I had a fruitful exchange with Eckhard Karden of Ford. I found him to be very open about technology, but he would not comment directly about Subaru when I brought up the subject of the Settlement. I suggested that there was concern about both the OE battery choices and the charging algorithms used in the basic smart alternator system in use on the SJ, and the updated algorithms in conjunction with the EFB and AS/S system in use with the SK Forester models. He sidestepped this as I figured he would, and just discussed matching batteries to applications in conjunction with matching an appropriate charging scheme with the battery design and it’s placement in the vehicle.
He was kind enough to provide a few book chapters (for my personal use) that he and his colleagues have authored. We both have accounts on ResearchGate so he suggested that I use that venue for full manuscript access to his work to avoid the paywall issue in the future.
So let’s get into it….
I set the stage by relating the community experience in past years with conventional flooded batteries and the general respect/love for AGM. Price isn’t an object here. Members of our car community are looking for the best, most robust solution. EFB is a relative newcomer to the scene, not well understood, not readily available in the aftermarket in the USA, and can cost more than a good AGM. We in North America are a bit late to the party with regards to the moderate discharge cycling thanks to AS/S, and at 4 years in many of us are now facing our first battery replacement.
I stated my assumptions about the SK system, and he confirmed most of them and added a few.
1) The Japanese makes use EFB almost exclusively in Start-Stop applications. They don’t see an advantage in AGM. You’ll only find limited use of AGM in non-starter systems (dual battery) on some hybrids.
2) The Q85 battery Subaru specifies for the SK is a high specific output product. A good choice. It meets stringent charge acceptance requirements under a wide range of conditions and has good discharge cycle life. Cost is a factor for vehicle producers, but consistency of operation, reliability, and safety are also top concerns. He has no issues with this selection.
3) AGM’s should be shielded from heat. They don’t like to be cycle charged hot. Underhood placement without thermal protection isn’t good.
4) Vehicles that offer a choice of batteries have battery selection provisions so that the cycle charging conditions are matched to the battery technology. This impacts temperature and applied voltage. Charging voltage is different for an AGM and must be reduced as battery case temperature rises. So the active use of a thermal sensor scaled to the AGM map is needed, as is thermal isolation. All this is critical considerations when constantly cycle-charging the battery. The vehicle has no way of anticipating or correcting for a misapplication of battery type.
5) If you can’t do a charging program shift, then you’d be wise to replace an EFB for an EFB, an AGM for an AGM.
So that’s the basics. But there were a few surprises.
1) Ford, like all manufactures, does extensive testing anticipating that owners will do squirrely things. Usually owners cheap-out and sub in a basic flooded cell or lower capacity battery. The results are as expected. Early death. But what about the notion that AGM is automatically better and thus a viable substitute? According to Eckhard, OEM-Grade AGM batteries (and this is a seriously important distinction!) are not as badly heat-vulnerable as initially expected, based on field experience. If continuously charge cycled hot (underhood), they will likely fail earlier than you had hoped, but probably not in a dangerous manner. But he reminded me that headspace pressurized hydrogen buildup is a hazard not to be ignored.
2) But what if you just MUST have an AGM? He suggested deciding based on location. If you live in a moderate climate like Michigan (Dearborn, maybe?), he might say go for a good OEM-grade AGM. But in Arizona or Texas heat he’d rather stay with the original recommended replacement battery type (EFB).
I did not dive into exception cases like disabling AS/S. He’s a huge advocate, and it seemed inappropriate. But I figure that if you do disable AS/S on your SK, you’ve basically recreated an SJ experience. If an AGM worked for you on your SJ, it might hold up fine in an SK. I’m not planning on going there, so I’ll stick with an EFB when it’s time for a change.
