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It's time for some energy geek stuff.
The first picture is a typical Li-Ion discharge curve. As you can see in that picture, the discharge curve for li-Ion batteries is non-linear. The top 10% and the bottom 15% occur at vastly different curves than the bulk of the middle of the battery. If you owned any battery operated power tools before the Li-Ion movement, you'll likely remember how they had a pretty linear discharge rate. They would work fine for a while then slowly loose power. Li-Ion powered tools will work pretty much to full power right up to the point where they stop.

I was curious to see the Wrangler 4XE discharge rate and if I could see that same discharge curve over the course of a 26 mile drive. It's a little hard to see in my graph but you can definitely see that the top 10% and bottom 15% are on a slightly different discharge curve. Keep in mind, I was tracking miles over %SOC and not voltage. So my curve appears to be a steady decline where the first picture has a flat middle. That's due to the first picture tracking voltage and not distance. I'm just using it as an example.
So why does that matter? Keep in mind, I couldn't actually see the bottom 15% of the battery since we can't see anything below the <1% point. Where I have a 0 marked on the %SOC line, that's actually 15%. But you can just barely detect in those last two dots that the %SOC is starting to drop faster. This is important to note and I'll get in to the geeky stuff as to why that matters now.

We know that there is a bottom 15% that we can't see. But why did Jeep pick the 15% mark? In the curve in the first picture, you can see a massive drop off in voltage. I won't ask you to learn Ohms law but what happens in electronics and electric l stuff is there is a relationship between voltage and current. In that bottom 15% of the battery, it is a lot easier to charge because of reduced resistance. The two motor/generators in the 4XE are able to quickly recharge the battery in that 15% range without taxing the engine too much. The battery can also release a good amount of energy in that range but it just can't do it for very long. That bottom 15% is where the real magic of hybrid mode happens.
If you put the vehicle in eSave + Hold mode in hold mode up in the 90% SOC neighborhood, the system has to work a lot harder to return the energy it is using when demand calls for it. If you go back to one of my previous posts where I said "The Wrangler 4XE is always a Hybrid." Even in eSave + Hold it is still going to use energy from the battery when we demand a little extra power from the vehicle. Then it will work to restore that energy at the 90% rate (or where ever it was when we selected eSave). One might think that it would be the same as when the vehicle is in hybrid mode at the <1% state but in the upper percentages of SOC, it's making the vehicle work harder, thus impacting gas milage more.
All of that is to say, this is why the Hybrid mode is the most efficient mode of operation for the Wrangler 4XE for longer trips. If you know that you are going to be going a longer distance than you can cover in electric mode, your best fuel efficiency will result from hybrid mode. Once we allow that battery to hit the <1% state (which is that bottom 15%) that's where the ease of power flow due to the lower internal battery resistance really makes some magic happen.

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Hope that helps.

Electrical engineers and anyone in the field, please feel free to add or correct my language. I don't ever want to miscommunicate the wrong information due to poor word selection.
 

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This is what I suspected and why we get so much hybrid assist once the battery is "depleted".

It's probably a similar reason that e-save will discharge and hold at 95%.
 
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