Dead 12 volt battery experience

My measurements, while parked in EV mode, showed more variation in voltage than what you’ve experienced. I don’t recall the exact numbers, however, just powering up the car brought voltage from ~12.7 to 14 and change. As I turned accessories on, the voltage increased as demand increased. The highest voltage was around 14.6.

A DC/DC converter/charger certainly has the ability to to control a charge beyond just zapping 14.6V constantly. That actually seems a tad high, as most alternator equipped vehicles put out 13.9-14.0V.

It is beyond peculiar, that the battery reads 12.6V while in HV, with the headlights set to Auto, but in all other observed situations the voltage is 14V or more.

 

I usually have the radio ON, via XM, and Climate Control active while driving, so that might explain the lack of incremental variations of the applied voltage on my monitoring.

I totally agree. I don't have the resources, or skills to investigate that, but the 12.6 v charge in HV seems really anemic or nonexistent to me, and since my infrequent long drives in HV mode sometimes end up at the airport, with my car being left for multiple days, I really don't want to risk a low battery upon my return, and I consider use of the daylight headlight a reasonable way to assure I'm giving it a rational boost during that trip, keeping in mind that I must switch the headlights OFF, or back to AUTO before parking the car. I'm considering the alternative of using something to block the daylight sensor during those HV daylight drives, leaving the headlights in AUTO mode, to avoid any possibility of leaving the headlights on after I leave the car.

I just remembered one more twist to the HV vs battery charge scenario - if I start the drive in HV mode, during daylight, with the headlights in AUTO mode, and during the drive, switch the car back to EV mode, the 12.6 V applied voltage does not go back up to the 14.6 V level that is normal with EV mode, until I stop, turn the car off and back on to resume driving. I can't say that is an absolute behavior, but I did notice it happen recently, and I just repeated the same pattern on a short drive and confirmed it again.

 

Thanks to all of the contributors on this thread. I was able to use much of the great information and hints to help a friend with their Clarity.

She had been out of the country for seven months leaving the Honda parked and locked in her garage. I alerted her prior to her return that she might have a dead battery with the Honda which of course was the case. She called me up upon arrival and let me know she couldn’t even open the car with the key fob. I told her how to pull out the actual metal key from the fob which she did. I asked her to plug in the EV charge cord but access to the door is electrically controlled and so no go.

I drove to her location in our own Clarity and proceeded to get the car started. My little Li-On power bank was successful in getting the electronics back on. Interestingly the alarm immediately was set off but silenced quickly with the key fob. Got the car turned on and knew at that time it should start charging. I was able to drive the car with all the previously mentioned warning messages. I drove it for about 10 minutes and shut it off. Oops, not enough energy yet to restart car, power bank again got me going. Note that HV battery was at 100%, had not lost appreciable energy in 7 months.

I failed to bring my multimeter or battery charger so decided to drive that Clarity back to my house 15 minutes away and pick up these. When I reached my home I checked battery voltage and found it to be less than 10V. So driving vehicle for several minutes did not recharge this very dead battery enough. I put the car on the battery charger for about 25 minutes and then checked the battery voltage to find it was now just under 12 V. Surprisingly this still was not enough to start the car so I again used the power bank to get the car started.

Drove car back to friends house at which time the HV battery had lost some of its capacity. Parked the car back in the garage and checked the battery voltage which was now a little over 12V. Then hooked up the EV charge cord which is on a 110V circuit and noticed battery voltage at about 13.3 V. So while the HV battery is being charged the 12V battery is receiving a trickle charge.

Takeaways: Invest in a power bank jump starter, I always carry it in our Clarity. Charge HV battery often to keep 12V battery at highest condition.



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@LarryC

Reference your statement..."I asked her to plug in the EV charge cord but access to the door is electrically controlled and so no go"...

There is a mechanical release for the charge door under the driver side dash behind the OBD port. The handle is yellow in the Clarity I drive.

Additionally, if you aren't aware...the mechanical release for the trunk is behind the rear seat driver's side and uses the small key stored in your fob.

And finally...the mechanical release for the petrol lid is inside the trunk, left side under the "warning...fire risk" cover.

Hope this helps.

 

I believe the manual charge-door release is only for use if the release-solenoid fails. My experience is that plugging into my EVSE doesn't charge the 12V battery so opening the charge door manually wouldn't help with a dead 12V battery. However, it's good that you pointed out that manual release because it's not obvious.

I had to open the hood and charge the 12V battery before anything else (eg. HV charging) was possible. If the 12V battery has enough charge to operate the car's electrical system, then the HV battery can finish charging the 12V battery, but only while the Clarity is "ON."

 

Actually the EVSE does trickle charge the 12V battery IF the HV battery is not at 100%. The charging to both batteries will cease when HV battery reaches 100%.


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But my experience was that no trickle charging takes place if the 12V battery is dead. Perhaps my experience might have been unique--it was certainly uncomfortable! If I had plugged in my Clarity PHEV periodically after the HV battery was full, would it have trickle charged the 12V battery to keep it viable? I don't like leaving the car "On" to have the HV battery charge the 12V battery--I'm afraid I'll forget and someone will drive off with my car.

 

Allegedly, no charging will occur when the HV battery is fully charged. If true in all cases, periodically plugging in with a fully charged HV battery will not charge the 12V battery.

It also seems logical that if a 12V power source is required to operate certain functions, those functions would not operate in the absence of a 12V power source. It’s odd, not for Honda, that connecting power to the battery charger would not charge either battery if the 12V battery was below a certain voltage.

Does that help clarify things, or just add to the confusion?

 

It's best to leave the traction battery at 50% if left unused for any significant length of time. In fact, BEV owners are advised not to charge to 100% unless necessary (e.g., road trips).

 

That limited-charge advice may hold for BEVs with large batteries with small buffers (Tesla), but not for electrified vehicles like my Clarity PHEV and my MINI Cooper SE with small batteries and relatively large buffers. Manufacturers of vehicles with small batteries know that owners will not be running outside to stop charging at 50% or 80%. For us, every trip qualifies as a "road trip."

In fact, my Honda Clarity Plug-In Hybrid Owners Manual says on page 466, "To help extend the lifespan of the battery, it is recommended that you fully charge the battery each time prior to driving."

I'm not sure if the buffer is large enough to make it OK to leave the battery 100% charged for a long period of time in storage, however. So you're probably right about that.

 

Indeed it does. Of course the devil in the room might point out that it doesn’t say, “Charge the battery fully and leave it to sit for a year.”

The manufacturer of the lithium batteries that I recently installed in my motorhome actually recommends to fully charge them, after which they can be stored for up to 2 years.

We know the target voltage for a full charge, it’s about 90%. Personally, I’m not going to sweat it if the car sits for a month or two at 90%. It will allegedly self discharge at ~2% a month, so 5 months of sitting will bring it down to a nice, safe 80%.

 

Yes, I read that part. I was providing some information that might help clear up your uncertainties.

 

Do you think your theory could prove my experience -
The car was inoperable due to 12v depleted two times last week. Both times it left overnight being charged and plugged in, and both times the 12v battery showed ~7v when I measured it after not being able to start the car.
The DC-DC converter is doing its work - while the car runs on HV mode it shows 14.55V on the 12v battery, ~13.5v while it actively charges the main HV battery.
A voltmeter logger showed the voltage drops by 0.2V or so every hour, and this process started 3-4 hours later after the HV battery was fully charged, and the car was still plugged in.
I took the car this morning to my dealership service, but I am very doubtful about their ability to find the culprit.
Anyone else experienced the same?

 

I posted this in another thread. It does seem to suggest “recharging” the HV battery prior to leaving the car idle for a period of time and then recharging again at no less than 3 month intervals.

 

I posted this in another thread.