Motor Watts v. Volts x Amps (Inconsistencies)

Zoumios

New Member
Quick discussion I have to bring up considering I know little to nothing about ebikes.

As I understand it, peak motor watts is designated by the volts x amps that the bike posses (ex: 36V x 15A = 540 watts)

Here's the quandary:

This E3 ProTour is listed as a 500 nominal watt ebike, yet it's battery watt hours is only 417.6. Isn't that a contradiction? If its peak watts, not even nominal, is 417.6, then how is it claiming 500? https://electricbikereview.com/izip/e3-protour/

Here's another scenario, though:

This Specialized Turbo Vado 5.0 is listed as only a 350 watt ebike, yet it pumps out 604.8 watts. Couldn't the nominal be considered higher? https://electricbikereview.com/specialized/turbo-vado-5-0/


In both instances, I assume the answer is not only easy but obvious for someone who actually knows what they are talking about.

Note: I am not directly comparing the bikes or claiming they are similar. I used them both as individual examples for this question.

Thank you in advance.
 

Larry Ganz

Active Member
If a 36V x 14A battery = 504 watt hours, that means that a 500 watt motor can run for 1 hour before the battery goes dry. Therefore, a 350 watt motor could go for an hour and 25 minutes at full power before the battery runs dry.

Similarly, if the E3 ProTour has a 500 watt motor and a 417WH battery, then it can run at full power for 50 minutes before it runs the battery dry (417WH/500W motor - 0.834 hours of operation).
 

pxpaulx

Well-Known Member
You're mixing two different details. The watt hours of the battery is a measure of the available energy capacity of the battery pack, as Larry described.

The peak motor output is a measure of how much of that battery energy can be drawn at a given moment from the battery.

So while the processes are tangentially linked, they are not a direct correlation to one another.
 

harryS

Well-Known Member
Supergoop, the safe working range of an 18650 cell is 2.5V to 4.2V. They don't die below 2.5 v. They keep working, but as they push out current, metallic deposits form inside. These are like metal filaments that short circuit the battery, create heat, and fires. For that reason, battery management systems and motor controllers are designed to shut the battery off at 3.0 volts. So the battery shuts off (dies), but the cells are still partially charged.

The AH rating of a cell is specified for discharge down to 2.5 volts. Accordingly, we never get to use the full AH rating of a battery. There isn't that much more power available running between 3.0 and 2.5 volts. My guess is between 10-20% is not used. Usually, people ignore that when computing range, and it doesn't matter.

Zoumis, as the other posts told you, you cannot mix watt-hours and watts.
-Watt-hours is the nominal voltage of a battery x amp-hours (AH).
-Watts is volts x amps.
-AH is a capacity number. For example, 10AH means a battery could supply 10 amps for 1 hour, or 20 amps for 30 minutes, except they shut off at 3 volts per cell, so derate the AH by 20%.
 

Mr. Coffee

Well-Known Member
I'm really surprised that e-bike batteries don't have a stabilizer circuit built into them to provide more constant output voltage. A lot of electronic circuits can't tolerate a dramatic voltage change so it is actually pretty common. I suppose there might be some efficiency losses but it sure sucks for me that I get the least amount of power when the battery is drawn down. It is always uphill to my home.