Simplified expert explanation please: batteries.

[allen-uk]

I know I've asked similar questions before, but please bear with me...

When my battery indicator says that I've got 75% of battery capacity left, what does it mean? Does it mean that I've got 75% of the Volts, or the Amp/hours, or the Watts that drive the motor?

And again harking back to an earlier thread, how/why does this translate into less oomph (real word possibly torque or maybe power) - in practical terms at 100% capacity I sail up certain hills, at 75% I have to push a lot harder.

If I weighed 30kg less (currently 120, gord), would I still sail up at 75%?

As Brecht said, so many questions (although I bet he never rode a Wisper).


Allen.

 

[NRG]

75% of the capacity either measured by the Ah or better the Wh (watt hours: volts x Ah) not the volts. The volts drop as capacity is used up, quickly at first then a slow plateau with a very sudden drop as capacity runs out.

The oomph drops off because the volts have dropped. The controller will be limited as to how many amps it can draw but this will be a constant. IE: it will allow the same current draw from the battery at the start of your ride as at the end, let's say 10 amps. But the voltage will be high at the start of your ride and will be lower at the end, so a 36v battery will be about 42 volt fresh off the charger and maybe 10v less near the end. This means the amount of power (watts) will be greater at the start than the end of your ride....in this example 420w vs 320w....so the bike feels perky when you set off but falls off towards the end.....

Weight matters: the lower the weight the better the power to weight ratio so less power required from the motor.

Hope this makes sense!

 

[onmebike]

Why don't these indicators reflect the useable capacity?

If 31v, the cut off voltage, registered as 0% and 41v or fully charged registered as 100%, 75% would be around 38.5v and there shouldn't be any noticeable loss of performance.

 

[NRG]

But the battery discharge curve is not linear, agreed though these indicator lights are very inaccurate and are only partly accurate when the battery has no load.

 

[flecc]

There's some additional information on this in my article in the Technical section:

Battery Meters
.

 

[allen-uk]

Thanks NRG, flecc. That starts to make sense.

I suppose that 'usable capacity' depends too much on subjective circumstances to be quantifiable (weight, fitness, how many big hills, etc).

A