How do I calculate Watts used per mile

[trevor brooker]

I know the voltage before & after ride & mileage is this sufficient information?
I have a 52v 17.5Ah battery. 100% = 58.8v 0% = 42v so 16.8v range.
if I start at 55.5v (55.5v-42v/16.8v = 80%) & use say 15% of the battery to cover 15 miles I can compare different assist levels
But without purchasing a watt meter can I estimate watts per mile?

 

[anotherkiwi]

A Watt meter is a painless way of calculation and belongs in every e-bikers kit.

 

[Deleted member 4366]

I know the voltage before & after ride & mileage is this sufficient information?
I have a 52v 17.5Ah battery. 100% = 58.8v 0% = 42v so 16.8v range.
if I start at 55.5v (55.5v-42v/16.8v = 80%) & use say 15% of the battery to cover 15 miles I can compare different assist levels
But without purchasing a watt meter can I estimate watts per mile?

Rate of voltage going down isn't constant, so that calculation doesn't work. If you want to know how your bike works, get a wattmeter.

 

[Woosh]

you can use the middlepoint of the voltage readings to calculate WH used per mile. Measure the voltage at the start and when you arrive at destination (you have to rest the battery for a minute or more for accurate assessment).

Let's say you have a 36V 15AH battery and ride 10 miles.
Say you read 41.5V. Measure the voltage Say you read 40.5V.
the middlepoint of your reading is (41.5V + 40.5V)/2 = 41V
Percentage used: (41.5V - 40.5V) / 10V = 10%
Power used: 15AH * 10% * 41V = 1.5AH * 41V = 61.5WH

watts per mile = 6.15WH//mile

To condense it in something simple for a 15AH battery:

WH = 1.5AH * Voltage / distance.

for a 10AH battery:
WH = (1AH) * Voltage /distance

 

[trevor brooker]

you can use the middlepoint of the voltage readings to calculate WH used per mile.

Let's say you have a 36V 15AH battery and ride 10 miles.
Say you read 41.5V. Measure the voltage Say you read 40.5V.
the middlepoint of your reading is (41.5V + 40.5V)/2 = 41V
Percentage used: (41.5V - 40.5V) / 10V = 10%
Power used: 15AH * 10% * 41V = 1.5AH * 41V = 61.5WH

I understood & followed everything but would just like to check something
Percentage used is based upon 36v battery (18650 x 10 cells)
so max voltage is 4.2v x 10 cells = 42v
so min voltage is 3.2v x 10 cells = 32v
so difference is 10v
hence in your example start volts less end volts divided by difference of 10v
so this would change for 48v or 52v batteries to `13v or 14v

 

[Woosh]

this would change for 48v or 52v batteries to `13v or 14v

that's correct.
For the same difference (start Volts - end Volts), you go farther, more miles, at the same speed with a 48V battery than a 36V battery or a 24V battery, more miles with a larger capacity versus a smaller capacity, more miles when the battery is fully charged than when it's depleted.