A Final Word on Bypassing BMS for Discharge

After many comments on my recent battery packs, everyone wants a thinner pack, which is understandable. Only problem is the BMS… The higher the Amp rating you go the more it costs and the bigger it gets. So, what is the final word on bypassing the BMS’s discharge (in conjunction with the VESC of course)? For those who have did it, how are your packs? Any thoughts on whether or not a ‘commercially’ available pack should have a bypassed BMS?

If I can recall I thought that the Space Cell has a 30a BMS but I cannot confirm.


Fairly sure all pro version space cells only use 10A BMSs just for charging

I think many (personal experience talking here) do that so they don’t have to spend as much and get a cheap one from China, but I now do not trust the cheap ones at all now (my pack got unbalanced, though I don’t I did damage to it since I caught it quickly)

So I’d say if it’s done just for the space requirements and it’s still have a quality BMS, that is just smaller, I think it’s a good way to slim it down. I thoroughly trust the VESC to properly handle the voltage cutoff and amperage discharge limits. If it’s done for the cheap aspect, and you go with a low quality China BMS, then you’re asking for trouble.


Just curious, how unbalanced did your pack get and did it occur during charge or discharge?

I had 2 sets of cells that were about .75 volts below the rest in my 10s4p Li-ion 25R pack. They were still above the limits for the cells, but below what they should have been. I believe it happened during charging as the charger cutoff below where it used to (at like 90% on my percentage display instead of like 99%) and that triggered me to check the voltages of each set with a multimeter.

The problem with BMS’s is that they seem to be unable to balance a pack if it gets too far out of balance and will not charge to full charge because of this. This happened to me one time when I replace one of the 2s Lipo packs in my build. the replacement was at storage charge while the other packs had considerable more charge on them. I wound up having to balance charge each pack individually with my hobby charger and then reconnect them to the bms. It worked fine after that. It is most likely that your pack went out of balance during discharge and your bms was unable to rebalance them. I asked a rep from Bestech and they told me that the bms I use keeps the cells balanced during discharge as well as charge cycles. I haven’t actually tested to prove this however I think that I will soon just to see if it is really true. If true, then this could be a really good argument for discharging through a bms.


Very valid point. There may be some real benefit to do balanced discharging as well, since the VESC cannot currently monitor that. I would be very interested in your findings. I haven’t really ridden my board since the unbalanced issue as i took it apart to address the issue with a regular balance charger and to work on actually finishing it (it was functional, but not in enclosures or anything, the VESCs and batteries and such were just rubber banded in place)

I would add to this discussion that the newer “smart” BMS’s that are in development around here (by @JTAG and @raphaelchang I believe) are quite small physically and can do everything we’d want them to do. They are on the more expensive side, so they be a great option if the only consideration is pack size (and not as much price)

It will be interesting to see what they come up with and as far as the price is concerned, if it’s good then it will be worth paying for just like a quality vesc is. I’m doing a little test right now with one of my builds that is under charging just a little. 41.8v and the green light comes on. I pulled the cover off and check the voltage of each cell. they all range from 4.18 to 4.19. I plugged the charger back in and after the green light came on again with the voltage at 41.8, I left the charger on and plugged in and after a little time the total voltage is up to 41.9 So apparently even after the charge light goes green, its still charging. I’m gonna leave it on for a while and see if it goes up to 42.0 I realize that 2/10 of volt is nothing over the entire pack. In your case where your only getting 90%charge, maybe if you just leave the charger on it will eventually balance out the pack.


@Namasaki @rpn314 I don’t currently have a BMS but will be soon taking the plunge. I have to say I thoroughly enjoyed your guys’ discussion. Thanks :grin:

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Well I left it on the charger for a little while but it never went above 41.9. I checked the cells afterwards and they are not perfectly balanced. they vary from 4.195 to 4.205 which is probably splitting hairs.

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@Namasaki that’s interesting, and contrary to all else I’ve read. If it’s true, that’s great! Can anyone verify this?

To add to this discussion @JLabs I current have a Bestech 80A BMS and the problem is that it’s huge and heavy. In the future I plan to bypass the BMS and use a good-quality (but physically smaller) BMS, assuming I can find one haha!

For a 10S I found this 60A BMS to sustain a dual (street, not mtb) build load ok, it’s not that big (112mm * 53mm and probably ~12mm thick, can’t remember) and it seems to do the job. I have 2, one is connected to a 8S5P (I didn’t connect the balance wires 9 and 10) pack and it kept my cells balanced for ~6 months / over 1000 km and they’re still inside the +/- 0.5v range.

I will be testing this theory very soon. I did notice that when I was riding my brakes down a steep hill with a full battery and my pack voltage went up to 42.1 or 42.2 that within seconds after I released the brake and was coasting, the BMS brought my voltage back down to 41.9

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I have a few problems with Battery Supports BMS. 1: when I ordered one, they couldn’t get my order through customs and after waiting too long I cancelled the order. 2: they don’t have a built in E-switch so you’ll need an external E-switch which means extra cost, more connections and more space to mount it. And since they don’t have a switch, it seems that they are always on so how do you reset it if one of the safety circuits trips?

Yes, I felt the need to have a switch too, I reset it once (desoldering/resoldering a battery wire and unplugging it) but I saw there wasn’t the need to in that case, the battery was low (I left lcd and vesc on for long time) but it just needed the (li-ion / brick) charger to slowly charge it to “revive” it, unplugging and replugging the bms was not needed but I agree it can be time consuming if that fails and it’s soldered

Being the owner of a space cell I’d advise against it. Mine has likely got dead cells in it as it won’t charge pass 92% and barely gets half the range it used to. Before the issue with 92% being the max battery capacity it wouldn’t charge pass 96%.

@barajabali @onloop Do you guys have experience with this? By that I mean bypassing the BMS’s discharge. Any comments?

(Jason I dont mean directly respond to the comment above)

As long as you’re using the vesc, bypassing the bms for discharge isn’t a big problem. Better to do it with larger batteries rather than smaller. Larger batteries are more stable and generally stay in balance with less effort

As large you refer to the amount in parallel? So its a no go for 2p but acceptable for 3p+?

Eh ‘large’ is sort of relative. Depends on the amps you want to pull in my opinion.

You’d really just have to do it and test it. I find that no bms discharge on my 7p batteries is totally fine.