Can't use new drill battery pack with Li-Ion and BMS

[netdiver]

Dear all,
I am trying to make a new battery pack for a Makita drill, going from a 6 cells Ni-MH (or Ni-Cad in origin) 7,2V to a two cell lithium 7,4 V.
I have enclosed, in the original battery package, two "litokala HG2 18650" cells (3000mAh battery with 20A discharge current) connected to a 20A BMS for 2 cells, that has input and output in common, but I think the BMS is cutting the power when using the battery in the drill.
I can't use the battery for more than one activation, and in this first shot the motor rotates well. At the second button press, the battery seems to be instantly discharged, the motor moves a little then stops.
If I recharge the battery for a minute, then I can start the drill again, but only for one time as before.
I think that this behaviour comes from the BMS.

Is it possible that the initial spike of current is higher than the 20A permitted by the BMS?
Is there any workaround I can use to overcome this limitation (apart getting a new BMS with a higher output current, that seems non-existent for 2 cells)?

Thanks in advance.
Lorenzo

[pcprogrammer]

Check the battery voltages with a multi meter. The 3.7V per cell is the nominal voltage, but when fully charged it is 4.2V per cell. Lower then the 3.7V means it has low charge, and it should not be below 2.5V or so.

Also check the specifications of the drill to see what current it can draw when driving a load.

With cells rated for 20A discharge it is not good to draw more then that for prolonged periods, but it can deliver as much as the internal resistance will allow when shorted. If the BMS indeed has current measurement and a cutoff FET for that, then it will just cut the voltage as long as the current is to high. It won't drain the battery, and if it does than the BMS is either connected wrongly or it is just plain crap.

Provide us with more information about the BMS you are using, like make and model or a picture of both the front and the back side with good resolution to be able top read the markings on the components.

[tunk]

Try to bypass the BMS and see if that works.
In a handheld vacuum cleaner I did replace 3 NiCd cells with
two 18650s in parallel. At constant load it did draw around
8A, so I thought a 13A BMS should be ok. It tripped, most
likely because of high start-up current.

[Hanein]

I think AI will help solve the problem , or contact the service center

[Buriedcode]

I think AI will help solve the problem , or contact the service center

I'm going with .. this is a bot?

[wasedadoc]

I think AI will help solve the problem , or contact the service center

I'm going with .. this is a bot?

The poster's only other post is valueless too.

[Buriedcode]

In reply to the OP - like others have mentioned, I suspect the startup current from your drill is higher than the cut-off current.  It's why I stopped trying to convert an 18V NiCd drill battery with Lithium before I even started - Finding a suitable BMS that will handle the high start up current, as well as a controller to ramp up the drill RPM slowly (to limit the initial current draw) was a hassle.

NiCD and NiMH naturally have a higher internal resistance which blunts the current surge, so the controller can just start the motor without any soft-start, the lower resistance of Lithium cells means this has to be done in circuitry.

One possible alternative would be to buy a lithium battery from other manufacturer and move the batteries and BMS/controller from that to the new drill.

[pcprogrammer]

In reply to the OP - like others have mentioned, I suspect the startup current from your drill is higher than the cut-off current. ...

Another option is to use the BMS only for charging the batteries and add a slow fuse with enough capacity (say 40 Amperes) to protect against to much prolonged over current. Have some detection for to low voltage that blinks a LED to warn that it is time to stop and recharge the batteries. For charging there is still the need for a constant current / constant voltage cycle.

Husqvarna uses this setup in their 36V BLi devices. The BLi200 battery pack has an 80 Ampere fuse in series with the batteries, and serial communication with either the charger or the device to signal a problem with the batteries. They use a 2 parallel, 5 series setup with VTC4 batteries. These batteries have a discharge rating of 30 Ampere. There is a BMS monitoring the individual cells and in case of an imbalance it tells the charger to signal an error and stop charging.

[netdiver]

Thank you all for the suggestions.

It's passed some time from my first tests, but I remember that the batteries were not discharged.

the suggestion by @pcprogrammer about making a different circuitry is out of my skills.

Surely I will try to bypass the bms as suggested by @tunk

In a video on youtube, as a side note, the youtuber said that he had used a capacitor on the BMS output to avoid the cutoff of BMS. Is it a feasible solution?

TIA ALL

[inse]

You will not be able to put a capacitor with the required capacity into your electric drill.
A botched solution would be to bypass the current shunt.
But don’t tell anyone I suggested that 😉