Shunt Modification Questions

[NZgeek]

Hi All,

After a month of enough drama to make an American Soap Opera, I'm finally fiddling with "Frankentraption" again. I want to reduce the 33-35 Amp peak draw on my 36V 12AH LiFePO4 to a more sensible level - I was thinking to 20 Amps (it's supposed to be a 500W DD Motor kit, and currently peaks at 1200+W draw).


Barrage of Questions:
1) I have 2 shunts, so the easy thing would be to remove one, to give me around 16-18ish Amps - Correct?

2) I could solder a little up the shunt to adjust it 20 Amps anyway - correct?

3) What are the cons? Does the shunt get hot, and I'm going to make it worse?

4) How will it likely affect my assist? I think I'll find an obvious reduction on the very steep hills (can pull 30+Amps continuous for 90 seconds+ at the moment), some reduction in acceleration but minimal reduction in top speed on level roads (I usually don't use the assist on dead flat roads, as it seems to do nothing over about 18MPH)

Sound ok? My battery BMS has been cutting out just before home, after 2 steep hills very close to each other (30+ Amps for 90 seconds, 20 second downhill, 30+ Amps for another 90 seconds), so by limiting the draw, I should be able to avoid the voltage sag that's cutting the battery out. Currently, if I ride with very conservative throttle on those hills, it makes it OK - Trying to ride up a steep hils and smoothly hold about 1/2 throttle is difficult, and I'd like to extend my batterys lifetime as much as I can!

Thanks for any input/advice/money you can offer .

Allan.

 

[NZgeek]

Haha... I'm too tired!

1) I need to SERIES the existing shunts, to increase the resistance! Not remove one!

Do I actually want to double it anyway? I can alter the overlap between the two, to adjust it though.

 

[Deleted member 4366]

Cut through one shunt with sharp side - cutters and pull the ends apart slightly to make a gap. This will double the resistance because the current only has one pipe to push it's way through instead of two. The controller will then halve it's demand for current, so the remaining shunt won't get hot, but you will only get 16 amps. You can then add a bit of solder to the remaining shunt to increase the current to what you want.

Naturally, you'll have proportionally less torque. Top speed down - hill won't be affected, but your lower power might affect the speed on the flat and up - hill.

 

[NZgeek]

You're a star

I like how the shunt is conveniently located in the easy to access end of the controller

I ended up having to fiddle it all a bit - Since I was in there, I thought I may as well aim for 20A. After a lot of trial and error (but no BANGs) I ended up desoldering one end of one shunt and the other end of the other one, overlapping them by increasing amounts to eventually get to 19.52A peak.

I don't want to ride it with the broken spoke and slight buckle, so I can't try it out until I've received the parts and built the new wheel

However, I did have a little play in the driveway and peaked at 19.88 with my lights on.

Thanks for the help

 

[NZgeek]

Awesome! Thanks.

I've ridden to work and back twice now.

Peak is under 20A everytime, hill climbing is only slightly reduced - maybe only 10% slower! It's less urgent off from a start, but plenty fast enough. I haven't noticed any real reduction in top/cruise speed assist, but I don't spend much time on flats anyway.

I used it heavily today, and it didn't cutout on the final hill (I was faster up the hill than the HUGE tractor that was catching me on the flat! - so certainly a success (for now at least!).

On a side note, one of the Theatre educators has a medical condition that makes it hard for him to ride hard/far, and he's looking into a kit for his bike

Cheers.