How do you calculate range? I know the normal way of (capacity)/(wh/mile)=mileage
and I know that capacity is (# of cells in series)x(# of cells in parallel)x(cell voltage)x(cell capacity)=pack capacity
but why do we use the nominal (3.6v for Li-ion) voltage? We know that higher voltage has efficiency gains, so why not 4.2v? Is this a compromise to avoid doing integrals for capacity as voltage drops? What’s the complicated version of this formula that gives the true capacity? How good is the approximation we use now?
I have a background in physics (bachelor’s degree) and I don’t see why we calculate at 50% voltage except to make conservative estimates. Integrals suck so please tell me I’m wrong here.
I think it was shartis that said you want to step up in voltage when the power wires can’t handle the current. I don’t know if there is any truth to this. Most of us aren’t even stressing 12awg.
Capacity doesn’t change with voltage. Maybe I’m misunderstanding what your’e saying
Regarding range. What works for me is ((capicty)x.85)/(wh/mile)=range… the .85 is going to change depending on your cuttoff. I usually have it at 2.9v. Most probably at 3.2v so maybe .7 for the calculation. If you wan’t all the capacity you have to set it at 2.5v cutoff but your battery well get ruined for sure.
3.6v is the nominal voltage e.g the average voltage through a discharge. lipo has a nominal voltage of 3.7v because it’s cutoff voltage is higher than lion.
I don’t really understand all your following questions, the equation is simple because we just use specs off the cell’s data sheet. We have ton make a few assumptions because real world is different to lab testing.
range varies a lot, there are too many factors in play. comparable data should just be Wh, but extra info for example battery config / p groups would also helps.
Mileage depends on wh/mile but also, the voltage of your battery falls faster or slower depending on how hard youre pushing your board (so wh/mile varies during the ride). So theoretically, you could make a function that gives you the voltage of the battery throught every given ride, but its impossible to predict the range if you dont know the wh/mile you will be doing exactly.
BUT, to answer your question, you dont need integrals to calculate the voltage of the pack throught out the discharge because you really should be looking at it as power stored in each individual cell.
EXAMPLE: An 8AH cell will drain to its minimum voltage if you apply to it a constant load of 8A for an hour. So in a perfect world it will discharge at 8A @ 3,7V for one hour. Which if you multiply its 29,6wh (this is the power capacity of the cell measured in watts it can discharge constantly in an hour). As we dont live in a perfect world the voltage of this battery will change throught the discharge so the amps the cells are able to discharge change throught the discharge as well: So at 4,2V the maximum discharge it will be 7A because 7x4,2 = 29,6wh.
I dont know if im explaining my point, im from spain and explaining this has been tremendously dificult XD
So the point is the voltage that matters is the nominal voltage, as in the end, its the total power of the cell that matters, if it holds 29,6wh doesnt matter at what voltage (for the range calculations) because in the end without taking in account heat losses and shit, from each cell you will extract a certain amount of wh
Think very well explained above. Look at cell discharge charts they show under different load how much time you can discharge them and the total usable Wh pulled out, the slower you draw current the less loss as heat so more usable Wh you get from each cell.
Also altitude changes over a ride end up at 0 if you do a loop but going up steep inclines will result in more amp draw and losses and high current charging will also lose more of the charge ‘in transit’ from esc to battery etc.
The 3.6V (times capacity in Ah, or amps per unit time) is a good average value for getting the right Wh since about half the time of the discharge it’s over this value and about half the time it’s below is another way to think about it.