Controller current draw vs. motor current draw

cellmate

10 mW
Joined
Nov 15, 2018
Messages
25
Hi all,

I'm considering doing a minimalist ebike conversion using the G310 kit from Grin:

https://www.ebikes.ca/shop/electric-bicycle-kits/bafang-g310-311/rear-g310-kit-basic-throttle.html

The kit comes with a 20A controller, and I was planning to do 2 x LiGo batteries. Each LiGo can handle 10A peak, 5A continuous. My usecase is to be riding motor off 90% of the time, and only give a light boost on 2-3 hills on my commute, at about 60% throttle for no more than 30-60 seconds at a time.

The Grin motor simulator says that with my weight, on the grade of hill I will ride, with me putting in 150w of human power, the motor will output produce about 207W which at 77% efficiency gives a 7.4A current draw at the battery. That is well within spec for 2x LiGo.

I went in to Grin in person (I live in Vancouver) and spoke to them about my plan, and they felt I would need 3x LiGOs. They said I would wear out the cells with 2x because the 20A controller would draw that much most of the time when climbing a hill, and even 30 second burst is considered "sustained" use.

Now I'm not sure what to believe. It doesn't seem physically possible to me that the motor could be drawing 7.4A but the controller is drawing close to 20A.

I don't think I would ruin the battery packs using them as I intend to. Obviously I could wreck them by trying to climb big hills without pedaling, but my usecase is hit the hill fast with momentum, keep pedaling hard, and apply a bit of thumb throttle for a short burst to take the edge off the hill.

Thoughts?
 
cellmate said:
Thoughts?

Sure. Do it your way but make sure you monitor with an ammeter (or Cycle Analyst). When you see that you actually end up using more throttle than you planned (just a guess on my part), go buy the third battery before you over-stress the two you have.
 
Can you take a screen shot of the simulation, or give the parameters you used? 357W to go up a 'hill' sounds low.
 
Depending on the controller and display, you can often limit the maximum power draw and thus save the batteries from abuse. I do that with my DC booster so that the controller doesn't ask the booster to supply more current than it is able to supply. When it does that, I get a system shut down. But with the KT controller and LCD5 display, I can limit the max amps that the controller can put out. But again, I'd make sure I had a way to monitor amp or watt draw so I could be sure the controller is doing what I want it to do.
 
Thanks for the responses guys.

Here's a sim screenshots at 20kph 60% throttle, withing the 10A sustained of 2xLiGo

20kph_4pct_grade.JPG


And another at 25kph 76% throttle, within the 20A Max of 2xLiGo

25kph_4pct_grade.JPG

Limiting the power probably won't be an option as I'm going for a super minimal build - just a thumb throttle, no screen of any kind.

As I've done more research on batteries, the shine of the LiGos has kinda worn off for me. I still like the packaging and simplicity of them, but the cells inside aren't worth the price they're asking. They're fairly low capacity AND low current. I could build a 10s2p battery out of Samsung 30Q batteries that could handle the full 20A draw without breaking a sweat with more capacity AND at a lower price.
 
Just double checking here: 4% grade and not 4 degrees? If Grin calculated using 4 degrees, then 3 modules would be more righter.
 
4% isn't much of a grade, and 20 kph isn't very fast. So that all seems about right. So the question comes down to whether you limit yourself to that speed and grade.

You might want to get one of those $15 inline watt meters. and wire it in temporarily to verify everything is doing what you think. Once you are sure, then you can move it back near the battery or controller and just spot check it every now and then to see what your peak amp draw is and confirm that your riding habits aren't creeping into using more juice than you think.

https://www.amazon.com/Tenergy-Precision-Meter-Analyzer-Backlight/dp/B017YCTRKK/ref=sr_1_3?ie=UTF8&qid=1542510403&sr=8-3&keywords=tenergy+amp+meter
 
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