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.

 

[slitespd]

Just charge up and then pay a bit of attention to the average milage per 1v that you are getting and go from there. There will always be variables no matter what you do, so simply try to ride within those variables. I get about 15 miles per volt in my terrain and I always leave a few volts in reserve before I reach cut-off voltage.

 

[Jeremy McCreary]

Just charge up and then pay a bit of attention to the average milage per 1v that you are getting and go from there. There will always be variables no matter what you do, so simply try to ride within those variables. I get about 15 miles per volt in my terrain and I always leave a few volts in reserve before I reach cut-off voltage.

You get the same miles per volt throughout your useful voltage range?

 

[slitespd]

You get the same miles per volt throughout your useful voltage range?

I can’t tell you because…………..the last thing I’m going to be concerned about are those perhaps slight variations. Fifteen miles per volt works for me!

 

[Slaphappygamer]

I can’t tell you because…………..the last thing I’m going to be concerned about are those perhaps slight variations. Fifteen miles per volt works for me!

So, from 54v to 48v you get 90 miles?

 

[Chargeride]

Mine always runs out a bastard way from home.

 

[slitespd]

So, from 54v to 48v you get 90 miles?

Yep.……… and that’s enough for me. I’ll very infrequently ride it to the limits in one go! I usually ride from 54v to 47v in several rides.

 

[Roamers]

Mine always runs out a bastard way from home.

Doesn't anyone have "estimated range" display?

 

[fooferdoggie]

Doesn't anyone have "estimated range" display?

sure my bosch nyon does it and if I mapped out a route it would let me know how much battery I would need. same with specialized. but it takes a very good well-managed battery to be able to do that kind of prediction.

 

[harryS]

Yep.……… and that’s enough for me. I’ll very infrequently ride it to the limits in one go! I usually ride from 54v to 47v.

That's really good. I believe you are on a BBS02B? You must be riding with minimal assist.

My BBS02B burns 8 watt-hour/mile (years of data) with a combo of PAS1/PAS2, I have a 15AH pack on my BBS02, rated for 720WH but it's more like 630 WH in real life,. I put on 38 miles last week in the cold. That would be 300 watt-hour..I've used up half the battery. It's at 48 volts. so that works out too, I'll put on a power meter when I recharge to see the actual AH. I know I'm putting it to the math majors.

As for estimated range, I know what my bikes and batteries can do, but it's always good to be home before I'm under 50%.

 

[PCeBiker]

I can’t tell you because…………..the last thing I’m going to be concerned about are those perhaps slight variations. Fifteen miles per volt works for me!

What would Really help with the fudge factor with any battery is to find out how far you get on your Last Volt before warnings and shutdown, then multipy that by your number of useful volts.

I usually ride from 54v to 47v.

54v-47v = 7v × 15mpv is 105 miles.

But lithium batteries are shut down (or slowed down) at 39v-42v so that's at least 12 usable volts, so 12×15mpv=180 miles.

That's just simply not going to happen.

Doing your math and testing using the FULL range of your battery yeilds more accurate results.

And using The LAST Useable Volt as your mpv parameter yeilds the most conservative range possible when the proverbial s*it hits the fan like how @Chargeride mentioned.

Knowing the maximum range possible and the minimum range on your last volt helps you guesstimate "50%" much more accurately

 

[Roamers]

sure my bosch nyon does it and if I mapped out a route it would let me know how much battery I would need. same with specialized. but it takes a very good well-managed battery to be able to do that kind of prediction.

I suppose as a recreational rider it's easy to periodically check range and adjust plan as necessary. Or plan grocery shopping within range; I know some need to travel further, but after 1 trip pretty easy to know capabilities.

 

[PCeBiker]

,.. but it takes a very good well-managed battery to be able to do that kind of prediction.

You not only need a very good well-managed battery to be able to do that kind of prediction.

You need to be a very good and well-managed rider and NEVER do anything out of your ordinary to mess up the results.

Like riding into a 30mph headwind.
Or riding up a 30° incline.
Or getting stuck in the mud and getting 0 mpv.

I got mud jammed between my fenders and tires and the wheels stopped turning.

I managed to get the damn thing back to the road where there was enough fiction from the asphalt to rotate the tires, but I was using 21 amps with at least 50 volts left in the battery and I only going 8-12 kph.

I could smell the mud heating up getting turned into Adobe bricks.