Battery SOH - bragging rights or bullshit?

[chl]

No one said that, ever !
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But people are not saving money by charging slower, so it would be incorrect to tell them so.

Huh? Yes they are!

If they charge with a lower current they are wasting less kWh to wire resistance, and that is what the utility charges for.

Lower current, less energy wasted to wire resistance.

It may be a small number of kWh per charge, but it adds up.

I have a 75ft run, so 150feet total of 6AWG with about 0.04 ohms per 100 feet, so 0.06 ohms total.

I did the numbers for my wiring at 80A vs 32A as an example.

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To add about 10kWh to the battery:

At 80A it takes about .52h (31 minutes) if there were no losses due to the wire resistance.
80A x 80A x 0.06 ohms x 1 hour = 384 Wh energy lost in 1 hour
But in 0.52 hours x 384W = 199.7Wh is lost due to wire resistance
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If I use a 32A charge current it would take about 1.3h if there were not wire resistance losses.
32A x 32A x 0.06 ohms x 1 hour = 61.44 Wh energy lost in 1 hour
In 1.3 hours x 61.44W = 80Wh lost due to wire resistance
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The 32A charge wastes less energy than the 80A charge for adding about the same kWh to the truck battery.

Numbers don't lie.

And the difference in energy per hour wasted is about a factor of six: 384/61.44.

So even though it takes longer to add energy to the battery, it is more energy efficient to use a lower current.

The utility charges for the energy you are using whether you add it to the truck or waste it.
The less you waste, the less you pay.

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Or put another way, it actually uses more energy at 80A than at 32A to add a kWh to the battery, and you pay for it.

Like I said, it may be a small amount of cost per charge, but over time it adds up.

Maybe time is more important to the owner than cost, then fine, use a higher current.
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Due to wire resistance in my example:

In one hour at 80A x 24V (19.2kW), 18.82kWh is added to the battery.
For every hour at 32A x 240V (7.68kW), 7.62kWh is added to the battery.

Wasted energy costs money.

Hope that makes more sense seeing the numbers?

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[chl]

FWIW, I don't charge to 100% unless I'm going on a trip outside my usual sphere. Example, a.friend that lives about 80 to 90 miles away, family that is 100 plus miles, etc. I want to be sure to use the extra charge above the 80% I typically charge to. Really, I have probably fully followed their criteria only a few times.

However I was responding to the person that appears to be having balancing issues. Their responses.suggested to me that they had never changed to 100% for balancing. So for that person, it seems prudent to give it a go to see if it improves their situation, or flushes out another problem.

Yes, I agree, it would be worth a try.

If there is a bad cell or two, it won't help/fix that issue.
But if there was a lot of BMS 'drift' causing the delta, then it might.

In the Leaf forum over the years it has been an issue and results were mixed.
With the Leaf, to see a balance improvement, the process of up and down between 100% and 10% had to be done three times with long pauses (5 hours or better overnight) in between!

But what people found out was a large delta was generally not fixable this way and indicated a bad cell or two. With the Leaf, there were people who would replace down to the cell pair level, quite a task!

 

[mr.Magoo]

Huh? Yes they are!

To add about 10kWh to the battery:

At 80A it takes about .52h (31 minutes) if there were no losses due to the wire resistance.
80A x 80A x 0.06 ohms x 1 hour = 384 Wh energy lost in 1 hour
But in 0.52 hours x 384W = 199.7Wh is lost due to wire resistance
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If I use a 32A charge current it would take about 1.3h if there were not wire resistance losses.
32A x 32A x 0.06 ohms x 1 hour = 61.44 Wh energy lost in 1 hour
In 1.3 hours x 61.44W = 80Wh lost due to wire resistance


Hope that makes more sense seeing the numbers?

I have to say it's interesting that you're so hyper focused on energy losses in the cable (which I'm not arguing with by the way, yes, they're real, yes, it's a factor, yes it costs money), yet you fail to realize the energy "losses" in the truck due to heating/cooling of the battery, pumps, keeping electronics alive, charging your 12V battery, etc.
It's a bit like saying you're letting your ICE car idle through the night because cold starts are so inefficient.

In your equation the delta between the two scenarios is 120W, so as long as the truck is using less than 120w to keep things alive you'd be correct that it is a net gain by charging slower, but the truck isn't using less than that, it's using a lot more, resulting in a net loss - which I've shown here:
https://www.f150lightningforum.com/...nitor-the-j1772-negotiation.34786/post-655408

Additionally, with your logic, L1 charing would be the best thing for efficiency since there's hardly any loss at all, yet it's been talked about a million times that it isn't all that great at all.

 

[chl]

I have to say it's interesting that you're so hyper focused on energy losses in the cable (which I'm not arguing with by the way, yes, they're real, yes, it's a factor, yes it costs money), yet you fail to realize the energy "losses" in the truck due to heating/cooling of the battery, pumps, keeping electronics alive, charging your 12V battery, etc.
It's a bit like saying you're letting your ICE car idle through the night because cold starts are so inefficient.

In your equation the delta between the two scenarios is 120W, so as long as the truck is using less than 120w to keep things alive you'd be correct that it is a net gain by charging slower, but the truck isn't using less than that, it's using a lot more, resulting in a net loss - which I've shown here:
https://www.f150lightningforum.com/...nitor-the-j1772-negotiation.34786/post-655408

Additionally, with your logic, L1 charing would be the best thing for efficiency since there's hardly any loss at all, yet it's been talked about a million times that it isn't all that great at all.

Incandescent light bulb is only 5% or so efficient at producing light - 95% goes to producing heat, so unless you are operating an 'Esay Bake Oven' using an incandescent bulb is wasting energy, and you pay the utility company for it.

In cold weather, when your truck is plugged into an EVSE, if needed, the system will draw energy from the utility through your EVSE and not from the battery just for battery warming.

But that is not wasted energy.

Whether you charge at L2 at 80A or 32A, the amount of energy used by the heater will be the same.

Unless you are preconditioning the truck for a departure, just maintaining the battery temp uses 1-2kW from what I have read.

So 1-2kW will be subtracted from the available power for charging whether charging at 32A or 80A.

With L1 charging when it's cold and the battery needs to be heated, the problem is it doesn't provide enough power, barely enough to just run the heater, at best 120V x 15A = 1.8kW.
NOTE: assuming a 120A 15A circuit.

If you run the numbers subtracting 2kw from the available power for the 32A L2, then
(240V x 32A) - 2kW = 5.68kW

To add 10kWh then would take: 10k/5.68k=1.8hours
In 1.8 hours, the wire energy losses would be 61.44 x 1.8 = 111Wh

For 80A and running the heater the available power for the battery would be
(240V x 80A) - 2kW = 17.2kW

To add 10kWh then would take: 10k/17.2k=0.58hours
In .58 hours, the wire energy losses would be 384 x 0.58 = 223Wh

So the 32A rate, even subtracting the energy needed for running the battery, would still be more efficient than the 80A rate.

 

[mr.Magoo]

Whether you charge at L2 at 80A or 32A, the amount of energy used by the heater will be the same.

Except that there's that other pesky factor called time.

If you charge twice as fast your auxiliary equipment will use half as much energy, no?
(Bit simplified, but I'm about to give up so...)

Heater is a bad example since it runs mainly in the beginning, but all the other electronics, pumps and whatnot, still runs regardless if it takes 1hr to charge or 12hrs.

Hopefully we can agree that using a 100W lightbulb for 12hrs consumes more energy overall/total compared to using the same bulb for 1hr?

 

[chl]

Except that there's that other pesky factor called time.

If you charge twice as fast your auxiliary equipment will use half as much energy, no?
(Bit simplified, but I'm about to give up so...)

Heater is a bad example since it runs mainly in the beginning, but all the other electronics, pumps and whatnot, still runs regardless if it takes 1hr to charge or 12hrs.

Hopefully we can agree that using a 100W lightbulb for 12hrs consumes more energy overall/total compared to using the same bulb for 1hr?

LOL, yes, to the light bulb question.

But not an exact analogy for the charging current question.

Compare a higher wattage / higher current bulb to a lower wattage lower / current bulb and then losses due to resistance start to become a significant thing - that is perhaps a better analogy.

Resistance losses rise with the square of the current.

Ironically, an incandescent bulb wastes energy through its resistance as heat because it is in fact a wire a purely resistive element.

Yes time is a factor, but it is a liner component of the calculation - current is an exponential component.

I get the point which I think is the bulb represent some of the overhead of the truck during charging that is the same for 32A and 80A.

I don't know the exact number but I have read it is somewhere between 100W and 400W depending on, what, battery cooling needs?
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NOTE: if it is the top number for both rates, 400W, the 32A charge rate would not be more efficient, if it were a lower number 200W for 32A and the higher number 400W for 80A perhaps because of extra cooling demands, then it could be depending on how long the charging lasts to reach the desired kWh added.

For example: to add 20kWh at 80A with 400W overhead:
20/18.416=1.086 hours; losses 1.086 x 784 = 851.4Wh

at 32A to add 20kWh with 200W:
20/7.418=2.7h; losses 2.7 x 261.44 = 706Wh

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So, what about cooling?

Doesn't the battery cooling system have work harder using more energy at a high current than at a low one to cool things down?

Isn't more energy expended cooling the battery etc. when charging at 80A then 32A?

If that is the case, then that is a bigger effect than the heat wasted due to wire resistance at high currents.

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I agree with another comment above that there is likely a sweet spot with an optimum charge rate for efficiency, because of the energy overhead of the truck systems during charging - power x time.

I think that is likely between 32A and 48A as some discussions have suggested on this and other forums, due to the factors mentioned.

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A really 'smart' truck (and/or EVSE) would be able to adjust the current for optimum efficiency and cost based on supply wire resistance (could use the voltage drop at a set current to calculate that), battery temperature, vehicle overhead during charging, utility rates, etc., as needed on the fly!

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