90% Charge (Better for the HVB) vs 80% charge (more economical)

[phidauex]

A few thoughts - as far as the battery, 80% is better for the battery than 90%, which is better than 100%. It won't matter right away, but it could matter over many years. Either way the holdback on the MachE battery is generous so even a reported 100% is probably more like 92% or 93% of the chemical SOC.

As far as energy required to go from 70-80% and 80-90%, no, both cases will use about the same amount of energy. Internal efficiency of batteries is pretty flat through most of their curve. Where this becomes less true is at the very margin, more like 97% to 100% of real charge (which you can't access on this car anyway).

Faster charging, however, is less efficient than slower charging, since energy losses in the battery are mostly due to ohmic heat generation, and those losses increase with the square of current, so doubling your charge rate is 4 times the lost energy. But again, you don't need to worry that much about it, because the home 240V L2 chargers are still only charging at around 0.15C, which is a very gentle charge rate. You might become concerned about it if your primary every-day charging was on a DCFC, but that won't be true for most people.

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[Eosin]

Ford recommends keeping the HVB at 90% charge for the health of the battery. However, it takes more energy (longer time) to charge from 80 to 90% - resulting in a higher cost from your electricity provider. It's my understanding that it's cheaper (and faster) to go from 70 to 80 % charge than it is to go from 80 to 90% charge even though its the same increment (10%). So what is everybody doing or plan on doing - charging to 90% (except for days when a complete charge is needed) or 80%? Thanks

You are absolutely correct about it costing more to charge from 80-90% than from 70-80%. Several people stated otherwise, but I'm quite sure they were simply guessing.

As the battery's state of charge increases, so does the it's resistance; therefore, it takes more energy to charge a battery when it is nearly full.

 

[Crilly]

What-another 10 cents. That would be a lot.

 

[Marcel]

You are absolutely correct about it costing more to charge from 80-90% than from 70-80%. Several people stated otherwise, but I'm quite sure they were simply guessing.

As the battery's state of charge increases, so does the it's resistance; therefore, it takes more energy to charge a battery when it is nearly full.

The only way to know for sure is measure the time it takes to charge the 10%, so stopwatch 70-80%, 80-90%, 90-100%.

I have done this frequently with my e-Golf and have a precise usage meter on my electricity, I see it's slowing down after 95% because of the levelling off in the individual cells...so only the last percentage takes more current but not much.

Charging to 90% should not cost extra?

 

[machefan]

FWIW the manual calls out max 90% for regular daily needs which in reality is 80% due to the buffer. If 80% is enough for daily use for ones needs then go with that.

 

[Eosin]

The only way to know for sure is measure the time it takes to charge the 10%, so stopwatch 70-80%, 80-90%, 90-100%.

I have done this frequently with my e-Golf and have a precise usage meter on my electricity, I see it's slowing down after 95% because of the levelling off in the individual cells...so only the last percentage takes more current but not much.

Charging to 90% should not cost extra?

Wow... That's really great your vehicle does this well with the charging curve, but there are probably subtle changes you aren't noticing as it's not physically possible for the charging efficiency to decrease over time.

Here is a graph for a Nissan Leaf on a level 2 charger which does not do as well as you report with your e golf. 1 hour of charge adds the 1st 40%, but it takes 1.5 hours to add the next 40% to reach a total charge of 80%. This is a more realistic curve shape reflecting decreasing charging efficiency over time.

This is physics. Kind of like gravity... It's there whether you have noticed it with your e golf or not. I suspect if you created a graph with as many data points as this you would be surprised. I suspect you just haven't noticed the change over time because you are either "eyeballing" it or using approximations when monitoring the charging data. If you kept precise data like in this graoh, you would see similar results.

How significant this efficiency drop is to each person is will vary, but there is certainly a cost increase to charging to higher percentages.

Some people will drive in an ICE vehicle past a gas station charging $2.75 to reach a station charging $2.74 because they don't want to waste that extra 1¢ per gallon they could otherwise save. Likewise, if the cost of charging increases by any amount over time, some people will find it relevant.

Saving a measly 10 cents every night over the course of a year will save $36.50 over the course of a year. That is like getting a few free "fill ups."

 

[machefan]

Here's the passage from the manual.

 

[timbop]

Thanks for this - so is the idea that some commercial fast chargers will let you go to 80% relatively quickly but the last 20 % can take 2 hours because they want a quick turnover of the chargers? or is there some other reason.

DC Fast chargers are able to supply (on average) 10 times the power (nominally 100kw) compared to AC L2 charging (on the order of 10kw). That generates a lot more heat, which is wasted energy. When the battery gets above 80% the DC power that the car accepts tapers down - in the case of the mach E it tapers down to 12kw (about the same as AC L2 charging). Since AC Level 2 charging is a much lower power level, it generates far less heat (and thus less waste). AC Level 2 charging will also decrease the power draw a little bit as the battery gets full, which adds to the charging time - but the net amount drawn in the end works out to about the same. So, AC level 2 charging from 70% to 80% is just slightly more wasteful than 80% to 90% - probably not enough to even notice.

 

[timbop]

Wow... That's really great your vehicle does this well with the charging curve, but there are probably subtle changes you aren't noticing as it's not physically possible for the charging efficiency to decrease over time.

Here is a graph for a Nissan Leaf on a level 2 charger which does not do as well as you report with your e golf. 1 hour of charge adds the 1st 40%, but it takes 1.5 hours to add the next 40% to reach a total charge of 80%. This is a more realistic curve shape reflecting decreasing charging efficiency over time.

This is physics. Kind of like gravity... It's there whether you have noticed it with your e golf or not. I suspect if you created a graph with as many data points as this you would be surprised. I suspect you just haven't noticed the change over time because you are either "eyeballing" it or using approximations when monitoring the charging data. If you kept precise data like in this graoh, you would see similar results.

How significant this efficiency drop is to each person is will vary, but there is certainly a cost increase to charging to higher percentages.

Some people will drive in an ICE vehicle past a gas station charging $2.75 to reach a station charging $2.74 because they don't want to waste that extra 1¢ per gallon they could otherwise save. Likewise, if the cost of charging increases by any amount over time, some people will find it relevant.

Saving a measly 10 cents every night over the course of a year will save $36.50 over the course of a year. That is like getting a few free "fill ups."

The leaf also doesn't have a liquid cooled battery management system. The MME should be affected less because it manages heat much better.

 

[Eosin]

The leaf also doesn't have a liquid cooled battery management system. The MME should be affected less because it manages heat much better.

It's the charging resistance which increases as the battery voltage increases. Similar to blowing up a balloon. Getting that last bit of air in is harder. I realize the physics aren't the same in my analogy, but the point stands.

 

[timbop]

The heat isn't the biggest limiting factor to level 2 charging curves... It's the charging resistance. Similar to blowing up a balloon. Getting that bit if air in is harder. The physics aren't the same I. My analogy, but the point stands.

Perhaps, but to answer the OP: in the end the difference is negligible.

 

[Eosin]

Perhaps, but to answer the OP: in the end the difference is negligible.

But science says it's not. You have no data or science to back that up. Your response makes it evident you are not familiar with battery physics... Which is fine. I'm not trying to rub that in your face. But for all that's good and holy, if you don't know battery physics well enough to apply some formulas, do the algebra, or at least collect data points to refute the science, then you probably shouldn't spread misinformation.

If the charging is even a few percentage points less efficient between 80 and 90% state of charge, then it will equate to a measurable cost difference over time.

No matter how confident you are in believing otherwise, it's still incorrect.

 

[generaltso]

If the charging is even a few percentage points less efficient between 80 and 90% state of charge, then it will equate to a measurable cost difference over time.

Please measure this on an MME and get back to us. The difference in efficiency you’re talking about is negligible.

 

[tomking29]

I will go with the recommended 90%. Maybe after I get to a normal routine I’ll adjust from there. 80% seems very low to me.

 

[ChasingCoral]

Ford recommends keeping the HVB at 90% charge for the health of the battery. However, it takes more energy (longer time) to charge from 80 to 90% - resulting in a higher cost from your electricity provider. It's my understanding that it's cheaper (and faster) to go from 70 to 80 % charge than it is to go from 80 to 90% charge even though its the same increment (10%). So what is everybody doing or plan on doing - charging to 90% (except for days when a complete charge is needed) or 80%? Thanks

First of all, let's clarify: are you talking about home AC charging (L1/L2 = 120/240v) or DC Fast Charging? I am not aware of anything that indicates 80-90% takes any more energy or time on L1 or L2 charging than 70-80%. I think it really comes down to battery life. Lithium batteries are generally happiest operating near 50%. They don't like full discharge or sitting around at 100% charge. For that reason, I plan to generally operate between 20-80%. Of course, with the reserve, I will actually have a larger margin than that. I have already set my charging default at home to charge to 80%. However, all of that is for enhanced battery life, not charging cost. It is easy to override the 80% default and the Mach E will charge to 100% on a given session without changing the default.

DCFC substantially tapers above 80% at EA stations. I'm not sure if we have seen how it compares at other DCFC stations but I think this is controlled by the Mach E to reduce battery damage. Ford does recommend not exceeding 80% on DCFC, again to enhance battery life.

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