Help about 1s 18650 BMS

[naskoialan]

Hi all,

I have following issue, we have broken bosch glassvac, and I open it the whole electronic was covered in som king of lacquer, and element names can not be read also no schematic is available. It have one liion 18650 battery and one small electric motor. So I decided to modify it to remove original electronic and to use 1s BMS for 18650 and later to add some kind of soft latching switch (now that is with hardware one). So I measured current when electric motor is connected to battery without bms and it was around 2,5- 2.7 A. Then I get 1S 5A BMS and connected it to battery I checked battery is now charged normally with voltmeter 4,2 volt, but issue is when I connect motor to BMS it try to spin made a quarter turn and stop.
When I connect mutimeter to measure current with BMS motor start working, so I suppose it is some kind of peak current in begging and BMS shut down load this is only my guess.
Can you advise me how I can proceed further I am out of ideas except to replace li ion with nimh battery and to simplify everything.
Best Regards,
Atanas

 

[Pommie]

The start up current of the motor is probably causing the BMS to shut down. Try adding a 1 ohm (3W) resistor in series with the motor. This will limit the start current to 4.2A but will make the motor run a little slower. If it works then maybe we can suggest a better solution.

Mike.

 

[naskoialan]

Thanks Mike,

I will try this and then report back here.

 

[naskoialan]

Hi Mike, I have added 1.5 ohm 5 watt resistor (beacuse I can not find 1 ohm in the shop around me) the motor started but resistor heated a lot and motor spin slower as you suggest. But when motor is running if I bypass resistor it keep running directly on BMS at higher speed. So probably I need some custom mosfet switching circuit to manage this.
And the other things that bother me originaly there is temperature sensor attached to battery, but bms did not support that.

Best Regards,
Atanas

 

[Nigel Goodwin]

Hi Mike, I have added 1.5 ohm 5 watt resistor (beacuse I can not find 1 ohm in the shop around me) the motor started but resistor heated a lot and motor spin slower as you suggest. But when motor is running if I bypass resistor it keep running directly on BMS at higher speed. So probably I need some custom mosfet switching circuit to manage this.
And the other things that bother me originaly there is temperature sensor attached to battery, but bms did not support that.

The temperature sensor is basically for the charging circuit, not for the BMS - and if it was, it would be part of the BMS, not part of the battery.

 

[Pommie]

A small timed relay circuit to short the resistor after a few seconds might be the answer. Something like the 5V version of this might suffice. If you want to build something yourself, google "soft start DC motor schematic".

Mike.
Edit, another option is a constant current source to limit the startup current to something like 3A.

 

[naskoialan]

Thanks for information, you helped me a lot.

I will try possibilities and will report here when I have result.

 

[Pommie]

Good luck with your project.

Mike.

 

[naskoialan]

Hi all,
I wanted to post update if someone else need info on such topic. So I am using inrush current limiting circuit from here 2.1 Active Inrush Current Limiter Circuit (P-MOSFET). The mosfet are NDP6020P and resistors R1, R2 are 10K, C1 is 1 uF. So motor now run fast and there is no need of resistor, fet heating but probably small radiator will needed. I am pretty new in mosfet teritory I was used mainly bipolar in past so it is possible that state of FET are not running in optimal state in this case.

Best Regards,
Atanas.