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HW 4.12 Change low voltage cutoff

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subodh.malgonde
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HW 4.12 Change low voltage cutoff

I am new to VESC and I have a Maytech VESC, HW 4.12. It which is rated 8-60V. Is there a way to change the low voltage cutoff to 7.5V? 

I want to make the VESC work with a 2S LiPo battery. I will use the VESC to control a 1/10th scale RC car which is designed to work with 2-3 S LiPo battery.

TheFallen
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The VESC cannot operate below 8V. Specifically the DRV8302 cannot operate below 8V. Honestly I doubt the VESC is the ideal choice for a 1:10 RC car. Unless you're willing to redesign the VESC I think you;d be best trying a different ESC.

subodh.malgonde
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Thanks for your reply @TheFallen. The VESC has been successfully used in RC cars such as the MIT Racecar, F1/10. All these projects use a 2S battery and the stock VESC. So this should be possible by changing some settings.

subodh.malgonde
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I tried exploring the VESC tool and I noticed these 4 settings:

  1. Minimum input voltage (default value 8V)
  2. Maximum input voltage (default value 57V)
  3. Battery cutoff start (default value 10V)
  4. Battery cutoff end (default value 8V)

So, if I understand this right:

  1. The VESC will only function if the input voltages between 8V and 57 V. This seems straightforward. 
  2. As the voltage falls below 10V, the power supplied to the motor will fall. 
  3. For voltage below 8V, the motor won't spin at all.

A fully charged 2S LiPO is 8.4V and nominal voltage is 7.4V. So for a 2S battery these cut-off settings are high and to make it work I have to:

  1. Set 'Minimum input voltage' to somewhere around 7.0V
  2. Set 'Battery cutoff start' to somewhere around 7.5V
  3. Set 'Battery cutoff end' to 'Minimum input voltage' i.e. 7.0V

Am I right in understanding this? Any suggestions?

TheFallen
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I see no mention of batteries used in either the MIT Racecar project or F1/10. Got any links? I did find an oblique reference to 4s in the MIT Racecar GitHub repo so perhaps they're using a 4s battery?

I'd personally use a >3s battery and then use the rpm & current limits to ensure the motor doesn't over speed or heat up too much.

The MIT Racecar is using a VESC-X or FOCBOX, why not use this if you're 100% certain they're using a 2s battery. If you're using different (also a little older) hardware is it such a shock it has different requirements? I'd also ask them to see what they did to get around this problem.

Your limits will probably work, but IIRC once you hit ~3.6V per cell you're down to 10% capacity. But you'll also suffer from battery droop at such low input voltages which will reduce the voltage under load to the point where you'll trigger low voltage cutout early.

EDIT:
Ahah, here it is: https://github.com/mit-racecar/hardware/blob/master/racecar-2.0/drawing_...
They're using a "Traxxas 2940X Series 3 3300mAh NiMH 7-Cell, 8.4V Battery"
That's using NiMH cells, so 10.5V fully charged, 8.75 50% charged, 7V fully discharged.
Bit different from 8.4V to 6V for a LiPo.

subodh.malgonde
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Oh! Thats a good catch yes

I failed to check properly before posting here. I checked the bill of materials for the F1/10 project again and it says:

  1. NiMH battery 3000mAh, 7 cell
  2. Li-Po battery 5000mAh, 3 cell 11.1V

So you are right. I will put my two 2S batteries in series instead of buying a new battery. (EDIT: This may be a bad idea because Traxxas motors are designed to work with a 2S or 3S battery)

I would have ended up damaging my VESC had it not been for you. Thank you smiley

TechAUmNu
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You can use any battery voltage you want as long as its high enough for the desired speed. Then use duty cycle limiting in VESC to stop the motor spinning as fast. 

I use 12s battery with motors designed for 4s and just limit duty to 30%.

TheFallen
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Motors don't really have a voltage limit, they have a power limit. Providing you monitor & limit the power/temperature you can run any motor outside its voltage specifications. And I'm always a big fan of a single common battery for robots.