Voltage drop off and your KWH per mile

therobby3

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I took my electric conversion trike for my first serious range test yesterday and unfortunately got stranded with a dead battery. =[ So I had two main questions in this thread. My first is about voltage drop for batteries.

Is voltage drop linear?
I built my battery out of tesla 21700 cells. Anyway, I went on a trip that was 28 miles one way. When I got to the 28 miles, my voltage went from around 108 volts down to around 95. However on the way home, it went from that 95 volts down to the low 70s, at which point it was dead. So that's quite a significant drop on the ride back. That means I dropped 13 volts during the first 28 miles. Then on the way home it died about 17 miles in, which means it dropped a further 21 volts in just 17 miles (95v down to around 74).

So my question is, is this normal? Do lithium ion batteries' voltages drop faster as they lower? meaning, it's not a linear drop off? In other words, a 109v fully charged battery, and 74v fully discharged, mathematically would be at 50% at 91.5 volts. But is this not really how the battery depletes in real world use?

If you have an electric build, what is your average watt hours per mile?
My machine is around 560 pounds, with a big 12kw QS hub motor in it. I was trying to drive very conservatively in order to conserve battery the whole trip. The math works out to around 150 watt hours per mile that I was getting. This was mostly back roads, with a speed of around 40 to 50mph. I'm a little disappointed with that number, I thought I would be able to get closer to the 100wh per mile mark. I don't have much regenerative braking settings enabled yet, but nonetheless I don't feel that would have made a significant difference given I was driving pretty easy. If anyone else has a similar motorcycle/scooter build, what types of WH per mile figures are you getting?
 
Are you running 26S?

My pouch cells have a near linear behaviour between 4,1V/Cell down to 3,3V/Cell.
Consumption is 80-160Wh/mile.
80Wh is with a lot motorway around 50mph.
160Wh on curvy motorcycle roads with near permanent full throttle. Power drawn from battery permanently around 11-13kW, Motor overheats in 15Minutes.
Scooter weight aprox. 380pounds + me 180pounds 5kw hub motor 21S5P (100Ah) SMVC72150
 
Thanks for the response. Yes, I am running 26s. Ok, so yours is fairly linear. What about below 3.3, or is that usually your cutoff?

80-160wh per mile for a 380 pound machine makes my 150wh sounds like perhaps it is about right. My concern was that I did the math looking at Zero motorcycles and the Harley Livewire and they were closer to 85 to 120wh per mile. However, my thought was that perhaps they are pretty optimistic in those ratings and that real life distance is quite a bit lower.
 
I haven't tested under 3,3V whilst driving.
Cut off is set to 3,0V.

When I build the pack, I made a static test with 83A continuous with an calibrated electronic load.
My 100Ah pack made 97,7Ah according to the calibrated load. The BMS said 2,4Ah left at SOC 3% with cut-off set to 3,0V/Cell under load.

At 3,4V the BMS stated SOC 20%
at 3,3V it said 10% SOC ,
at 3,2V it said 6% ,
At 3.1V the cells started to drift extremly


The voltage drop with 83A at my cells is 0,05V/Cell
During driving the controller draws up to 170A.

Screenshot_20211006-222855.jpg
Screenshot_20211006-222822.jpg
 
Wh/mile is depending on your throttle , more than on anything else
 
Ok thanks for the info! It sounds like judging by yours, that it is a fairly linear drop off down to around 3.1v, then anything under that it pretty poor and drops off fast.
 
I would regular only go to unloaded 3,3V/Cell.
Beyond this there is only 10% left.
 
Capacity is definitely not linear, w/voltage.
See - Capacity mapping
Different cell formulations can vary widely!
 
dominik h said:
Wh/mile is depending on your throttle , more than on anything else

* Average throttle value / ride, to be more accurate.

I run anywhere from 60-160 wH/mile when going fast and like 20-40wH/mile when slow or taking my time. ON my bikes.

In any instant, wH/mile can be in the hundreds easily.. with high power builds... a single pull, in 20 sec, might be 8-900watt Hours /mile easily. This is why you look at the average throttle value for the ride. Any instant may be very high. On datalog I have seen ridiculous instant wH/mile reading.. but for most miles you do not get that much... once you have traveled at least a mile... This means if I continue the rate of discharge I will be at this wH/ mile ( 800? 900?) but the speed picks up, current drops, and load ( acceleration and tractive) are lesser thn me and my drag race... Drag race is short time. Short track.. small wH/mile average but huge wH/mile instant... the goal of the drag racer is to increase this "average" as much as possible without reaching a compliance of the voltage from overloading the battery.. ( and losing volts)...


On a typical power bike, I would expect to see 40-60 wH/mile with typical weight and speeds.. But how much sag, is Ok? Depends on the IR of the pack vs the loss load and power of the controller...

A very lossy system might sag very much .. power wasted before the controller... Or.. a very lossy system might be in the controller: Asking too many amps for the battery to comply with, and the battery then breaches compliance voltage and begins to sag badly after this. And create heat..

....many cell have different "SOC vs Voltage" curve and many cell have " stiffness"... Typically a trade off for power vs energy density in the numbers.

With mostly all lithium 3.2v is essentially dead... 3.0v / cell empty empty. The weakest cell group ( in holding capacity) will stop the discharge through the BMS usually. When you go all the way don, the worst group stops the discharge first... BMS says " oh no a group is to low" and stops the discharge.. so.. not all groups are reaching 3.0v... some are still at 3.2v... ect.. but the worst group stops the charge and then the whole battery says " Im done" from the BMS.
 
Ok, so it doesn't look completely linear for sure, but it does look fairly close judging by the charts. At least down to around 3.1v, which would align with what dominik said in his experience.

@Dogdipstick Just to be sure, when you say you get 60-160wh on your "bikes", are you referring to bicycles or motorcycles/scooters?
 
therobby3 said:
on your "bikes", are you referring to bicycles or motorcycles/scooters?

Light motorcycles disguised as bicycles with cranks and operational PAS systems....I like to pedal, I do.... Have a amp dial and 3 limit preset, ... but.... But the availability of power upon demand. I often go for miles at 4-7mph pedaling with little as 2A input.... but have the amps on a dial if I need to go fast. When the whole system is working... Lol.

I do have a bunch of graphs showing human vs bike power. From the torque sensor, shunt, controller, ect.. and the logger on the CAv3.14
 
DrkAngel said:
Capacity is definitely not linear, w/voltage.
See - Capacity mapping
Different cell formulations can vary widely!
DrkAngel said:
Swing 5300

"cycle life" indicates DOD as major factor in usable life!

file.php


3.40V recommended as reasonable static DOD, but, 3.50V might double usable life! ... ?

Added scale to measure mAh incrementally.

file.php


4.2 - 4.1V 100mAh
4.1- 4.0V 450mAh
4.0 - 3.9V 450mAh
3.9 - 3.8V 500mAh
3.8 - 3.7V 525mAh
3.7 - 3.6V 700mAh
3.6 - 3.5V 1100mAh
3.5 - 3.4V 650mAh
3.4 - 3.3V 380mAh
3.3 - 3.2V 130mAh
3.2 - 3.1V 80mAh
3.1 = 3.0V 40mAh

Using Paint
Created scale from 1064mAh
Compered 10 to 4 segments
10/4 - shrunk 40%
1000/1064 - shrunk to 94% for accurate scale
copied and pasted to original picture
"copied" and dragged to measure between voltage lines to measure mAh

I have previously noted that specific chemical formulations of Lithium cells produce differing optimal discharge voltages.
This minimal discharge rate graph indicate 3 distinct optimal discharge voltage points. (18650 cells)

file.php

Red - Most common have reasonably steady output till 3.6V
Green - Loses reasonable use at ~3.4V
Blue - With the highest capacity, has decent output till ~3.2V

This seems to indicate 3 distinctly separate "formulations", each with their own specific optimal discharge voltages.

Discharging deeper supplies minimal mAh of saggy-pitiful voltage. And likely ... inflicts excessive cell damage for minimal return!

Note: this graph was made at a .2A discharge rate, so optimal discharge voltages are near the cells true resting voltages rather than the cell readings under typical discharge rates.
After determining your cells optimal discharged voltage, take cells to that point, apply typical discharge rate, note voltage sag and determine that as pack "empty" voltage during "typical use".
 
Related ... somewhat!
Please notice ... the 2900mAh cells imply a great increase in capacity over the 2500mAh cells, not true!
The 2900mAh graph shows a marked lower voltage below 3.8V. This nearly equalizes the actual (usable) mWh of electricity!

file.php


Additionally, I prefer the higher sustained voltage for a more even performance profile.
 
I have a theory why the batter discharges faster as the SOC goes down.

My QS205 powered Vespa has a 72V battery.

100% SOC is 84V, 50% SOC is about 72V, and 0% SOC 60V.

The motor controller will draw as many amps needed to produce the power being asked for. As
volts go down, amps go up.

If power demand is, for example, 2000W, then the Amperage needed is:
2000/84 = 23.8A for 100% SOC
2000/72 = 27.7A for 50% SOC
2000/60 = 33.3A for 0% SOC


As the amperage goes up, energy wasted as heat in both the internal resistance of battery, and also in the wiring goes up as well, thus a greater amount of power (watts) needs to be draw from the battery to make up for.
 
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