open another thread to avoid derailing this thread further.

author: Connecteur
author_rank: Frequent Contributor
timestamp: March 30, 2025, 07:31:16 pm
content: Quote from: UnijunctionTransistor on March 30, 2025, 07:23:31 pmTry to mind which thread you're responding to.

author: bdunham7
author_rank: Super Contributor
timestamp: March 30, 2025, 07:46:49 pm
content: Quote from: Connecteur on March 30, 2025, 04:17:10 pmYou'd have to tap into that HV hybrid battery and unless there is a commercially available (and tested) solution I doubt anyone wants to step up and hack something in there. Besides that you have the issue of your 12V battery being destroyed by the full discharges. Our EV was going dead on occasion and the 12V batteries were not lasting, probably due to being partially discharged repeatedly even when it didn't go all the way to zero. The solution was to pull the fuse for the telematics module. I would suggest that the solution to your problem would be easier to implement if you were to determine what is causing that drain and put a relay in so that it is fully disconnected when the car is turned off. If I needed the telematics module to operate, I'd do just that. I certainly don't need it to operate when I'm not driving the car. If you wanted to give it some alone time after driving so that it can update or whatever, you could put a 20 minute timer (or whatever) so it stays on for a bit.

author: Connecteur
author_rank: Frequent Contributor
timestamp: March 30, 2025, 10:24:26 pm
content: Quote from: bdunham7 on March 30, 2025, 07:46:49 pmThe hybrid battery is already tapped into. The DC-DC converter is switched on when the car starts, which supplies charging power to the 12 volt system. What I tried to explain was that I needed some sort of controller to switch on the DC-DC converter so that the car's control system can come on. As it is, when the 12V battery falls below about 11.5V, it will not start up, and only gives you a "battery too low" type message, requiring a boost or a service call to the dealer, even though there is plenty of power in the main hybrid battery. The objective is to get the computer system to switch on, so that the DC-DC converter can do it's job supplying charging power to the 12V system. I can't be any clearer than that.Where did you get the idea that my 12 V battery was being destroyed? My original battery was fine, but the dealer replaced it anyway for good will I assume. The battery goes dead unpredictably, and I want to avoid the hassle and degradation to the battery by boosting each time it happens, and it has happened at the most unexpected times.It should be a simple matter for the controller to sense that the 12V battery is below the threshold for starting the car, and turn on the DC-DC converter, so that the 12V system can function and charge the 12V battery. In the extremely unlikely case that the hybrid battery is below the lower charge threshold, the controller should be prevented from engaging the hybrid battery to boost the 12V system through the converter.

author: langwadt
author_rank: Super Contributor
timestamp: March 30, 2025, 10:31:57 pm
content: Quote from: Connecteur on March 29, 2025, 04:07:01 pmnot only Prius, not so long ago my dads hybrid Yaris also refused to start because of a low battery

author: Xena E
author_rank: Frequent Contributor
timestamp: March 31, 2025, 06:54:35 am
content: Remove the inverter/ DC-DC converter assembly from the car.The part you're after is in the lower section of the assembly.There is a possibility that the discharge problem noted is caused by MLCCs on the DC-DC converter board.If not, the hack that the TS wants to do can be done on that board.You will need special service tools, and it has to be done carefully as a mistake will allow coolant to leak onto the electronics in the unit.If you have to replace the unit upper board it will need to be reprogrammed by Toyota... so don't touch it.When you have the converter out, come back, posting photographs and I will tell you what you need to do for the repair and/or your hack, (not recomended)...For cars 2017 onwards, follow this video:-X

author: Jeroen3
author_rank: Super Contributor
timestamp: March 31, 2025, 07:39:33 am
content: So to prevent this issue in a Hyundai i10 parked long term at a holiday home we've just fitted a solar panel on the trunk shelf and wired it to the permanent live of the trunk light.Nothing fancy.If there is room for an extra small 12V battery somewhere you could also see if an Cyrix-CT can help. That way you don't have to touch any of the the HV stuff, which may cause workshops to bail if they see mods.

author: Xena E
author_rank: Frequent Contributor
timestamp: March 31, 2025, 08:13:38 am
content: Quote from: Jeroen3 on March 31, 2025, 07:39:33 amAbsolutely correct, simple solutions first.... however reading the thread the TS doesn't seem to want to do simple.Toyota Auris pool car at work was randomly doing this, often dead in a few days: it was a bad door switch.It's not the fact that these things happen, its the catch 22 situation, inconvenience, and expense that it often puts the owner.X

author: Siwastaja
author_rank: Super Contributor
timestamp: March 31, 2025, 09:03:38 am
content: Bad door switches are likely one of the most common culprits. I have had them, nearly any car have at some point when they are aging.I have had flat battery (in totally non-hybrid 1980's and 1990's gasoline cars) maybe ~4 times total. Reasons always being me forgetting the lights on (maybe twice), or bad door switch (maybe twice, too).Door switch can be pretty ugly in that it works intermittently so you see the interior lights turn off, and then after you are gone the lights turn on for the whole night.Also remember the light switch of the trunk.Simply because a modern hybrid car is very complex piece of engineering, do not assume that the failures need to be in these complicated parts. A classic mechanical failure that has stayed the same for 50 years isstillthe likely culprit.

author: cv007
author_rank: Frequent Contributor
timestamp: March 31, 2025, 05:36:38 pm
content: You have fuse panels- while measuring parasitic current pull fuses one by one to maybe at least narrow down the culprit. Even if finding/fixing the problem doesn't prevent the occurrence of the occasional low battery condition, or the need to find a solution to that particular problem, it at least would get you back to par with most of the other Prius owners. Maybe it will take time/patience to find, but I would think it would be time well spent to get yourself a 'normal' working Prius.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: March 31, 2025, 05:37:28 pm
content: Quote from: Connecteur on March 30, 2025, 07:31:16 pmThis was an unwarranted low blow. I had only attempted to show courtesy to your thread, that was all.Chill out.

author: Gyro
author_rank: Super Contributor
timestamp: March 31, 2025, 09:00:56 pm
content: Quote from: Siwastaja on March 31, 2025, 09:03:38 amThe glove box light switch is another classic one, especially when it is loaded full of crap that distorts it slightly.