Calling all math majors

[jobtraklite]

I've been trying to figure out how much range I have left at any point in a ride with a 48V battery.

Assumptions: max voltage when fully charged = 54.6
Low voltage cutoff = 39.0
Usable volts = 54.6-39.0 = 15.6

Current status
mileage 20
voltage reading 47.0

So % of usable voltage used = 100% x (54.6 - 47.0)/15.6 = 48.7%

Expected total mileage = 20/.48.7 = 41.1 miles
Remaining miles = 41.1 - 20 = 21.1 miles

ASSUMING THE USUAL FACTORS REMAIN THE SAME: SPEED, TERAIN, WIND, ETC.

Do I flunk the test?

 

[Roamers]

Do I flunk the test?

Easy to check hypothesis; ride it and see how much further you go.

 

[Mike_V]

Yes, there are too many dynamic variables you didn't consider, obviously setting yourself up in some passive aggressive manner light new member.
Ambient temperature and battery pack temperature over time for example since chemical reactions generate the electricity.

 

[Jeremy McCreary]

I've been trying to figure out how much range I have left at any point in a ride with a 48V battery.

Assumptions: max voltage when fully charged = 54.6
Low voltage cutoff = 39.0
Usable volts = 54.6-39.0 = 15.6

Current status
mileage 20
voltage reading 47.0

So % of usable voltage used = 100% x (54.6 - 47.0)/15.6 = 48.7%

Expected total mileage = 20/48.7 = 41.1 miles
Remaining miles = 41.1 - 20 = 21.1 miles

ASSUMING THE USUAL FACTORS REMAIN THE SAME: SPEED, TERAIN, WIND, ETC.

Do I flunk the test?

Before it's a math issue, it's an engineering issue. You're assuming that voltage is a linear function of distance traveled at constant ground speed, slope, headwind, surface properties, assist level, and rider power — with all these parameters exactly the same before and after reaching the intermediate voltage of interest (47.0V in your example).

Do you have any data supporting that unlikely assumption in your bike's case? And do you ever actually ride that way in real life?

If no to either, the results would be meaningless.

 

[Chargeride]

The wind refuses your parameter.

 

[Slaphappygamer]

The wind refuses your parameter.

Gravity agrees.

 

[harryS]

Familiarize yourself with how your ebike runs, A lithium battery is nonlinear. You get far more miles for 54-to-48V than from 48-to-40V. Just go out and ride it. You'll see.

 

[6zfshdb]

First of all, you don't want to discharge your battery to the LVC point on a regular basis. I would use 42V instead of 39 for any calculations. The simple fact is, the last volt won't get you as far as the first. If you ride out until the voltage drops to 50%, you will likely not have enough juice to get back.

As others have said, there are too many variables in addition to those you listed, including temperature and battery age, to be able to make any accurate math calculations. All lithium batteries loose capacity at lower temperatures. Older batteries can have an unpredictable voltage drop as the load varies, which can activate the LVC even though significant charge remains.

I gave up trying to calculate remaining range. Through experimentation, I made my own chart to get a rough idea of how much farther I can go. When in doubt, I carry a spare battery to eliminate range anxiety.