I'm building a small robot (for the context, with a pi zero, a PCA9685 motor controller, 8 small servo motors a regulator and an Ubec) fully powered from a 2S 18650 battery.

I've inserted a (cheap) 2S BMS between the (homemade) battery pack (with a small power switch included, and removable 18650 batteries that I remove to charge from an external charger) and the rest of the electronics. My problem is that unless I put some voltage to BMS output (P+ and P-, which are also used when charging) to "boot it up", the BMS refuses to output anything. Once "booted", I can remove the voltage.

I've seen people talkinging about "reseting" or "activating" BMS by charging it in order to make it output current... However I don't want to include a charging plug in my battery pack since I want them to be removable (as I'm too lazy and don't trust much these BMS for charging, since they don't seem to do balancing)...

So is there a way to workaround this "activation" step so that I can just switch on my battery power button and it works?

For reference I've tried with these 2 BMS but had the same issue: https://www.aliexpress.com/item/32827942991.html https://www.nkon.nl/sk/k/2S-BMS-PCB-2MOS-2004-D.pdf

Thanks for your help

  • 1
    \$\begingroup\$ honestly, your BMS's job is to be reliable at managing your batteries. If you bought two that you don't trust (I wouldn't either, no datasheet, no purchase), and that don't have a necessary feature – buy a better one. aliexpress is a terrible source of electronics – no datasheets in any language I can read and specifications on the product page don't necessarily have much to do with the sold product – so that's what I'd change first. \$\endgroup\$ Commented Jan 25, 2020 at 9:47
  • \$\begingroup\$ Hello. Did you try momentarily shorting the P- with the B- using a switch? I highly doubt it breaks the BMS...but I'd try it out if you have multiple cheap ones and they don't work as intended. \$\endgroup\$
    – Natsu Kage
    Commented Jan 25, 2020 at 9:53
  • \$\begingroup\$ @MarcusMüller Well it's a first robot (a little smars quadpod) just to learn how that works. I probably won't use it much and as a consequence I didn't want to invest too much... Maybe a bad idea indeed. \$\endgroup\$
    – rtmv
    Commented Jan 25, 2020 at 10:25
  • \$\begingroup\$ @NatsuKage I thought about that but that wouldn't mean bypassing the entire BMS (and making it completely useless)? Actually I'm not sure it doesn't work as itnended. Well since there are no real doc, I don't know what's the intended behaviour I guess! But I'm wondering if it's role is not to be always plugged in (and maybe have a poweroff switch "after" the BMS). In that case I'm screwed.. \$\endgroup\$
    – rtmv
    Commented Jan 25, 2020 at 10:25
  • \$\begingroup\$ I'm saying to only short the B- and P- with a momentary switch. (OFF-MOM) So only half a second or so. This may be enough to make it come out of asleep mode. Once it's reset this way it will function as normal. Only thing that might make this fail is that the BMS needs a voltage higher than B- on the P- terminal to get out of sleep mode. \$\endgroup\$
    – Natsu Kage
    Commented Jan 25, 2020 at 10:41

1 Answer 1


The simplest workaround to an un-activated BMS (sleep mode) is to supply it with power from it's own batteries. To achieve this, a push switch/momentary switch can be used between the P- and B- terminals of the BMS.

Block schematic for standard 2S BMS

Note: This will work only if the cells have adequate voltage. The BMS will return to sleep mode if the individual voltages are too high or too low.


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