Why so much buffer/reserve kWh?

[Triggerhappy007]

Yeah that was worded poorly above I don't drive as many miles as that made it seem. My mileage didn't increase until about the last few months I owned the Tesla through currently with the Mach-E this is supposed to been a temporary thing and it's just been dragged out. Currently it's about a 110 mi round trip 5 days and 1 day is 150 MI.

Lol, we're so alike. I have a 110 miles commute 5 days a week. I don't drive much the other 2 days.

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[andrew]

I'm really not worried about the battery at all. it's guaranteed for 8-years and I have never in my life owned a car that long anyway

Yes, but Ford is worried about it. They are the ones on the hook for that 8 year warranty. Perhaps the only reason they are giving you that guarantee is because they are controlling the quality of the battery. I don’t have all the facts, but I’m pretty sure that buffer plays a part in the 8 year warranty.

 

[Crazypostman]

Yes, but Ford is worried about it. They are the ones on the hook for that 8 year warranty. Perhaps the only reason they are giving you that guarantee is because they are controlling the quality of the battery. I don’t have all the facts, but I’m pretty sure that buffer plays a part in the 8 year warranty.

You are 100% correct
The battery can do much more meaning a much smaller buffer would technically still be safe. A much higher charging rate would be safe. But Ford doesn't want to be on the hook for the warranties. I almost wish there was a waiver we could sign that reduced the warranty to four years but unlocked a whole lot more capabilities.

 

[BMT1071]

You are 100% correct
The battery can do much more meaning a much smaller buffer would technically still be safe. A much higher charging rate would be safe. But Ford doesn't want to be on the hook for the warranties. I almost wish there was a waiver we could sign that reduced the warranty to four years but unlocked a whole lot more capabilities.

That would only work if you were legally bound to keep the car for 8 years. Or cover the warranty from years 4-8 for the next owner. The 8/100 warranty isn't something the manufacturers came up with on their own. It is a government requirement. Like the emission control component warranty on ICE cars.

 

[Crazypostman]

That would only work if you were legally bound to keep the car for 8 years. Or cover the warranty from years 4-8 for the next owner. The 8/100 warranty isn't something the manufacturers came up with on their own. It is a government requirement. Like the emission control component warranty on ICE cars.

Yeah I don't know the legality of it it was just a screwball idea. Maybe if when you sold it you were just legally obligated to sell it with the knowledge that the manufacturer warranty had expired

 

[RickMachE]

Yeah I don't know the legality of it it was just a screwball idea. Maybe if when you sold it you were just legally obligated to sell it with the knowledge that the manufacturer warranty had expired

Yeah, that would work, because so many are such upstanding citizens.

 

[dtbaker61]

I made a post a few weeks ago talking about the range not really being up to what it should be. Of course all of the apologist that know more than me came out letting me know how I was wrong it was the temperatures and the HVAC usage. Well today I just wanted to make a post about capacity we're not even going to talk about miles. So the model I have has a 76kWh gross and 70kWh usable battery pack. I know the BMS doesn't know the exact kWh remaining but it's a pretty good guess based on voltages.

So here's where my problem, when I charge the car to 100% and it cuts off the car shows that I have 65.61 kWh remaining. When I drive down to 0% I have 3.34kWh remaining. ( I assume those 3.3kWh are usable still below zero but that's risky so I'm not going to count those as "usable" )

So really from 100% to 0% I only have 61.7 kWh usable.

(I'm using displayed state of charge numbers not BMS. BMS shows 95% when car stops charging full, and 5% when the car shows 0%)

So that leaves 14.3kWh of reserve/buffer.
10.96 if I use those 3.34kWh that are below 0%.

That seems excessive to me! I'm not a battery engineer, but other car manufacturers do fine with 5-7kWh battery protection buffer.

What's the highest kwh that any of y'all have been able to pull out of an Sr?

Okay let the flogging commence!

PS. I actually hope my numbers are wrong somewhere. Or I'm misunderstanding something.

The more room you leave at both ends of the charge cycle, the better for Lithium... especially a pack that consists of many many cells that are not perfectly balanced at both ends because cells have slightly different capacity.

most Battery Management Systems (BMS) are capable of top balancing on the fly by sensing as individual or parallel strings of cells approach 'full' voltage, and shunting energy around the full strings to balance the rest so all end up at the 'full' voltage at the end of a 100% charge. Most BMS have some internal current limits they can handle with shunting... which is why charging 'slows down' as pack voltage starts climbing from the flat middle working voltage to ramping up rapidly at the end. Hence the 80% ramp down in DCFC in particular, although the charge current is reduced with L2 charging as well.

With a top balanced pack after a full charge, because not all cells are exactly the same capacity, the goal is to make sure you never drive the weakest cell into the danger zone under load... because if it hits 0 while under load, it will fry, short internally, and NEVER RECOVER. This reduces capacity permanently.

Since pack voltage changes very little in the 20%-80% state of charge with Li, the system 'guesses' where you are by counting kWhr in/out since last 'full charge'. Since there is *some* internal resistance, the absolute state of charge drifts from the guess until a full charge sets a new reference 'full' and resets the kWhr counter.

When actual state of charge starts dropping below 20% or so, voltage starts dropping as well, and depending how many cells/strings are being monitored, when the weakest string approaches the danger zone, the system throws more and more urgent warnings to avoid permanent damage to the lowest capacity cell(s).

It is a REALLY REALLY REALLY bad idea to squeeze out the last few kWhr for the last few miles. risk of permanent damage and reduced capacity is high. This is why the 0% cutoff is conservative... to support the 8 yr/100k mile warranty.

 

[dtbaker61]

You are 100% correct
The battery can do much more meaning a much smaller buffer would technically still be safe. A much higher charging rate would be safe. But Ford doesn't want to be on the hook for the warranties. I almost wish there was a waiver we could sign that reduced the warranty to four years but unlocked a whole lot more capabilities.

higher charge and discharge rates are possible, but with absolutely predictably reduced life/capacity of the pack.

If you'd be willing to replace a pack for $28k after four years/50k when it could go 8/100k.... I'd say 'go ahead', but sign a waiver, and disclose to whoever buys your car.

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