SOC vs Range

[Calvin H-C]

Yesterday we had the opportunity to do a 475 km (295 mi) road trip in the SR F150 Lightning pulling a trailer on an unusually cold day that started at -22 C (-8 F) and "warmed" up to -12 C (10 F). I've done this trip in my Focus Electric (FFE) in mild weather without a trailer, and found both had about the same range (about 180 km or 112 miles) on a full charge.

We arrived at our destination with about 3 km range on the guess-o-meter (GOM), what ICE drivers would call "driving on fumes". We thought we had enough surplus charge from our final stop, but despite over eight years EV experience, we ended up cutting it a bit too close.

It's rare for us to drive the Lightning down to 10% SOC, let alone less than 5%. Definately more common for my FFE to arrive home with the battery display orange, which occurs when it gets down to 16 km (10 mi) on the GOM. Not that I do this frequently, but perhaps a few times per year, especially in colder weather.

Here's the observation I noticed having now seen a near-zero to 100% charge cycle on both vehicles...

With the FFE, as it charges the GOM and SOC increase pretty much linearly. In recent cold weather, if a full charge has my GOM showing 100 km (62 mi), then a 20% SOC would show 20 km range, a 60% SOC would show 60 km, and so on. In mild weather, when the FFE might have about 200 km (124 mi) range when fully charged, the "GOM to SOC" ratio is about 2 over the entire charge.

With the Lightning, this ratio is not linear. At the low end of SOC, the GOM would climb less than 1 km for each percentage of SOC climb. As the battery charged, each percentage increase would add a bit more to the GOM than it did earlier. I recall the Lightning showing something like 23 km when the SOC was 25%, but by 40%, the GOM showed about 52 km. As I write this, the Lightning is at 99% SOC and the GOM shows 175 km.

Just an interesting observation from dealing with two EVs. I suspect this has to do with different battery management software.

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[Heliian]

Lol, yes, the lightning is incredibly more advanced than the ffe.

However, don't trust the gom, use your current efficiency and soc to do your own calculations.

 

[Maquis]

Did you enter your destination in the nav? That gives me better GOM performance, especially in cold weather.

 

[Firn]

I have been working on this myself. I have been logging data since August (a few million rows, probably over 50,000,000 cells.

This plots how much energy is in each 1% of SOC by temp.

This plots it in SOC, color coding for temp. You can see that lower temps have less energy remaining for any given SOC.

 

[Calvin H-C]

However, don't trust the gom, use your current efficiency and soc to do your own calculations.

I fully understand that the G in GOM stands for "Guess".

It did with ICE cars as well, but as long as one is aware it is not a guarantee, it does a pretty damn good job of giving one an idea of knowing whether a trip will be non-stop or not. We never had gas gauges that showed percentage, but now that we have battery management systems that can, SOC has become the holy grail of knowing if you can make the trip, and it really isn't. If one is 20 miles from home, and the GOM says you have 35 or 40 remaining, it's a pretty safe bet you'll make it. If anyone is complaining they didn't make a 20 mile trip when the GOM started out at 25, then they are an idiot.

Rant aside, I was not commenting on absolute GOM values, but how the ratio of the GOM to the SOC changes more significantly with the Lightning compared to the FFE. I'm wondering if there is a comparable difference with other EVs on the market.

 

[ZeusDriver]

With the Lightning, this ratio is not linear. At the low end of SOC, the GOM would climb less than 1 km for each percentage of SOC climb. As the battery charged, each percentage increase would add a bit more to the GOM than it did earlier. I recall the Lightning showing something like 23 km when the SOC was 25%, but by 40%, the GOM showed about 52 km. As I write this, the Lightning is at 99% SOC and the GOM shows 175 km.

I suspect that this has something to do with displayed vs real SOC and also displayed SOC vs GOM.

Many systems for measuring SOC are effectively coulombmeters: they are measuring amp seconds in vs amp seconds out. One reason that a simple voltage measurement (as an indicator of SOC) does not work well for any of the lithium batteries is that the voltage/charge level is not linear (although it is pretty close much of the time). Amp seconds in vs amp seconds out works pretty well, by comparison.

Even though the voltage/charge level curve is not linear, it is nevertheless fairly flat, with the voltage for a 90% charged lifepo cell being, say 3.35 per cell and 20% being 3.20. So it is easy to ignore that difference, and just use amp seconds in versus out. Doing so gets you close enough in most cases. I have an electric outboard motor which is equipped with a coulombmeter, and it works very well for its intended purpose.

However, power is measured in watts (amps * volts) and the truck's ability to do work requires watt seconds. I wonder if the Ford SOC indication is really correct. Clearly there is some math missing, because 10% soc difference at low charge and higher charge should get you exactly the same amount of work accomplished.

Of course, the conservatism of the GOM when charge is low, could just be programmed in to make it less likely that people get stranded.

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