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Bypassing BMS of space cell to pull more amps


#1

So I’m getting ready to mod my space cell to pull 60 amps instead of 30 amps.

BMS can tax a max of 30 amps, so I need to pull power before the BMS instead of after.

Do I need the bms powered off while pulling 60 amps before it?
Maybe my image how electricity flows is incorrect. But the way I see it is, if I’m pulling 60 amps before the bms, the 60 amps will go through the bms still if the bms is powered on. If it’s powered off, there would be no issue. Is this a correct interpretation?

This would imply I can’t power the battery while the board is on.

I just want to make sure I don’t blow out the bms by wiring around the bms.

Would buying a 60a 10s li-ion bms be a better idea to maintain functionality or is it unnecessary?


Custom 18650 Battery (Pre-Build)
Help me understand the Space Cell pro 4
#2

I have never done this before, so this is an experiment, I imagine just solder another set of wires directly onto the battery where the existing discharge wires are coming from, bypassing the BMS.

Please be sure to set the current limit in the vesc so you don’t cook your pack trying to pull too many amps.

then you will have two sets of discharge wires, one that is BMS limited ones that are not. this way you can choose.


#3

Could you imagine this damaging the cells, by not pulling through the bms?

I’m going to buy a bms in the group buy on here that can pull 80 amps, so everything can be wired through a bms, but in the mean time I still need power, so I’ll try out what you suggested.


#4

yes, it is possible to damage cells if you discharge them too rapidly.


#5

And that can be taken care of by setting the max dischanrge in the VESC.

Is the a max amps that these particular cells should be discharged at?

I know you said set them to 60, I was curious if they can take 80 continuously?


#6

the vesc can control current from battery.


#7

Yes, I get that.

But is 60a the max I should discharge from these cells? Or are they capable of 80a?


#8

soldering a set of leads directly from BMS where the pack leads go into the BMS should work, it will basically put the ESC in parallel with the BMS, which will allow you to balance charge and discharge while bypassing the BMSs safeties during riding discharge.

Yes there is a very decent chance of over discharging your cells this way. I wouldn’t pull more than 60 from them if you want them to last any length of time. i believe the cells individually are only capable of 25Amps burst, which would put them somewhere around 75A burst in the space cell’s 3P config. I could be slightly off on that though. Either way expecting 80 amps out of them is potentially dangerous.


#9

Could I over discharge if I set the lower voltage limit on the VESC? Are taking about the battery become unbalanced, even if I balance them from the bms at each charge?


#10

@evoheyax

Have you had any luck with this project?

Did you decide where to draw the line on output? Are you thinking 60 amps?


#11

Yes, its working great so far. I set each vesc to max of 25 amps, so 50 amps total. This DID fix my hill climbing problems.

Before:

After


#12

Glad to see you got this working!

Have you put it through its paces yet?

Also, the new PRO4 states a peak output of 80Amps. If you had the PRO4 would you have gone higher than 25Amps per vesc?


#13

Yes. I would do 50 amps per motor if possible.

I did 1 good 5 mile hike with plenty of hills on it, and a bunch of flat ground testing. I can’t say if this will last, its just kinda a hoping game now.

Since these cells can burst at 75 amps, I’m assuming the constant is closer to 45 or 50 amps, thus why I decided to settle on 25 each.


#14

Very cool, can you elaborate on the improvements? Better hill climbing in regards to temperature? Noticeable speed increase?


#15

I was never able to determine what caused the roll backing of speed on hills. There is a chance that allowing 25 amps instead of 15 is not really playing any role here, seeing as I also have chaka’s heatsinks on these vescs. So it might have been a temperature thing all along. But I can go up hills of 7% grade on my route to school without an issues of slowing down. The ONLY limit I hit during my commute was a low voltage limit, which is a weakness of the battery again. Once I get to 30%, a hard trigger plug on a hill will sag it right down to 0%, causing the vesc to rollback. This is not too big of a deal, since I am over most of the hills by the time I starting having this problem.


#16

might need to add another 10 cells.


#17

Is that possible (considering the cells I have are not new anymore))?

Either way, I was planning on build my own pack sooner or later. I know I running the cells hard like this, and I don’t expect more than 400 cycles. Been through 4 days of commuting (8 cycles, 5 miles per cycle, total of 40 miles). Had an issue today with the pins for the canbus port (I think that’s the right name) of one of the vesc’s broke off. I had to solder the wires to the bottom of the port, and used lots of hot glue to keep the wires between the vesc’s in place. So for now, all is working great.


#18

I dont really have a clue but I think you might as well check and reinforce the connections between the cells, because (theoretically) if these are made out of normal nickel strips they might only support around 45A continuous, depends there are different ones out there, but I didn’t find any which would support 60A yet.

If you have a 3P and each single cell supports 20A continuous you could get 60 out of a “cell” while being sure you are not going to damage the actual cells.

just some thoughts guys…


#19

Hello :slight_smile:

How is the battery holding up so far ?

I’m planing on using this technique on my battery packs ( future and current ).

IMO with the VESC voltage limit and choosing a safe overhead for li-ion. The discharging function of a BMS is just not that necessary.

That way we can have smaller bms in the battery pack and bypass it to discharge.


#20

So far so good. I have done around 100 charge and discharges like this and no issues what’s so ever. 50 amps is for sure within these cells potential for continuous discharge. I do expect the battery to last less though. The VESC does a great job of slowly cutting me off as I approach the end of my space cells charge.