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Battery Range vs Capacity

Posted: Mon Apr 30, 2018 5:32 am
by talbert99
I'm in my third month with my FFE, and I am struggling to understand where the 33KW battery capacity is. I have a 56 mile commute to work - and commonly use 16 to 19 KW to get to work. (that is what the vehicle reports). I have a charging station at work - and to bring the car up to 100% it will usually take 19-21 KW (Chargepoint usually shows 3KW more than the vehicle states that it has consumed).

What I'm struggling with - is ex: today I consumed 15.8 KW and the vehicle stated I had 24 Miles left. My avg. consumption is 284W/Mile which means I have

16 consumed
6.8 left
Total is 23KW? what happened to my 33KW battery? I called up Ford and asked for an explanation and was directed to take the vehicle in for service. The service tech stated that there were no errors and Ford stated i'm within parametric limits. The service guy was nice - but didn't have any answers for me - other than - the vehicle needs to learn how I drive and stated I should come back in at 5K miles.

So where are the missing 10KW? I live in the NE and am concerned when I start using HVAC - that 24 miles will be consumed up pretty quick and i won't be able to make the 56 mile trek on a single charge. Does anyone have an answer? This is pretty simple math - addition/subtraction - no algebra and it just doesn't add up. I would appreciate a good explanation. btw, i've tried charging early morning - or when I get home - no difference. (a buddy stated that may I should have the battery warm ?)



Re: Battery Range vs Capacity

Posted: Mon Apr 30, 2018 6:49 am
by triangles
First the difference you see in the charge station power used to charge and what your car says you've used is caused by charging inefficiencies. Since charging isn't 100% efficient you only get about 85-90% of the energy out of the battery that you put in while charging. The difference is lost as heat. Further 33.5 kWh is the maximum capacity of the battery. You will never be able to use this full capacity because doing so would ruin the battery. Lithium ion batteries degrade quickly when kept fully charged (4.2V per cell) and it is even worse to let the battery sit depleted or nearly depleted. If it were possible to fully deplete the battery it is very likely it would never hold a charge again. By default when the car says it is 100% charged it is only really 90% (about 4.02V/cell if I remember correctly). You will often see this referred to as 90% SOC which means "State Of Charge." So if you see someone refer to a battery % SOC you know they mean what the actual battery charge level is not what the car says. On the other end when the car says the battery is depleted it really has about 5% left. These limits protect the battery from you prematurely killing it. If you plan to keep the car long term avoid running the battery very low as much as possible. Also less important is don't let it sit fully charged all the time. Since the car already limits you to 90% it's not that big of a deal but the battery would be happier long term if it was only at say 80%

Temperature very much so affects battery capacity. Capacity most noticeable starts dropping at about 50F and really takes a hit at freezing and below. IIRC the battery is happiest between 70F and 95F. When it's warm you will have a max usable battery capacity somewhere in the upper 20's kWh, probably about 28-29kWh. Being in the 280's Wh/mi means it's still cool where you live or you are driving at high speeds on the freeway. For reference my winter power consumption can go as high as 350 Wh/mi. I've even flirted with 400 Wh/mi a couple times. Since electric motors are so efficient, fuel economy is the opposite of gasoline cars. You will go much farther in the city than at high speeds on the freeway. Your 33.5 kWh FFE is rated at 115miles of range. This is somewhere about 250 Wh/mi (.250kWh/mi * 115mi = 28.75kWh). That is under ideal conditions. It isn't too hard to beat 250 Wh/mi in the summer. If you've been using heat, that uses several times more energy than the AC. Depending on how much AC you're using, that only takes about 10-15% off your range. If you keep off the expressway you should have a range of about 100 mi with AC. With age and usage your battery capacity will decrease. That is the unfortunate nature of batteries.

Also as a side note for winter conditions: Charging a cold soaked battery is not good for the battery. Trying the DC quick charge a cold battery is even worse! If you can, plug in immediately after driving while your battery is still warm. Once charged, letting the battery cold soak in the winter if you won't be driving the car for a while will not harm the battery. It's just trying to charge a cold soaked battery that is not good. I don't claim to fully understand why charging a cold battery is bad, but I think it has something to do with the cold affecting how the chemical reactions work in the battery. I may have it wrong but I think it results in some of the lithium ions plating out as lithium metal which is not a reversible reaction. Someone more knowledgeable correct me if I have that wrong.

Re: Battery Range vs Capacity

Posted: Mon Apr 30, 2018 7:01 am
by triangles
Also we call that gauge that says how many miles you have left the Guess-O-Meter (GOM). There's a reason it is called the Guess-O-Meter. It can wildly fluctuate and I only ever take it seriously when I am below 1/3 battery. If you drive consistently it will be fairly accurate. If you don't then it won't be very accurate. This can be made worse if part of your commute is very efficient and part very inefficient like 80mph on the freeway.

Also for colder months if you can leave it plugged in (L2) there are battery heaters that will maintain some minimal heat level in the battery.

Re: Battery Range vs Capacity

Posted: Mon Apr 30, 2018 7:39 am
by triangles
As far as your math goes, where are you getting your 6.8kWh remaining capacity from?

I really recommend against doing this because IMHO it puts a lot of unnecessary wear and tear on the battery, but if you absolutely must know how many kWh your battery can provide you can do a run down test. This is done by charging to 100% resetting the trip meter, rolling the windows down and putting the heat on high until you get the battery depleted message. Read the gauge on how much energy you used and that is your usable capacity.

I prefer a less accurate method that is easier on the battery. I drive normally until I have about 10% battery capacity left. I use the kWh used as an estimate of 90% of capacity and do the math to find what 100% would be. This is way less accurate because the % battery remaining is based on what the battery voltage is and is not truly a measure of having 10% capacity left.

Re: Battery Range vs Capacity

Posted: Mon Apr 30, 2018 8:14 am
by Akua
Well Put Triangles!

That first paragraph should be saved for future questions people have about battery life and range!

Re: Battery Range vs Capacity

Posted: Mon Apr 30, 2018 8:18 am
by Jon_h
An easy way to get an estimate of (usable) capacity is to use Forscan and an OBD2 tool. Estimated energy is right there.