Does volts / mile really indicate anything?

[LovinEbike]

I'm a newbie who therefore have "miles per charge" interest. Of course there are variables; bike weight, rider weight, amount of pedal assist, terrain, etc. For sake of discussion, I have been holding those variables fairly constant and monitoring my mileage and charge depletion (my display shows current voltage).

I get a little more than 10v of use per charge and that has been pretty consistently giving me about 42 miles (again with my personal variables). So my questions:

1) is it fair to approximate 4 miles per 1v of charge, given my particular parameters/consistent variables

2) can those numbers (for my situation) then be used to approximate miles remaining? For example my battery completely turns off when I drop below 42v. Therefore when I see I am at 44v I know I have about 8 miles of assist left (2v x 4miles per v). Again consistent variables in my ride patterns will be a big factor. If those last 2v are on a steep incline, the results would obviously be different than a downhill finish.

Thanks

 

[Stefan Mikes]

What are you talking about? The only parameter that really matters is Wh/mi (Watt-hour per mile).

 

[6zfshdb]

Keep in mind volts per mile isn't linear. It will vary with the battery state of charge. The first volt will give you more miles than the last. A better way to look at it is % charge remaining as indicated in charts like this one:

If you were to graph these numbers, you'll get a curve rather than a straight line.

When your voltage drops to 50%, you actually have 35 to 40% charge remaining.

 

[LovinEbike]

I am talking about is how to use the information that my bike display gives me during my ride. Specifically my display show voltage remaining.

So my comment was about using that information to determine mileage remaining. Thought that was pretty clear

 

[LovinEbike]

Thanks 6zsfhdb, that was exactly the guidance I was looking for. Too many variables to try for a precise indication of mileage remaining at a certain battery voltage left, but figured the numbers would provide some guidance on the mileage remaining.

 

[6zfshdb]

Thanks 6zsfhdb, that was exactly the guidance I was looking for. Too many variables to try for a precise indication of mileage remaining at a certain battery voltage left, but figured the numbers would provide some guidance on the mileage remaining.

As Stefan mentions above, a more accurate way to track mileage is to measure watt hours consumed. Watt hours per mile is linear but it is difficult to measure on most e-bikes without a more specialized gauge.

BTW, welcome to the forum! You've come to the right place to ask your questions.

 

[m@Robertson]

Thanks 6zsfhdb, that was exactly the guidance I was looking for. Too many variables to try for a precise indication of mileage remaining at a certain battery voltage left, but figured the numbers would provide some guidance on the mileage remaining.

Unfortunately, you can add to the variables @6zfshdb lists: the characteristics of the cells in your pack, and their performance change as they age. Particularly the former. Mathematically correct charts like he linked are inherently imperfect, but realistically keeping an eye on your voltage level (and intuitively understanding what '55.4' means, for example) is the best real-time, glance-down-at-the-handlebars-and-know-something method available.

These charts have more information for you to overload on, including links to a source that shows battery discharge curves for individual cell brands/models.

Li-Ion Ebike Battery Charge Charts

Whats an 80% charge on a 48v battery? These charts give answers to questions like this on common ebike battery voltages: 36v, 48v, 52v, 60v and 72v.

talesontwowheels.com

 

[LovinEbike]

Thanks to all. I believe Robertson summed this up the best with his comment:

"...best real-time, glance down at the handlebars and know something method available..."

 

[rich c]

One of my favorite features on Bosch systems is the range calculation. If I'm 10 miles from home and it tells me I have 6 miles of range left, I'm shutting off the assist on flat pavement, never get above ECO mode, etc. It lets me make the riding adjustments and have never come home with a dead battery in 6 years.

 

[scrambler]

I am talking about is how to use the information that my bike display gives me during my ride. Specifically my display show voltage remaining.

So my comment was about using that information to determine mileage remaining. Thought that was pretty clear

Normally display have setting options that allow you to display either a capacity meter graphic, a % of charge number, a voltage or a combination of these.
Mine for example always display a capacity bar showing how much of the battery is left as an icon, and gives me the choice to display either Volt or battery charge percentage.

So check your display manual and what options it offers.

As was said previously volt per mile is not good as a lithium battery discharge curve is very flat, so it stays around the same voltage for most of the discharge, then starts dropping quickly.

If you have no option for a charge left icon and can only see voltage, you would need to learn or have a card to see what capacity each voltage corresponds to for your battery (each battery is slightly different)

 

[fooferdoggie]

One of my favorite features on Bosch systems is the range calculation. If I'm 10 miles from home and it tells me I have 6 miles of range left, I'm shutting off the assist on flat pavement, never get above ECO mode, etc. It lets me make the riding adjustments and have never come home with a dead battery in 6 years.

my old nyon shows watts per mile its really handy to see how much juice your actually using.l but the new one does not have that.

 

[tomjasz]

These charts have more information for you to overload on, including links to a source that shows battery discharge curves for individual cell brands/m

You my friend are a brilliant fellow. Those charts are incredibly helpful, WOW, just WOW!

 

[Gionnirocket]

Curious why all the charts that I've seen on here have 3v/cell or 39v on a 48v battery as 0% as all the manufacturer battery spec sheets that I've seen have a 2.5v - 2.75 as LV cutoff.
I realize the need to err on the side of caution, but I find the inaccuracy annoying

 

[scrambler]

Not an inaccuracy, but a strong recommendation.
Going below 3V reduces the battery life significantly

 

[m@Robertson]

You my friend are a brilliant fellow. Those charts are incredibly helpful, WOW, just WOW!

you haven't seen those before? I think they were my first page on the site. They are the most popular by a long shot. Google has picked them up.

Just this morning I got a comment from someone telling me all the voltages are off. Thats a first. He has not noticed I think that they were published in 2019 and in fact he can do the math himself on voltages thanks to the info on those pages that displays the entire calc method and all inputs.

 

[Gionnirocket]

Not an inaccuracy, but a strong recommendation.
Going below 3V reduces the battery life significantly

Understood... and I'm not suggesting to anyone to do so
But a strong recommendation doesn't equate to an accurate measurement

 

[m@Robertson]

Understood... and I'm not suggesting to anyone to do so
But a strong recommendation doesn't equate to an accurate measurement

Yeah but he's on the money there. 3.0v is technically considered to be a 0% charge (Anything less is below the minimum operating range for an 18650 cell) and I would argue you are going to kill the pack dead as a doornail if you let it go down that far. On storebought bikes I bet the Low Voltage Cutoff in the controller is set up for more than that. Go that low on a pack and you stand a risk of the BMS refusing to accept a charge at all. At which point you have to disassemble the battery and manually apply current to the cells to get them up over the he's-dead-Jim threshold.

 

[Gionnirocket]

Yeah but he's on the money there. 3.0v is technically considered to be a 0% charge (Anything less is below the minimum operating range for an 18650 cell) and I would argue you are going to kill the pack dead as a doornail if you let it go down that far. On storebought bikes I bet the Low Voltage Cutoff in the controller is set up for more than that. Go that low on a pack and you stand a risk of the BMS refusing to accept a charge at all. At which point you have to disassemble the battery and manually apply current to the cells to get them up over the he's-dead-Jim threshold.

I do agree that going below 3v is not recommended, especially if you don't know the details of your specific pack.. but it is not below the minimum operating range of most if not all 18650 cells.
Considering that LV cutoff happens under some load and within seconds... V bounce back at rest will occur.
I know I'm being anal and splitting hairs here, but in my mind it should read as it is in your best interest to not drop below 10% / 3v

 

[scrambler]

All that said, all cell capacity specs are based on 2.5V, so all the advertised Battery capacity are actually not the usable capacity.
But as long as everyone does the same assumption, we can still compare them using that method

 

[m@Robertson]

I do agree that going below 3v is not recommended, especially if you don't know the details of your specific pack.. but it is not below the minimum operating range of most if not all 18650 cells.
Considering that LV cutoff happens under some load and within seconds... V bounce back at rest will occur.
I know I'm being anal and splitting hairs here, but in my mind it should read as it is in your best interest to not drop below 10% / 3v

Yeah you're right. I had to go back and look at what I did with those charts and check some specs. It is common to use 3.0v, but thats usually to set the LVC on at least some controllers. Its wrong to say thats the minimum for the cell itself.

However, looking at whats out there in the world, it does look as if 3v is the standard. the Luna chart linked above uses 3v as well. We can tell that from the 55.4v 80% charge value on a 52v pack. Its pretty much universal that 55.4v is an 80% charge on a 14S pack so the 3.0v rule of thumb is widespread I think. I've seen other sources saying the same thing now that I am looking around. Its the safe generic number to use absent knowing the exact specs of your cell.