I left the FEH garaged for 10 days while I was on a trip. Came home to find it would not crank and under hood battery was low (∼9vdc). I charged the under hood battery and was able to read the HV battery SoC as 30%, and the car started right up. This is the first time ever that I have seen both the under hood battery and HV battery discharged at the same time. I had the battery shop test the 12v battery and they said it was "good".

So my question is, when the FEH is parked with key OFF, is there any connection between the two batteries? If the HV battery was self-discharging to a low state (which it does occasionally) could that also discharge the 12v battery over time? And conversely could a failing 12v battery draw down the HV battery? Anybody know?

 

Go read your owner's manual. It explicitly states that the 12V should be disconnected during extended periods of storage (>30 days). Failure to do so will cause the BOTH the 12V and HV battery to discharge. I can only surmise that by keeping the BCM powered via the 12V system, it in some way discharges the blocks through their monitoring circuit.

I posted on GH concerning a MMH owner who left his 12V plugged in and on trickle charger for 6 months while away at his winter home. 12V was fine upon his return, but his HV battery was completely flat. No HV jump start on that year. Was an '09 with only 64K miles and should have been under warranty, but Ford denied any warranty for failing to follow the owner's manual procedures. I had to disassemble the battery and manually charge it.

IMHO, your 12V is NOT "good." A "good" 12V will not go flat in 10 days (9V is beyond flat), and this incident has further damaged your battery substantially. Replace it. Don't care how old it is. It's going to get you sooner or later. Next time you're going to be gone for more than a week.

The only way your 12V is "good" is if you have a parasitic draw on the 12V. You should track that down and fix it.

 

Also wierd things happen as these get older.

Like a door switch fail.
If it thinks you opened a car door it will "wake up" the car for 10 minutes and pressurize the brakes. Over and over.

Or car alarm circuit fail. Actuvates horn and blinking lights over and over.

My 2005 FEH started doing this at 10+ years but we drove it every day so it got recharged every day.

The HV batteries will self discharge faster and faster with each passing day. At 10 years losing 1% a day is common.
And remember- you PARK IT at only 40% to 53%. Never 100%! And the car won't start below 20%!

So when your hybrid is 10+ years old you may only be able to park it for 20 to 33 days in best case scenarios.

 

the car won't start below 20%!

I just started my 2005 FEH with all 03% SOC today. Did not need a jump start. I'm the original owner followed the forum a lot during its heyday in 2006+ when GaryG, gpsman1, DesertDog (Carl from Scanguage) Bill Winey, etc were posting and providing a lot of helpful information. Also interesting tic to tac replies. I got almost 200,000 miles on my FEH and have in the last year noted a significant drop in "resting" SOC if it sits for 2-3 days. Starts up and the SOC climbs back up to the 53% range. Still can pull 32-35mpg (no significant headwind and more summer like temperatures) on the highway at 55-63mph. Still burn more E85 than E10 without issue.

 

Good to see some old familiar names back!

I'm coming up on 100k with my '07 FEH. Nothing but routine service so far, even the original brakes. But if I keep it will probably need a fairly extensive 100k mile service.
If I get another hybrid it'll be the Toyota RAV4.

The only obvious symptom of the HV battery degrading is with regenerative braking being weak all the time. Instead of regen charging the battery up to 60% SoC on a downgrade the ICE will spin up.

 

I got a 2017 CMax last June with all 301 miles on it for almost 40% off the retail price. The CMax gets much better mpg than the FEH but has a serious flaw with its air cooled HVB pack that dates back to 2010 design change with the FEH. In short, it does not keep the HVB in a cooled ranged during the hot summer days. Sitting in the parking lot during the hot summer day, the HVB acts as a heat sink. Charging with the level 2 (240 amps) utilizes 9 amps and will also heat up the battery pack. One has to monitor the HVB pack in an attempt to minimize degrading it. Some have degraded the pack over 1.0 kWh (out of a new usable 5.6kWh capacity) within the first 20,000 miles. Charging from the wall outlet (level 1) utilized around 3 amps and can actually lower the HVB pack temperature if done during the night. MY CMax also has some electrical issues that sometimes arise and have not been resolved at the dealership.

AS for my 2005 FEH, at around 105,000 miles a speed sensor went causing driving ability issues. The famous window vent on the rear cargo windows also went out around that time. The I had numerous suspension replacement parts done for the next 75 thousand miles. A body part rusted and had to be replaced to pass the state inspection. The low SOC has really developed this year and I need to take it to the dealership for a review/limited workup on if it is the battery cells finally wearing out/dying after 15 years and nearing 200,000 miles.

 

Car won't start at 20% has been proven to be false: Ford designed the HVB to last 15 years and it is showing the aging

 

I just found an OLD laptop in my garage I forgot I had.
Got it in 1998. Still has Windows98 on it.

It had not been plugged in at all since 2005.
I plugged it in and the battery got warm and recharged!!!

I'm posting here because the laptop battery was NiMH.
The same battery chemistry as the FEH.

It does not power that laptop very long (less than 1 hour) but I am shocked it recharged at all after 21 years. AND after sitting at dead zero for more than 10 years.

(The photos I had in this laptop are all 640x480 and 200 KB each.).