Controller shunt mods

[fechter]

oh ok cool got it about the shunt resistor, but I still need to break the trace for the wiper connect, correct?

Ive seen complicated work arounds for measuring current with an arduino, seems this type of shunt could do the same thing?

Yes, you need to cut the trace to insert the divider.

An Arduino A/D input would not have very good resolution at the voltage level on the shunt. It would work better if the signal was first amplified about 10x. Typical circuits I've seen use an LM358 or similar op amp to boost the signal ahead of the Arduino.

Another option is to get an inexpensive power meter. There are several out there. I use a TK15 coulomb meter on one of my bikes and like it other than the screen is pretty small. I use it mainly to keep track of my battery level.

 

[newbiehere]

for the pot in parallel, how does it not take on the a large current, like the shunt is doing?

 

[fechter]

The shunt resistance is about 1milliohm. The pot can be somewhere between 1 ohm and 100 ohms and it will take about 1/1000 of the current if it's 1 ohm. Another way to look at it is the voltage across the shunt at 100 Amps would be around 100mV. This is nothing for the pot. I used a 10 ohm pot.

 

[newbiehere]

oh ok cool, got it

 

[newbiehere]

so if the voltage is very low, then a pot can be used? in the other post, it was mentioned that I would need a huge pot or rheostat to control the current. here we are not controlling the current (fixed) but instead the voltage so a large current can still move across the pot?

 

[fechter]

The big shunt resistor on the controller board has to carry the full current. But at the rated current, the voltage across it is still very small. The pot is only carrying a tiny amount of current and is just changing the voltage that the MCU "sees".

If you wanted to make the shunt resistor on the board variable, that's where it would get difficult.

 

[newbiehere]

Oh I see, thanks

 

[Yungyakz]

so i dremel tool off some of the trace, then put a pot in between? if its a 3 terminal pot, how do I connect the terminals?

Sorry for the delay, holidays are interfering with the advancement of technology again.

The way you did it will work but you if you turn the pot too far, you can totally defeat the current limiting and possibly blow something up. I've used that method before and it will be OK if you just dial it in carefully and leave it alone after that.

There are many other ways to do this. Below is just one. By using a pot and a fixed resistor, you can limit the maximum increase. In the circuit below, the pot will adjust the current limit from stock level (100%) to 2x (200%). The pot and the fixed resistor are the same value, which gives the 2:1 range. With a different fixed resistor, you could make the range whatever you want.

2x variable shunt divider.jpg

I'd like to increase a controller from 25A to ~34A.
I'll use the shunt modification, but like to be a bit precise.
The controller, currently use 2x 5 mohm shunts.

So, please tel me if I'm right :

2x5mohms= 2.5 mohms

2.5 mohms -> 25A
??? mohms -> 34A

From 25A to 34A, it's +36%
So, 2.5 mohms - 36% = 1.6 mohms

3x 5 mohms =~1.67 mohms

I finally just need to add a 3rd 5 milliohms shunt on my controller to reach my goal ?

To get 34A, the shunt would be about 1.84 milliohm. (2.5 x 25/34)

If you add one 5 milliohm to the existing, your limit would be about 37.5A (that's probably close enough)

This is great mod to do on chineese controllers and i have a 15 fet 1kw controller and done few mods like ebs and adding solder to the shunt etc but really want to try it but I dont want to cut the trace and then not be able to solder wire wire to the cut trace. Is there any pictures on how it is done and how the trace was cut and wire soldered to it. I have a backup controller theese controllers are cheap but I dont want to mess things up.

 

[AlexTheTech]

imma just say that this has been a hugehelp, now i just need to burn it out XD

 

[joshuahuang]

how come when the bike accelerates from a stop it draws peak 40a but when you accelerate from say 15 mph it draws less? Is it because the phase current is lower than the battery current?

 

[amberwolf]

If it's a hubmotor, the motor is already spinning, generating a voltage (BEMF) that itself will limit the possible max current you could get.

Also, depending on speed, vehicle air resistance/rolling resistnace, terrain, wind, etc., then if you're already moving, generally it won't take as much power to accelerate from that speed than it would from a stop.

You can go to http://ebikes.ca/simulator and play with different setups to learn about this. Using custom batteries and controllers that essentially allow you "infinite" current will let you see what is limiting--the controller/battery, or the situation.

 

[DogDipstick]

how come when the bike accelerates from a stop it draws peak 40a but when you accelerate from say 15 mph it draws less? Is it because the phase current is lower than the battery current?

Yeah.. but to pull over (X)Kw on light e bike, the speed is what you need to keep on pulling tq. Yeah Amps= acceleration, and 8-16mph/second is fast acceleration.. you will have to be around there for the amps to be pulled.

Yea hills will do it. Hills multiply load by the (delta) change in angle.

I have an 8.5Kv, but only pulls 2000w at 40mph on a 80lb bike top. I have the capabilities in battery for 300A contin. I top out, load drops, and I sit there at top speed.... If I had a higher Kv motor I would pull more current, but that is my top.

Kind wish I had more Kv, so I could ride in the max output range for my ebike. Fact is, I could have 2000000 watts, available, based on spec but I will only pull 2000 at top speed...

Also, wind increases with speed exponentially. I hit the wall of wind and top out.

I do pull 8MPH/second on startup and that is 6500w.



I want to know if it is a good idea to shunt mod a Grin 20A SINE controller, and can you measure the shunt resistance to therefore afterwords, to correctly input the number to the CA3.

Best shunt mod thread ever.