Calculating watt hours used

P

PRW

100 kW
Joined
Nov 10, 2011
Messages
1,046
Location
Melbourne, Australia
so, on a recent ride, my voltage dropped from 75v to 60v over 30km. Battery is a 18s6p 30Q.

I have calculated 64.8v x 18AH = 1166WH available; voltage drop possible say 75 - (18*3.2) = 17.4v

Dropped 10v/ 17.4v = used 10/17.4 * 1166 = 57% of 1166 = used 670WH, therefore 22WH/ km.

Is there a simpler and more accurate way of calculating this?

thanks
 
Measure it directly with a shunt based coulomb counting wattmeter.

In Ah not Wh
 
Because voltage drop is not linear with many lithium battery types, calculating it does not work so great. There tends to be a flat spot in the middle of the discharge that means a quite a few watt hours can discharge with a very small voltage drop. Then near the end of the discharge, voltage drops off a cliff if you are charting it.

Any kind of watt meter is better. Ideally an expensive Cycleanalyst, but even a cheap one will work fine as long as it goes to your working full charge voltage. Some have 60v max.

Once you have a watt meter, then you can do test rides to find out what your real world capacity is, in watt hours or ah. Once you know that, simple math tells you when to turn for home, or that you need to turn for home AND slow down a bunch.
 
I use these el cheapo meters to measure watts consumed when I do a capacity test, I plan on putting one inline with the battery to see how much ah I use. As long as my BMS does not turn off the power, I should have the reading history on it.
There are some which takes a micro usb connector for backup power and will save the history, I think my other one show me MAX amps.

Note: the wires DO get HOT around 20a sustained,

I discharged a 12v at 23a for 3 hours and the wire was 100F

https://www.amazon.com/Proshopping-Precision-Analyzer-Checker-Measurement/dp/B07Z8XJ71K

your VOLTAGE is more than the max for this unit, please research something similar with higher voltage rating.
 
As a temporary measure, just time your charger.
If your charger is 2 amps, and takes 3 hours, it put in 6ah of energy. If your cells are rated at 12Ah, you know your battery was approximately half empty.
Colin
 
dogman dan said:
Because voltage drop is not linear with many lithium battery types, calculating it does not work so great. There tends to be a flat spot in the middle of the discharge that means a quite a few watt hours can discharge with a very small voltage drop. Then near the end of the discharge, voltage drops off a cliff if you are charting it.

Any kind of watt meter is better. Ideally an expensive Cycleanalyst, but even a cheap one will work fine as long as it goes to your working full charge voltage. Some have 60v max.

Once you have a watt meter, then you can do test rides to find out what your real world capacity is, in watt hours or ah. Once you know that, simple math tells you when to turn for home, or that you need to turn for home AND slow down a bunch.
Click to expand...
thanks - one of my bikes has a CA, I didn't realise it showed AH/ WH! will have a look, and a watt meter for the other
 
gobi said:
I use these el cheapo meters to measure watts consumed when I do a capacity test, I plan on putting one inline with the battery to see how much ah I use. As long as my BMS does not turn off the power, I should have the reading history on it.
There are some which takes a micro usb connector for backup power and will save the history, I think my other one show me MAX amps.

Note: the wires DO get HOT around 20a sustained,

I discharged a 12v at 23a for 3 hours and the wire was 100F

https://www.amazon.com/Proshopping-Precision-Analyzer-Checker-Measurement/dp/B07Z8XJ71K

your VOLTAGE is more than the max for this unit, please research something similar with higher voltage rating.
Click to expand...
thanks, will research - and thanks for the voltage warning
 
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