Questions About Kill A Watt Readings

[LimboJim]

I have a few ebikes with different motor systems: Shimano e8000, Bosch CX, Yamama PW, and Brose T. The first three have 4A chargers, the last 5A. I also own a 2015 iZip Sumo with CurrieTech/TranzX motor; that bike came with a 2A charger.

I started monitoring the amp draw on my Kill A Watt meter as I charge them from below 50%, and noticed that the first three all show about 1.4 - 1.5 amps until they hit 85-90%. Then, the amp readings start dropping, all the way to 0.2 amps as they approach full.

The TranzX charger, however, charged around 1 amp, then slowly dropped off as it approached full. So here are my questions:

Why don't I see 4 amp readings from 4A chargers? And why do 4A chargers show just 1.5 amp max, while 2A shows 1 amp?

 

[Deleted member 4210]

By the way, your kill-a-watt is on the AC side of the circuit, whereas that '4 amp' rating of your charger may actually represent the current flow on the DC output side of the charger. So your readings may actually be correct for the AC current that is flowing into the charger, where it's then converted ( rectified) to DC before it flows into the battery.

 

[Deleted member 4210]

And charge rates are always slower from full depletion (first 10 to 20%) and at the end of the charging cycle, or last 10 to 20%, or roughly after you get the battery to 80 to 85% charged until fully charged. Cells or rather the pack, is being 'balanced ' during that slower rate of charging at the end.

 

[Ravi Kempaiah]

I have a few ebikes with different motor systems: Shimano e8000, Bosch CX, Yamama PW, and Brose T. The first three have 4A chargers, the last 5A. I also own a 2015 iZip Sumo with CurrieTech/TranzX motor; that bike came with a 2A charger.

I started monitoring the amp draw on my Kill A Watt meter as I charge them from below 50%, and noticed that the first three all show about 1.4 - 1.5 amps until they hit 85-90%. Then, the amp readings start dropping, all the way to 0.2 amps as they approach full.

It has nothing to do with the cell balancing. Where you place your kill-a-watt meters make the difference. Do you splice your charger cable coming out of the charger and then feed it to the kill-A-watt meter? If you do that, you will see 4A.
The charger takes 120V and 1 to1.5A and converts that to 42V and 4A output.

120V * 1.4A (at the input) = 42 * 4 (a the output) = 168 W
Energy is always conserved. First law of thermodynamics

 

[LimboJim]

Amazing technology! Blows my mind learning how it all works, but the basic laws of electricity have always eluded me.

Chargers rectify current, I guess, so that my eMTBs can rectify me!

 

[Alex M]

"Low" 1.4A input is because this is before AC-DC conversion, as explained.

Your charger is 90% efficient at best, so some of the energy will be lost to heat. It would draw more than 1.5A input to make 4A output.

Balancing could be one of the reasons of slowing down as it's nearing 100%. Who knows what else. Battery is a chemical machine, electricity is a by-product. Not simple at all.

 

[Ravi Kempaiah]

First law of thermodynamics is application of the concept of 'conservation ' using energy, heat, and work.

Without you, there wouldn't be any entertainment on EBR ??

 

[LimboJim]

It's all making sense to me now! Thanks for all the infotainment... Just let's not delve into personal attacks, mmk?

Now here's another potential can of worms: how does Ohm's law fit into this equation?

I'm kinda kidding, but impedance (imp-E-dance) and resistance were two concepts that always flustered me, making my attempts at studying electricity as appealing as a colonoscopy.

Now that I'm all in for ebikes at 50+ y.o., however, both seem necessary!

 

[Alex M]

Ohm's law is always there. Efficiency (heat loss) is the first thing that comes to mind. But don't worry about it.

What is important, is watts in = watts out. AC W=I(amps)*U(volts) = DC W=I(amps)*U(volts). More-less. Give or take the losses.

Battery probably has <100% efficiency as well, you need to put in a little more WH than what you've taken out when riding.