Voltage Drop during battery discharge

[SteveRuss]

Granted, I haven't actually taken a meter with me to check this out but. I've been riding quite a lot recently and i'm starting to wonder whether my battery is actually supplying less power to the motor as the battery discharges below a certain point.

After about 35 miles ride yesterday, my battery was down to about half and the assistance felt normal. A couple of days before, I rode around 30 miles on full power assistance (wind and lots of steep long hills) and I was on about 1/4 battery by the time I got home.

The thing that I can't figure out is. Is it me running out of energy and feeling like the bike is assisting less efficiently or is it the fact that the battery/controller isn't able to maintain the 36volts and/or current flow? Sometimes it feels like it's slowly fading away but it could just be me..

 

[trex]

the maximum power you get is proportional to the square of the battery voltage.
When the battery is half drained, its voltage sags for about 8%-10%, the maximum power you get at the wheels drops about 15%. The drop accelerates with further battery drain. When your battery has only 20% remaining, the maximum power drops by about 30%.

 

[SteveRuss]

the maximum power you get is proportional to the square of the battery voltage.
When the battery is half drained, its voltage sags for about 8%-10%, the maximum power you get at the wheels drops about 15%. The drop accelerates with further battery drain. When your battery has only 20% remaining, the maximum power drops by about 30%.

Thanks trex. That feels about right then.

In fact, it feels like i've lost a lot more than 30% but that would explain that i'm knackered by then as well. More so than the battery!!

Cheers. Steve

 

[Deleted member 4366]

It's a bit simpler than that: Your battery is 42v when fully charged and 31v when empty. It goes down more or less evenly when you use it (not LiFePO4 or lipo). Your controller limits your current to a maximum of say 15A. When fully charged you get 42 x 15 watts = 630w and when empty, you get 31 x 15 watts = 465w.

That's power from the battery, but with less power, you can't maintain such a high speed, so your motor becomes less efficient. You therefore become a double loser, losing both input and output power.

 

[SteveRuss]

It goes down more or less evenly when you use it (not LiFePO4 or lipo).

I've been thinking about this . Do LiFePO4 lipo batteries maintain their voltage over discharge then? (presumably with a definite drop off at the end!). If so, aren't they the more obvious choice for Pedelecs?

Cheers.

 

[trex]

No. LiFePO4 has lower voltage per cell, forcing you to carry 10% extra weight, making these batteries less (gravimetrically) efficient. Their flat discharge character also makes their battery meter a less reliable indicator. Their good points: very safe and can last a very long time.

 

[Deleted member 4366]

I've been thinking about this . Do LiFePO4 lipo batteries maintain their voltage over discharge then? (presumably with a definite drop off at the end!). If so, aren't they the more obvious choice for Pedelecs?

Cheers.

A fully charged LiFePO4 battery is 43.8v. As soon as you start using it, the voltage drops to about 39.6v. They then go down very slowly to about 37v, after which they drop like a stone. Basically, they give up all their charge between 3.3v and 3.1v per cell (12 cells).

This characteristic could be useful for ebikes, and is one of the reasons they were more popular a few years ago, but the the latest 18650 Li-ion cells are nearly half the weight of LiFePO4, so it's no contest.

 

[SteveRuss]

I liked the comments but just to say thanks to the engineer/enthusiasts here for all that good stuff information..