Looks like most have left this forum, given up or managed to sell off their dying HCH II's. You are the lucky ones. I've had mine for sale for months and I'm only asking mid 10's for an 06 with 49K and another year of Hondacare warranty. Kelley Blue Book says I should get mid 13's, but people have figured this car out and are wary. I keep dropping the price because it keeps breaking down and I'm tired of having it towed. Maybe someone can offer some advice for my dealer - because they are baffled.

The big battery died about 2 years ago and the software update reduced the MPG to around 29 now. Shortly after that, the car died on the road. It just sputtered down to a stop and the 12V battery was dead. This has happened EIGHT times in the past 20 months. I can jump the car to get it started, then it dies again within a minute or two - as if the 12V is not being charged by the DC to DC.

1st time; They replaced the 12V and said the battery had just died.
2nd time; Said the battery was fine and the car was fine.
3rd time; DC to DC needed updating per recall, fine now.
4th time; Replaced the 12V and said everything is fine.
5th time; Said the wiring was wrong and it's fine now.
6th time; Said they had no idea what happened, but it's fine now.
7th time; recall needed for software update, fine now.
8th time; 12V is very strong now, but they had to jump it...fine now.

The dealer is using something outside the vehicle to charge the battery, then runs it for a while and says the car is fixed, but they can never tell me why it keeps dying.

Do any of you guys have any clues?

 

I'd monitor 12V in the car while you drive- either with scangauge or a multimeter. When driving typically DC-DC is on and charging at 13.6+ volts. Under light load and when 12V battery is fully charged, it may shut the charger off for a bit, but as soon as a load comes on, like headlights or fans etc it should come back up. So either the converter is not charging your 12V or you have some drain somewhere else discharging the battery

 

You mentioned fans, does the HVAC (blower) fan run off the 12V battery or off the big one?

 

I had to have my Battery Control Module replaced under warranty last year but my symptoms weren't quite like yours. In fact, even with a defective BCM and disabled IMA, my system still was able to keep DC-DC charging active and I drove the car for nearly 2 months before Honda finally ordered the part and replaced it.

Everything runs off the 12v battery from what I can see. You need enough 12v power to turn the computer on to send the "Start" command to the IMA system to start the car, for example (even though the IMA pack actually starts the engine). Only IMA motor power is pulled from the HV pack as far as I know. While the engine is running, the IMA system supplies 12v charging to the battery and 12v system (so just like in a regular car, you can disconnect the battery while it's running and DC-DC charging will keep your electronics on).

Sounds like you know what is going on with DC-DC charging dropping out, but convincing Honda to replace more parts under warranty until it works can be a tough challenge. The best advice I can give is constantly monitor 12v circuits during different types of driving. Try disconnecting the 12v battery with the car running, that's been an easy age-old test of your alternator. If the car stalls or can't supply enough juice for say headlights, you've just proven DC-DC converter failure. Hope this helps...