Was working on getting the garden ready this weekend and had the 12v battery go dead. Since I was actively present while the whole thing unfolded, I though I'd pass along the experience.

I had the back doors and the lift gate open, the switch was in the ACC position and the radio was on. No other drains except at the beginning when the interior lighting was on from closing/opening doors.

I listened to the FM radio for at least 2 hours and then it acted as it if were losing the station's signal. This lasted for about 5 seconds and then everything went black.

I opened the front door and turned the key off/on. No clicks from the relays, no pressurizing from the brake system, and no display on anything. same thing on a few more key off/on cycles.

I went inside and grabbed the plug-in battery charger, attached it to the battery. Cut the switch off/on and nothing. Off/on again and the relays clicked and the FEH started right up. I promptly disconnected the charger. I let the FEH run, it went though several ICE-ON/EV cycles as I finished up my work and listened to the radio. After about 30 minutes, I turned the switch off. Let it sit for 5 minutes and then restarted without an issue. It has restarted at least 5 times since then without issue.

During this whole process (from the radio cutting out until after the last restart) the battery indicator (little glass eye on the battery itself) was a dim RED. I remember it being dim GREEN earlier in it's life. I've had my FEH since August 2007. So the battery is over 42 months old. I'm betting once you factor in lead time from the battery manufacturer and shelf/build time at Ford, the battery is about 4yrs old.

In any case, I'll keep an eye on it and know that a replacement 12v is in my near future. Fair enough.

I also need to check the water fill level in the battery. I'll do that this afternoon when I have access to the vehicle again. I'll also check the RED eye/GREEN eye situation.

 

I just had to replace mine. No issues starting mine. I was driving up into the mountains and it started hesitating like it was missing. Turned around and when to the dealership as I thought it was fuel related since i had just changed the fuel filter. Service Dept.checked the codes and said it was the 12v battery. I was quite surprised.

It's been about a month and no problems and it runs like a top.

 

This is a fascinating process for me. Thanks to the
two of you for sharing your experiences. IMO as an
old engineer the 12vdc low voltage battery in the
FEH is being used in a different set of circumstances
then it was primarily designed to do. And that
being supplying a big surge ( 100's of amps ) to
turn a large DC motor to crank a flywheel to start
a car. All of us have experienced the dying car
battery syndrome. Over time the cranking gets
slower and slower telling us that it's time to
change the battery. But in the case of the FEH
the lead acid battery goes over the edge
of it's power curve quickly and you don't know
it's coming till it happens. IMO I would suggest
that as your battery gets to 36-months of age
that about every 6 months just take it into
any of the auto parts stores and they can
check your battery. It's all usually free.
If you have your FEH maintenanced on a regular
basis checking the battery under a load should
be a check off item. The last check for my 06
FEH was at a Ford dealer and they certified it
to 500A cranking power. And they also confirmed
that the charging system is working. My battery
is about a year old.

Just in case I purchased a Peak Portable Power
System 450 Plus.

http://www.peakauto.com/electronics-..._systems.shtml

An extremely useful device not only for starting
the neighbor's car when his battery died, but it
powers lots of other things. Along with the jumper
cables it includes 115VAC, USB and +12vdc outputs.

 

I had thought that the switch in the floor well was for using the traction battery to charge the 12v battery. It turns out that the opposite is true; the switch is used to charge the traction battery FROM the 12v if the traction battery goes flat. I'm not sure how the main battery could go that flat, considering the protections built in.

 

I think, and I'm only guessing here, that if the HV
( traction battery if you wish ) and the LV 12vdc
batteries both go dead then if you connect something
like my Peak Power Station ( 12dc jumper ) to the
LV terminals and go through the 8min recharge process
with the switch in the floor well. Then the HV battery
will be recharged enough that and you can start the FEH.
I think.

 

Yes, that is the way I read the manual. But I still don't know how the HV battery would go that flat.

 

I also am of the understanding that so long as
the LV battery has some juice in it that it takes
very little energy from the HV battery to spin
up the ICE.

 

I found with about 3 weeks' of storage it will go pretty flat. It probably goes even faster in cold climates.
I've also found that an AGM (dry-cell) 12v battery solves these types of issues since they can be deep discharged repeatedly.

 

You are saying the High Voltage battery will go flat in 3 weeks? What is drawing on that voltage when the FEH is not turned on? The electronics are all run by the 12v system.

Here is what the 2008 Owners Manual States:

"Low and high voltage battery – storage

Your vehicle must be started and run for a minimum of 10 minutes once
a month in order to maintain the high voltage battery charge. This will
maintain the high voltage battery but it is not enough to maintain the
low voltage (underhood) battery and additional low voltage (underhood)
battery charging may be required after 60 days.

If your vehicle is to be stored for 30 days or longer, the low voltage
(underhood) battery negative terminal must be disconnected. Failure to
do this could damage your vehicle’s batteries."

I don't know if this means it will be depleted, or just begin to lose charge.

 

I would say that the manual is very conservative.
I doubt there is much draw from the LV battery when
the FEH is off. I typically can go a week or more without
using my FEH and don't seem to encounter a problem.
60-days of storage or beyond I would certainly start that
storage period with a fully charged LV & HV batteries.