I'll be heading back home for college break for about a month and plan to leave my 2010 FEH at university. I'm worried about discharge of the HV battery while I'm gone, as to my knowledge NiMH batteries generally discharge pretty quickly. The battery is in good health, although obviously not brand new; I'm not sure what the exact capacity is, but I would estimate it at about 50% of design at least (likely more).
Is one month long enough for self-discharge to deplete the battery? I have ForSCAN, so I can always run a balance to charge it up past 50%. I'm also open to building a jump starter, as I want one anyway to have in case the battery craps out. The few options I were considering were:

• Use as is: I'll have an opportunity as is to let it sit a weekend, and might be able to just check SoC before and after my trip. However, I don't know if the battery does SoC estimation with charge counting, which wouldn't measure self-discharge.
• Forscan balance before departure: last time I ran it, it left the battery with a higher SoC than it normally would have after driving around. This might give be enough buffer to last a month.
• Build jump starter: I already work with HV electronics as a hobby (tesla coiling, among other things) and have access to several HV supplies, so the most difficult part of this would be tapping into the battery "vestigial" terminals.

Any advice here? As an addendum: the method I've seen circulating these forums for charging the battery seems to be a purely constant-current system with no voltage limit. Wouldn't this overcharge the battery? I do know that (most?) NiMH cells have catalysts to tolerate overcharging at <0.1 C or so, but I'm not sure if this is the case here.

 

Good luck buddy seems that you're on the right track

 

Run balance. Terminate it prematurely when you hear the engine transition from being loaded to being driven.
Disconnect 12V
Disconnect HV harness
Disconnect data cable.
Remove service plug.

NiMH is over-charge tolerant. A full charge from empty to full would take 24 hours at the typical 350mA constant current charge, and it should only be done with the battery cover OFF and a box fan blowing down through the cells.

 

Sounds good. What's the typical % SoC self-discharge, month to month on these batteries in that configuration? My main worry is that, given that this is a very old battery (13y), the capacity loss from age would mean it would self-discharge below starting threshold in storage. (Alternatively, how long can I expect to let it sit like this before it's too dead to restart the car?)

 

Impossible to know. Variables include climate, terrain, driver, miles, A/C use, etc.

It's a legit worry. I've supplied you with the process that minimizes the potential for self discharge. It either will or won't make it.

 

Can you go into more detail about disconnecting the data cable on a 2008 FEH? Not sure where this connection is. Is it important to do this in a particular sequence?

 

Order:

Run balance. Terminate it prematurely when you hear the engine transition from being loaded to being driven.
Disconnect 12V
Remove service plug
Disconnect HV harness
Disconnect data cable.

Data cable is the large square plug connected to the front of the battery on the driver's side. 10mm bolt to remove.

 

Of course! Don't know why I was thinking in front of the 12v battery.

 

I would get a good battery maintainer for the 12v battery and leave it connected for the month. You can use a small solar panel as well. When you return, see if it starts. If not, there is a button on the passenger side that you press and after about 8 minutes or so, the 12v battery gives the hybrid battery a kick start. That button is discussed in your owner's manual.

 

Ford removed that button and charger starting with the '09 model year. The OP has a 2010, it's not there.

A good battery maintainer is a good idea, the solar panel is not for a battery that won't be monitored. It can overcharge the battery as it will continue to charge a fully charged battery.