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Discussion Starter · #1 ·
I believe I read that with my Prius Prime, the 12v battery was always being trickled charged while the Prime was plugged in (even after the traction battery was done charging). Is this true on the Clarity?

I'm asking because I want to leave my dash cam with parking mode recording on. However, that would mean that the vast majority of the time, when I'm parked in my garage, the dash cam would be needlessly on and running down my 12v battery. There is a limiter on how much the dash cam can drain the battery before it shuts off, but it seems ridiculous to let it do this day after day. However, I leave my Clarity plugged in in my garage. If it's always trickle charging, I don't need to worry about the dash cam running down the 12v battery while I'm parked in my garage with the Clarity plugged in.

Thanks!
 

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2021 PHEV Touring HB, CA
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Put a good quality Battery Tender-like device on the 12V auxiliary battery, using a conveniently placed wire extension with an in-line fuse and waterproof connector. I did this for my Honda S2000, and placed the connector at the back of the hood, next to the windshield, and tucked it in the drain relief channel when not using it. Plugging and unplugging took about thirty seconds, tops.
 

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The 12V battery is only charged while the main battery is actively being charged. The rest of the time the 12V does not get charged even though you are plugged in. Another option besides a battery tender is there are small lithium backup batteries made for this purpose that you can install under the dash, they power the dashcam when the car is turned off, and they are wired in so that the backup battery gets charged whenever the car is running.
 

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Discussion Starter · #4 ·
The 12V battery is only charged while the main battery is actively being charged. The rest of the time the 12V does not get charged even though you are plugged in. Another option besides a battery tender is there are small lithium backup batteries made for this purpose that you can install under the dash, they power the dashcam when the car is turned off, and they are wired in so that the backup battery gets charged whenever the car is running.
Thank you for that information. That's exactly what I needed to know.

I am aware of those backup batteries and I'm considering them. However, I'm putting two dash cam systems in. I'll have a front camera in the front, and another front camera in the back. Then I'll use the rear camera from each system on the side windows (in the little fixed triangular window behind the back seat). That way I'll have coverage in all directions.

I know that may seem excessive, but my existing system just saved me almost $10K from damage from gravel and debris that fell off the back of a dump truck in front of me on the freeway. My adjuster tallied everything that was damaged and said I was entitled to almost $10K that they will recover from the truck company with the help of the dash cam footage. The damage is a bunch of fairly light scratches and chips. The adjuster said that if he were me he would not have the damage fixed and just keep the money..

So the camera system makes great sense. I caught up with the dump truck driver and he started to argue with me. Then I told him I have it all on dash cam, and he just told me to call the number on the back of the truck. Dash cams can cut through all the BS.

Anyway, two systems would mean two batteries, so I'm a bit reluctant to do that. Plus, those batteries will still be charging and discharging all the time. I think the average lithium battery has about 500 charge cycles (I may be wrong about that), so I'd need new batteries in less than two years.
 

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Discussion Starter · #5 ·
Put a good quality Battery Tender-like device on the 12V auxiliary battery, using a conveniently placed wire extension with an in-line fuse and waterproof connector. I did this for my Honda S2000, and placed the connector at the back of the hood, next to the windshield, and tucked it in the drain relief channel when not using it. Plugging and unplugging took about thirty seconds, tops.
Yes, I was thinking about that. I just really don't want to have to mess with connecting that every time, but it is an option.

I could also have a power switch installed in the power line to the dash cam which would allow me to turn it off entirely when I'm parked in my car. However, that creates a real risk of forgetting to turn it back on when I'm driving, which could be a very costly mistake and defeat the whole purpose of having a dash cam when I need it most.
 

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I would look for a way to piggyback off of something that turns off when you shut off the car. One of the guys here might be able to tell you a fuse pin for something like the seat heater or radio that you could parallel off of. I don't know the car well enough but the Clarity geeks here do.
 

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This is interesting. So the 12V keeps getting charged while in charging state while plugged in...but not if Charged/Left plugged in.

Is the 12v battery always recharged when you Hold Break and press +Power Button? I haven't heard the engine kick on once except when I manually turned on HV on the highway so I was assuming that the 12v is always being charged for lights, radio etc while driving.

My other question is this....could the 12v faults when the car is parked for several weeks, come from the fact that there a LTE radio inside the car? We know a low LTE signal technically uses more battery power on a cell phone because it's constantly searching for cell towers... is this why these cars "die" so easily if parked for several weeks in a garage never turned on?
 

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This is interesting. So the 12V keeps getting charged while in charging state while plugged in...but not if Charged/Left plugged in.

Is the 12v battery always recharged when you Hold Break and press +Power Button? I haven't heard the engine kick on once except when I manually turned on HV on the highway so I was assuming that the 12v is always being charged for lights, radio etc while driving.

My other question is this....could the 12v faults when the car is parked for several weeks, come from the fact that there a LTE radio inside the car? We know a low LTE signal technically uses more battery power on a cell phone because it's constantly searching for cell towers... is this why these cars "die" so easily if parked for several weeks in a garage never turned on?
When you hold the brake pedal and press the power button this is known as READY mode, which it says on the instrument panel just below the shift indicator symbol (P, D, N, R). Whenever the car is in READY mode the HV battery charges the 12V battery. The gas engine has nothing to do with charging the 12V battery, at least not directly.

The LTE Telematics unit runs off of 12V and will use a little bit of power as it "listens" for an incoming call from Honda. A call which will likely never come if your car is sitting undriven, nevertheless it keeps listening 24/7 waiting for a possible remote climate request to come in. It probably uses a similar amount of power as a smartphone sitting idle with the screen turned off and no calls or text messages coming in.

The smartkey system also uses a little bit of power as it listens for the keyfob. My Prius had a way to turn that off when you went out of town. Clarity doesn't seem to have that, although it uses so little power it probably makes little difference.

My car makes odd sounds sometimes while sitting in the garage even if not plugged in. Some people think it's the battery management system, maybe but I suppose it could be other things also. Whatever it is might be running off of the 12V not the HV battery, but I don't know. As far as I know the HV system is only active while charging, when in READY mode, and while remote climate is running.

Some people have said they didn't use or charge their Clarity for several weeks and it started right up. However based on the number of people who have returned home to dead batteries after a week or two, I would not be surprised if Clarity uses more 12V power than is typical. However even so I suspect that their 12V battery was already pretty weak to die in that short of time, and the slightly higher drain of the Clarity probably exacerbates the situation.

I also suspect that people who go three weeks and say "the 12V battery was fine", well what they are really saying is just that the car started. It's possible that their 12V was in fact drained down pretty far, and they may have been closer to being discharged than they realized, but they would have no way to know that since it takes very little power to start the car (i.e. get it into READY mode). Unlike a gas car where the 12V battery has to spin the starter motor and the engine, so you will usually get a sense of the battery getting weak. With the Clarity (any hybrid actually) if you have difficulty getting the car into READY mode then the 12V battery is probably pretty close to being dead and should be fully charged before trying again to start the car, otherwise you run the risk of permanently killing the 12V, or at least greatly shortening its life.

One way to charge the 12V in that situation if you don't have a battery charger but you do have jumper cables or a jumpstart battery, is to connect the jumper cables, then plug in the charge cable and get charging started. Once charging is started you can then disconnect the jumper cables, and let it charge for a couple of hours before trying to start the car again.
 

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Discussion Starter · #9 ·
When you hold the brake pedal and press the power button this is known as READY mode, which it says on the instrument panel just below the shift indicator symbol (P, D, N, R). Whenever the car is in READY mode the HV battery charges the 12V battery. The gas engine has nothing to do with charging the 12V battery, at least not directly.

The LTE Telematics unit runs off of 12V and will use a little bit of power as it "listens" for an incoming call from Honda. A call which will likely never come if your car is sitting undriven, nevertheless it keeps listening 24/7 waiting for a possible remote climate request to come in. It probably uses a similar amount of power as a smartphone sitting idle with the screen turned off and no calls or text messages coming in.

The smartkey system also uses a little bit of power as it listens for the keyfob. My Prius had a way to turn that off when you went out of town. Clarity doesn't seem to have that, although it uses so little power it probably makes little difference.

My car makes odd sounds sometimes while sitting in the garage even if not plugged in. Some people think it's the battery management system, maybe but I suppose it could be other things also. Whatever it is might be running off of the 12V not the HV battery, but I don't know. As far as I know the HV system is only active while charging, when in READY mode, and while remote climate is running.

Some people have said they didn't use or charge their Clarity for several weeks and it started right up. However based on the number of people who have returned home to dead batteries after a week or two, I would not be surprised if Clarity uses more 12V power than is typical. However even so I suspect that their 12V battery was already pretty weak to die in that short of time, and the slightly higher drain of the Clarity probably exacerbates the situation.

I also suspect that people who go three weeks and say "the 12V battery was fine", well what they are really saying is just that the car started. It's possible that their 12V was in fact drained down pretty far, and they may have been closer to being discharged than they realized, but they would have no way to know that since it takes very little power to start the car (i.e. get it into READY mode). Unlike a gas car where the 12V battery has to spin the starter motor and the engine, so you will usually get a sense of the battery getting weak. With the Clarity (any hybrid actually) if you have difficulty getting the car into READY mode then the 12V battery is probably pretty close to being dead and should be fully charged before trying again to start the car, otherwise you run the risk of permanently killing the 12V, or at least greatly shortening its life.

One way to charge the 12V in that situation if you don't have a battery charger but you do have jumper cables or a jumpstart battery, is to connect the jumper cables, then plug in the charge cable and get charging started. Once charging is started you can then disconnect the jumper cables, and let it charge for a couple of hours before trying to start the car again.
I don't think you'll get much of a charge by doing what you suggest in your last paragraph. First, I think that would only work at all if your traction battery needs charging because (according to what others have said that I believe is correct), your 12v battery will only get charged at all while your traction battery is charging. Second, it sounds like it is a trickle charge. A couple of hours of charging will do something, but not much. I have a battery charger and one of those rechargeable portable jump starters that you can also plug USB devices into. You don't need much of a jump starter because the 12v system would usually only a tiny amount of power as no engine is being cranked. If your traction battery is dead, I suppose it might need to crank the engine, but the Clarity's engine is small enough that, even in that scenario, any portable jump starter pack should do. I used one with my Prius Prime a few times with no issues.

Things like the LTE and keyless proximity sensor create what the 12v industry calls "parasitic drain." The parasitic drain is the amount of 12v power your car is using while it's off. If you add an alarm (which I always do) and a dash camera (which I also always do), the parasitic drain gets higher. I've never had any problem with the 12v battery in the Clarity getting too discharged, and I frequently leave my Clarity sitting for days at a time. Of course, who knows how low the charge is actually getting during that period. Ultimately, as long as I can get at least three years out of a 12v car battery, I'm happy. In my experience, many car batteries fail in that span of time, anyway.

If the Clarity is like the Prius Prime, I believe that when it is in "READY" mode, the traction battery is charging the 12v battery. Also, if it is like the Prius Prime, when the traction battery gets low, if the Clarity is in READY mode, it will start its engine to charge the traction battery. This can allow you to use your Clarity as a type of generator. The downside is that if your Clarity gets to the point where it's running its engine, it will create exhaust that can kill you if it is in your garage or near a tent, etc., so be very cautious about doing this. Also, you have to leave your key in your Clarity to keep it in READY mode.
 

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I don't think you'll get much of a charge by doing what you suggest in your last paragraph. First, I think that would only work at all if your traction battery needs charging because (according to what others have said that I believe is correct), your 12v battery will only get charged at all while your traction battery is charging. Second, it sounds like it is a trickle charge. A couple of hours of charging will do something, but not much. I have a battery charger and one of those rechargeable portable jump starters that you can also plug USB devices into. You don't need much of a jump starter because the 12v system would usually only a tiny amount of power as no engine is being cranked. If your traction battery is dead, I suppose it might need to crank the engine, but the Clarity's engine is small enough that, even in that scenario, any portable jump starter pack should do. I used one with my Prius Prime a few times with no issues.

Things like the LTE and keyless proximity sensor create what the 12v industry calls "parasitic drain." The parasitic drain is the amount of 12v power your car is using while it's off. If you add an alarm (which I always do) and a dash camera (which I also always do), the parasitic drain gets higher. I've never had any problem with the 12v battery in the Clarity getting too discharged, and I frequently leave my Clarity sitting for days at a time. Of course, who knows how low the charge is actually getting during that period. Ultimately, as long as I can get at least three years out of a 12v car battery, I'm happy. In my experience, many car batteries fail in that span of time, anyway.

If the Clarity is like the Prius Prime, I believe that when it is in "READY" mode, the traction battery is charging the 12v battery. Also, if it is like the Prius Prime, when the traction battery gets low, if the Clarity is in READY mode, it will start its engine to charge the traction battery. This can allow you to use your Clarity as a type of generator. The downside is that if your Clarity gets to the point where it's running its engine, it will create exhaust that can kill you if it is in your garage or near a tent, etc., so be very cautious about doing this. Also, you have to leave your key in your Clarity to keep it in READY mode.
A regular battery charger would be the best way to do it, but surprisingly not a lot of people have one which is why I mentioned the HV charging method as an alternative, although it does require either jumper cables or a portable battery, because if the 12V is too weak to get the car into READY mode, then it's unlikely it will be able to get the system powered up enough to start charging.

I was going to mention jump starting the car and then leaving the car in READY mode as another method but it required all of the details that you mentioned, along with other "ifs" that make explaining all of that somewhat lengthy, and I felt my post was already way too long. The first if is whether the car is outdoors or not. And if outdoors is it okay to leave it unattended since you can't lock the door. As for exhaust, carport might be okay, but enclosed garage even with the door open, probably not because wind could blow the exhaust back into the garage. Unless you can first ascertain the level of the HV battery, if the HV battery is nearly full then it would take many hours to drain the HV battery while sitting in READY mode. Of course making sure climate is off, and also you need to start the car with the parking brake set to keep the DRL's from coming on, to minimize the drain on the HV battery. All of this is difficult to do when the car is being jump started, but it would be an option if as you mentioned the HV battery was nearly full and all that you had was a level 2 charger. But if a level 1 charger is available you should be able to get at least an hour or two of charging unless the HV battery was already full.

The 12V battery gets charged three different ways, or at least 13+ volts can be seen in three situations, basically any time that the HV system is running, which is in READY mode, while charging the HV battery, and while preconditioning. I wouldn't think that 12V battery charging works differently in any of those modes, but someone would have to connect an ammeter to measure actual current to see whether that is the case.
 

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Is jumping the 12v with a portable battery jumper the same as other cars?

Connect charger to positive first, then negative.
Press any button if necessary on the remote jump starter.
Go inside the Clarity and hold break pedal and press start to enable "Ready" mode?
Then, to disconnect the charger, remove the negative connector first, then positive?

Finally, I assume I should leave it in Ready mode for about 20-30 minutes to keep the 12v charged in either EV or HV mode.
 

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I don't think you'll get much of a charge by doing what you suggest in your last paragraph. First, I think that would only work at all if your traction battery needs charging because (according to what others have said that I believe is correct), your 12v battery will only get charged at all while your traction battery is charging. Second, it sounds like it is a trickle charge. A couple of hours of charging will do something, but not much. I have a battery charger and one of those rechargeable portable jump starters that you can also plug USB devices into. You don't need much of a jump starter because the 12v system would usually only a tiny amount of power as no engine is being cranked. If your traction battery is dead, I suppose it might need to crank the engine, but the Clarity's engine is small enough that, even in that scenario, any portable jump starter pack should do. I used one with my Prius Prime a few times with no issues.

Things like the LTE and keyless proximity sensor create what the 12v industry calls "parasitic drain." The parasitic drain is the amount of 12v power your car is using while it's off. If you add an alarm (which I always do) and a dash camera (which I also always do), the parasitic drain gets higher. I've never had any problem with the 12v battery in the Clarity getting too discharged, and I frequently leave my Clarity sitting for days at a time. Of course, who knows how low the charge is actually getting during that period. Ultimately, as long as I can get at least three years out of a 12v car battery, I'm happy. In my experience, many car batteries fail in that span of time, anyway.

If the Clarity is like the Prius Prime, I believe that when it is in "READY" mode, the traction battery is charging the 12v battery. Also, if it is like the Prius Prime, when the traction battery gets low, if the Clarity is in READY mode, it will start its engine to charge the traction battery. This can allow you to use your Clarity as a type of generator. The downside is that if your Clarity gets to the point where it's running its engine, it will create exhaust that can kill you if it is in your garage or near a tent, etc., so be very cautious about doing this. Also, you have to leave your key in your Clarity to keep it in READY mode.
My Clarity had drained the battery once due to parasitic drain. I let it sit in the garage for several weeks without driving it. The battery went dead and I couldn’t even unlock the car with the key fob. I put a charger on it just long enough for it to get enough charge so I could turn it on in “ready” mode. Disconnected the charger and took it out for a drive. While driving it charged up he battery sufficiently.
 
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