Why can't I "jump start" my Clarity via the main battery?

[bbct]

Yesterday my Clarity would not "turn on". All the indicators flickered and flashed, the map lights dimmed and flicked a bit. The dash would glow dimly then stop. Eventually it would tell me to push the button for accessory mode (not sure why). Any attempt to turn on the vehicle or turn on accessory mode was met by more flickering of lights and dash indicators. The brake would not release, I could not open the trunk, I could not take the car out of park...

I called the dealer service and he indicated it sounded like a dead 12v, and a jump might fix it. Sure enough, I used another vehicle to "jump" it. As soon as the batteries were connected, all seemed fine, and I "started" the vehicle. I know that leaving the vehicle in the On mode will charge the 12V. Perhaps I accidentally left it in accessory mode and killed the battery, I really don't know what did it. It seems fine now.

So here is my main question - my main drive-train battery was 80% charged. It charges the 12V when the vehicle is on. So why doesn't Honda provide a little switch, perhaps a rocker switch you need to press and hold even, to activate this circuit to "jump start" the 12v battery with technology that is already built into the vehicle? Why make me get another vehicle and jumper cables when there is this huge electric power supply built right into my vehicle?

Secondary question - even if I did leave accessory mode on, the car knows how much juice is left in the 12V battery, and how much it needs to "start" the vehicle. How hard would it be to shut off accessory mode if the battery gets below, say, 10V or whatever the main computer can live with? If it is already supposed to do this, it failed.

 

[Cash Traylor]

The 12 Volt battery is required to close the HV battery contactors that are always "open" in two conditions: Vehicle off and not charging, and if the 12volt battery is disconnected. The reason for this is NHTSA requirements for emergency responders to have a common way in passenger vehicles to "make them safe." In the normal car world after a crash the SOP for the rescue workers is to cut the 12volt battery lead. This disables the airbags (if not deployed) which are very dangerous to workers on damaged vehicles. It also helps prevent further shorts and fire risk. The same can be said for our cars where cutting only the 12volt battery out of the picture accomplishes the same. The downside is that you have to have that battery to "access" the high voltage stored in the Lithium pack. Now, on our cars there are other color coded HV lines that are indicated for cutting, however those are "icing" as fare as the NHTSA is concerned from the OEM.

The Clarity has an accessory timer that should have shut your 12 volt battery off. Depleting the 12volt battery should not be possibly unless you have installed (yes, I can say this because I HAVE installed such equipment) aftermarket things that draw from the "HOT" battery buss. Things like plug in ODBII dongles (that are always on, that port is always HOT unless it's fuse has blown), direct hot buss connections for add on car dash cams etc. Those can deplete the battery. The 12volt battery in the Clarity should last months between "power charge cycles" under normal lead acid self discharge and parasite "cpu/memory" vehicle loads. If your 12 volt battery is defective and has a shorting cell then that can cause your issues. I would recommend getting it tested at Honda. Now I say this kinda tongue and cheek. The "standard" car battery test is really not that helpful as you don't need "cranking amps" (it literally takes only a few amps to close those contactors/relays) you need "reserve capacity" which is a measure of usable amp hours over time. In my industry it is actually called a "capacity check" where the battery is drained as a constant load over a set time against its "ratings." That is expensive and not feasible here, but my batteries are $5000 each so worth it and required by regulations. For our case the battery "short current" test is useful enough for battery health and the criteria that Honda will use to prorate your battery for replacement. They can also do a parasitic load test to see if you have any unusual battery drain (this is common in most all vehicles as the manufacturer publishes a maximum current drain when the vehicle is in standby) and they can detect problems from there.

* Now of an interesting side note. Unlike other cars that require significant 12volt Cranking Amps to turn the starter and get things going, you really need very little current. So, a very small portable 12 volt battery start pack can get your Clarity going in this case. Note it has to have leads to connect to your battery directly, not through the cigarette lighter / power ports as those are on a relay that disconnects them from the battery buss. But you get the idea, something that would fit in your glove box would work.
Cheap and small : https://www.harborfreight.com/lithium-ion-jump-starter-and-power-pack-62749.html

Good luck, but no you will never be able to use the HV battery to jump your 12 volt battery due to "safety regulations" in design criteria.

Cheers,
Cash

 

[Mowcowbell]

That would be a good idea... think the Hyundai Ioniq has that switch that lets you 'jump' the 12v. For my Honda, I carry around a small lithium-ion jump pack in case I need a jump start. They're not expensive and definitely work.

And for 12v battery maintenance I hook up my Battery Tender about once a month and let it trickle charge the battery overnight.

 

[TomL]

The 12 Volt battery is required to close the HV battery contactors that are always "open" in two conditions: Vehicle off and not charging, and if the 12volt battery is disconnected. The reason for this is NHTSA requirements for emergency responders to have a common way in passenger vehicles to "make them safe." In the normal car world after a crash the SOP for the rescue workers is to cut the 12volt battery lead. This disables the airbags (if not deployed) which are very dangerous to workers on damaged vehicles. It also helps prevent further shorts and fire risk. The same can be said for our cars where cutting only the 12volt battery out of the picture accomplishes the same. The downside is that you have to have that battery to "access" the high voltage stored in the Lithium pack. Now, on our cars there are other color coded HV lines that are indicated for cutting, however those are "icing" as fare as the NHTSA is concerned from the OEM.

The Clarity has an accessory timer that should have shut your 12 volt battery off. Depleting the 12volt battery should not be possibly unless you have installed (yes, I can say this because I HAVE installed such equipment) aftermarket things that draw from the "HOT" battery buss. Things like plug in ODBII dongles (that are always on, that port is always HOT unless it's fuse has blown), direct hot buss connections for add on car dash cams etc. Those can deplete the battery. The 12volt battery in the Clarity should last months between "power charge cycles" under normal lead acid self discharge and parasite "cpu/memory" vehicle loads. If your 12 volt battery is defective and has a shorting cell then that can cause your issues. I would recommend getting it tested at Honda. Now I say this kinda tongue and cheek. The "standard" car battery test is really not that helpful as you don't need "cranking amps" (it literally takes only a few amps to close those contactors/relays) you need "reserve capacity" which is a measure of usable amp hours over time. In my industry it is actually called a "capacity check" where the battery is drained as a constant load over a set time against its "ratings." That is expensive and not feasible here, but my batteries are $5000 each so worth it and required by regulations. For our case the battery "short current" test is useful enough for battery health and the criteria that Honda will use to prorate your battery for replacement. They can also do a parasitic load test to see if you have any unusual battery drain (this is common in most all vehicles as the manufacturer publishes a maximum current drain when the vehicle is in standby) and they can detect problems from there.

* Now of an interesting side note. Unlike other cars that require significant 12volt Cranking Amps to turn the starter and get things going, you really need very little current. So, a very small portable 12 volt battery start pack can get your Clarity going in this case. Note it has to have leads to connect to your battery directly, not through the cigarette lighter / power ports as those are on a relay that disconnects them from the battery buss. But you get the idea, something that would fit in your glove box would work.
Cheap and small : https://www.harborfreight.com/lithium-ion-jump-starter-and-power-pack-62749.html

Good luck, but no you will never be able to use the HV battery to jump your 12 volt battery due to "safety regulations" in design criteria.

Cheers,
Cash

Thank you Crash Traylor. Most enlightening.

 

[css28]

That would be a good idea... think the Hyundai Ioniq has that switch that lets you 'jump' the 12v.

The key difference with the Ioniq is that the 12V battery is internal to the traction battery case, so there's no external wiring between the two. For original purchasers, the traction battery (with integrated 12V battery) has a lifetime warranty.

 

[DucRider]

The key difference with the Ioniq is that the 12V battery is internal to the traction battery case, so there's no external wiring between the two. For original purchasers, the traction battery (with integrated 12V battery) has a lifetime warranty.

IIRC, that is only on the PHEV (and Hybrid?) but not the Electric. It is an electronically separate li-ion battery contained in the main battery pack. It does have internal "jumper cables" that allow a button on the dash to be pushed to recharge the 12V portion if it dies. Lower right button:

 

[sniwallof]

... but not the Electric. ...

Had my hopes up that I missed something in the Hyundai Ionic electric, but no such luck, and many Ioniq electric drivers carry GB40's or equivalent. Little 12V battery is up front under the hood, '19 Limited (heat pump is behind the 12V battery, and hp valves are over the left wheel well):

Seems Hyundai dropped the heat pump from 2020 U.S. limited, odd. European versions (3 trim levels) mid and highest level got the heat pump.

 

[petteyg359]

The controller to detect the charge level of the 12V and whether it needs boosting from the big battery is probably powered by the 12V itself. If the battery is dead, said controller can't close the relay to charge it.

 

[vicw]

It's been 2+ years since we bought our 2018 Clarity, and this forum continues to teach me something new almost every time I check in. I am thankful for @Cash Traylor for his excellent clarification of the 12V battery vulnerability on the Clarity.

This thread reminded me of an incident we had some years ago with our 35 month old Odyssey, when the battery failed while we were picking up our grandson from school. We waited for hours for roadside service to arrive, and when he did, the tech refused to jump start us, as unsafe to do. The only option he offered was a tow to a nearby garage, which we declined. We finally reached our daughter at work, and she gave us a jump start from her car. The next morning I took the car into our local Honda dealer, who replaced the battery as a warranty item, with no charge to us.

I didn't know that such a compact car charger, as @Cash Traylor described, was available, and I purchased one from Amazon. Sudden battery failures are always a potential risk, irrespective of the age of the battery, and I am expecting that it can help us avoid another future wasted afternoon, if our 12V battery dies on us.

 

[TomL]

It's been 2+ years since we bought our 2018 Clarity, and this forum continues to teach me something new almost every time I check in. I am thankful for @Cash Traylor for his excellent clarification of the 12V battery vulnerability on the Clarity.

This thread reminded me of an incident we had some years ago with our 35 month old Odyssey, when the battery failed while we were picking up our grandson from school. We waited for hours for roadside service to arrive, and when he did, the tech refused to jump start us, as unsafe to do. The only option he offered was a tow to a nearby garage, which we declined. We finally reached our daughter at work, and she gave us a jump start from her car. The next morning I took the car into our local Honda dealer, who replaced the battery as a warranty item, with no charge to us.

I didn't know that such a compact car charger, as @Cash Traylor described, was available, and I purchased one from Amazon. Sudden battery failures are always a potential risk, irrespective of the age of the battery, and I am expecting that it can help us avoid another future wasted afternoon, if our 12V battery dies on us.

After @CashTrayler's contribution I also bought a compact jump starter on Amazon catching a lightning deal at 50% off for $49.

 

[Mowcowbell]

The Clarity is not unique in being dependent on a old-school 12v battery.

Even the Tesla Model 3 has one. It's tucked under a plastic cover underneath the wiper blades. A Tesla with a dead 12v battery is just as unable to start as any other vehicle. At least the Tesla's computer will warn you of low 12v battery to give some time to replace it.

 

[eneka]

The Clarity has an accessory timer that should have shut your 12 volt battery off. Depleting the 12volt battery should not be possibly unless you have installed (yes, I can say this because I HAVE installed such equipment) aftermarket things that draw from the "HOT" battery buss.

fwiw, the 12v battery can most certainly be drained if you keep the ignition on "acc" mode. I accidentally left it in that state and came up to a car with a completely dead 12v battery. Also learned it was pretty much impossible to move the vehicle when that happens..push button gear shifter + electric parking brake. Luckily it jumps right back up with a battery pack and charged the 12v battery fairly quickly.

 

[petteyg359]

fwiw, the 12v battery can most certainly be drained if you keep the ignition on "acc" mode. I accidentally left it in that state and came up to a car with a completely dead 12v battery. Also learned it was pretty much impossible to move the vehicle when that happens..push button gear shifter + electric parking brake. Luckily it jumps right back up with a battery pack and charged the 12v battery fairly quickly.

I'd like to point out a dead 12V battery will also prevent you from moving an older car. My 2001 CR-V, for example. There is an electronic lock on the gear shift; you cannot move it without the key turned to IV (all power on (A/C, power windows, lights)) or the engine running. If you parked it and the battery dies enough to not operate that lock, you need a jump to get it into neutral.

 

[eneka]

I'd like to point out a dead 12V battery will also prevent you from moving an older car. My 2001 CR-V, for example. There is an electronic lock on the gear shift; you cannot move it without the key turned to IV (all power on (A/C, power windows, lights)) or the engine running. If you parked it and the battery dies enough to not operate that lock, you need a jump to get it into neutral.

Past Hondas have a little switch next to the gear shifter where you can pop off the cover, and stick in your key/screwdriver to disengage the shift lock manually move the gear shifter into neutral. Our '05 Odyssey and '11 CRV both have this. Heres the 2001 crv owners manual

 

[JCA]

fwiw, the 12v battery can most certainly be drained if you keep the ignition on "acc" mode. I accidentally left it in that state and came up to a car with a completely dead 12v battery. Also learned it was pretty much impossible to move the vehicle when that happens..push button gear shifter + electric parking brake. Luckily it jumps right back up with a battery pack and charged the 12v battery fairly quickly.

The 12V battery will also drain in "On" mode (press Power twice without foot on brake), even more so with the fan and other components on, just like any other car. Someone here found that out the hard way The high voltage battery is only engaged and keeping the 12V battery charged when in "Ready" mode (or while the car is charging).

 

[Arctiic]

The Clarity has an accessory timer that should have shut your 12 volt battery off. Depleting the 12volt battery should not be possibly unless you have installed (yes, I can say this because I HAVE installed such equipment) aftermarket things that draw from the "HOT" battery buss.

Just wanted to point out that it appears rooting the HMI and using a modified kernel can also seemingly disable the shut-off you've mentioned (I have nothing aftermarket installed hardware-wise).

 

[The Gadgeteer]

My jump pack is in the trunk in the lower compartment. Good thing there is a way to get into the truck with a dead battery.

Open driver’s door with the manual key, open back door from inside, use manual key in keyhole behind backseat headrest, trunk pops open.

 

[su_A_ve]

These battery packs (I got a 600A peak Gooloo) are incredible and must have. Got it a couple of years ago for a cross country road trip. Used it a few times to jump my parents car (new Subaru with under powered factory battery).

And a few days ago, I noticed a neighbor trying to push start an Odyssey (facepalm). Came out and we was amazed about the little battery pack. Two minutes.

 

[Landshark]

Just wanted to point out that it appears rooting the HMI and using a modified kernel can also seemingly disable the shut-off you've mentioned (I have nothing aftermarket installed hardware-wise).

I’m pretty sure I’ll be safe from the risk of a seemingly disabled accessory timer. Out of curiosity, how would one root the HMI and modify a kernel?