Your first warning is with the brakes. Remember the buzz sound when you open a car door?
This is the 12v battery pressurizing the hydraulic fluid.
The sound will change and sound weak or sick.
This takes a few amps.... not like the relay.
HTH.
My 12v was bad after 5 years. Heat and cold stresses these just like any other.
John

 

But in the end pressurizing the brake system doesn't take near the amps that starting an engine does. So my take is that these batteries will age out.

I took specific gravities on mine when new and did so the other day. Mines still going strong by these readings. Interestingly, the same two cells were 1 Std Dev out as when new; 1 hi & 1 lo.

 

Something I want too confirm, does a weak 12V battery decrease MPG? Yes it does! My 12V OEM battery was doing fine in my '09 FEH I thought, but when I parked it for a two weeks, it ran dead. I slowly recharged it at 2 amps, and again it ran dead in less than two weeks. Before the first battery drain, I noticed a unusual drop in MPG. I purchased a new OEM battery today and will update.

GaryG

 

Is your DC>DC charger working? A parasitic drain can cause that also.

The dealer replaced my 12V battery during a few repeated visits for a transaxle leak but I had no problem with it. Numerous short drives probably killed mine.

 

I would expect that behavior to indicate the computer losing its memory forcing the system to relearn the calibrations, which would result in increased ICE running....

 

It would have to be bone dead as you can keep the KAL memory going with a 9V alkaline battery. They sell little adapters just for that.

 

Guys, it was not KAM because the drop in mileage occurred weeks before parking my FEH for three months. The tires and rims exchanged dropped my mileage some, but I thought the E10 was getting old and was adding to the problem. When I noticed the dome light didn't come On, I knew the 12V battery was dead. I recharged it and monitored it, and a week later it was dead again.

I decided Monday to just replace the battery and see today if the mileage got better. Sure enough, I averaged mid 60mpg today and things are back where I expect the '09 to be with the Limited rims and newer Michelins. The '09 has the newer Lambda wideband H02 sensors, so disconnecting the 12V battery and losing KAM is not near as bad as the older FEH.

There is no Jump start switch on the '09 FEH, but I thought the BCM would not let the HV battery drop below 32%. After 3 months of not starting my FEH, it started great today. The SoC was at 32% just like I thought it would be, so I let it idle and charge to 40% before putting it in gear. My day ended with the NAV MPG at a 69.8mpg tank average. Everything got reset including the SG, so I had to set-up it when I noticed it went to sleep on me. Set the SGII Fuel to Hybrid, engine to 2.5L and I was back in business. I now only have one SGII in my '09 FEH because I put the second in my '12 Focus and got a new one for the '11 Explorer. I let my '05 FEH go with my oldest SGII to the new owner. You could not program SoC with that SGII version anyway.

There is no question in my mind now, that a 12V battery going south in an FEH, the mileage will drop. That extra charging the 12V battery with the generator and engine running was the cause.

GaryG

 

I know this problem well. Just got a brand new Honda Dealer 100 month warrantied 12 volt battery in Jan 2012 and my 2003HCH didn't start twice in the last week(3/15/2012) but I think the TIRE discussion guys have the answer. Pump up your tires. It seems to help everything. Going out to test now.

 

Just replaced my OEM 12v battery......failed to trip the relays just once the other day, but did on a second try. Drove straight to get a replacement. 78 months on that original motorcraft....dealer said it was ready for a change back in December, but I'm sure that was based on a traditional "Cold Cranking Amps" test.

I have noticed a significant improvement in FE as reported by the dash display....and no other variable has changed. Even the weather has been about the same.

 

I've had my '09 FEH parked and not started for 5 months now. The new 12V battery went completely dead during this time so I put a trickle charger on it last night. It started fine this morning, but the SGII had to reconnect, so I couldn't get a SoC reading right away. When the HV SoC displayed, I still had 40%. The '09 FEH does not have the jump-start switch, but it's now safe to say, it must have a better system to prevent it from draining itself. When I parked the FEH, it had 50% SoC. The 12V battery didn't have a full charge when I started it today, so the HV battery must have dumped a good amount of SoC into the 12V battery.

The reason I'm providing this information, is for those that have the '09 and newer FEH without the jump-start switch. You should feel safer when parking your FEH for long periods of time. I've mainly parked my FEH because I love driving my '11 Explorer that has average 23.9mpg with the A/C.

GaryG