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4S 18650 with BMS No Start
#1
The 4s pack will start and run the small12vdc motor without issue. However, when the battery pack is connected to the BMS which provides cell balance and charging as well as protection) the motor will only momentarily start but immediately shuts down. I discovered that with two different BMS 4S boards had the same issue. Apparently, the di/dt in-rush current and the reverse spike caused the the BMS to immediately shutdown. Has anyone else experienced this and discovered a way around it? I’ve tried Schottky diodes in reverse in an attempt to dampen the reverse current but it’s not reliable using even a PN junction diode.
Looking for comment regarding similar experience and mitigation suggestions.

Thanks
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#2
You're right, the motor startup current is tripping the BMS over current/short circuit protection.
Reverse spikes aren't likely to be the issue, it's the current needed to spin up the motor + turbine blade in the vacuum.
Like we suggested in this thread:
https://secondlifestorage.com/showthread.php?tid=9953
Some options are:
- use a BMS rated for higher current.
- use a pwm soft start circuit to ramp the motor up to speed.
- bypass the BMS current sense resistor (short with copper link) & use fuse for over current instead.
- only use the BMS for charging/balancing not motor load ie wire it so the motor feed goes directly to the battery via a fuse for protection.

I'd suggest using a BMS with short protection on the motor isn't much use, yes for charging & balancing, but the motor side doesn't need it & is causing you problems anyway.
I'd wire it like option 4 ie connect motor to battery directly (via switch + fuse, etc), & just use the BMS to charge & balance.
I'd also add a "snubber network" across the motor terminal to calm electrical noise from the brushes.
Running off solar, DIY & electronics fan :-)
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