The problem is that there are many microprocessors that must remain powered all of the time. Alarm system, IA key transponders, keypad on the B post, etc. Although the drain on the battery is small it is always there. If the car is driven a lot of miles (more than 25) on a daily basis there should not be a problem. If the car can sit for a week at a time without being driven then the 12 volt battery will be slowly discharged. The design problem, IMHO as an electrical engineer, is that a wet cell lead acid battery was the wrong choice for this car. Because of the way they are designed, for cars with starter motors, they are not meant to ever be discharged to less than about 80% SOC. Their typical use is to supply a large burst of current to start the engine and then be immediately recharged. When a "starter" battery is significantly discharged physical damage to the lead plates happens. The damage can not be undone. Material comes off of the plates and collects at the bottom of the cell. When the pile of debris gets high enough to reach the bottom of the plates the cell shorts out and that is the end of the battery. The better battery for this application is an AGM (absorbed glass mat) battery. They are not damaged by low SOC levels and have a higher output voltage at low SOC levels. I put one in my 2013 Energi and have not seen the low battery warning message since the installation. It is not an easy installation because of the limited space. I had to remove the battery tray and cut the end off of it to allow the larger AGM battery to fit. The problem for Ford is that an AGM battery costs a lot more than a wet cell battery. I have more than one car and the Energi can sit for a week without being driven. I charge the 12 volt battery with a battery charger that has an AGM setting and can measure the SOC. I have seen the SOC as low as 37% without getting the low battery warning.
While I agree that AGM batteries are much better than wet cell batteries I don't think the wet cells on their own are the root cause for the problem. I also agree the load from the electronics which are still powered up when the key is off is a contributing factor. The FFH's small 12V battery size and battery quality issues are the main culprits. The vast majority of cars sold today use wet cell batteries. These cars also need to power loads when the key is off to run microprocessors, alarms, IA key transponders, keypads etc just like the FFH. The difference is they have a larger standard size wet cell battery.
The FFH's engineers probably thought since the 12V battery does not need to run the starter they could get away with a smaller battery and save cost and weight. They probably didn't think as much about the load on the battery over an extended period of time when the key is off.
I also think the FFH batteries have had higher than normal quality problems. The FFH 12V battery is smaller than a standard battery and there are probably a limited number of battery vendors that are set up to make this small battery. So when Ford started seeing the battery quality issues it was not easy for them to just switch to another battery vendor to solve the quality issue.
Edited by Automate, 16 April 2015 - 03:50 PM.