Need help rebuilt battery pack

So one of my BMS circuits finally arrived. It is the 2S one for my Eureka stick vac. I assembled the new battery pack using two LG 50T 21700 cells to replace five sub-C NiCD (or maybe they are NiMH) cells. I connected everything properly but no power. I then "woke" the BMS circuit by applying 8V briefly then I read voltage on the input/output terminals, but when I try to turn on the vacuum it only very briefly turns the motor then it shuts down and there is no voltage. When I bypassed the BMS and the vacuum starts and runs just fine on just the two 21700 cells. The BMS is rated to do 8A with 15A cut-off, and I measured the vac pulling around 5A when running and a bit higher during start-up. So I don't know what is going on. I then hooked the new battery pack (with the BMS) to an old 12V drill and it runs that fine. I even applied some pressure resistance to the drill while using my clamp meter and confirmed the pack could supply peaks of over 7A and 6A sustained (kind of briefly). Anyone have any idea why the new battery pack and BMS won't run the vacuum?

To be clear, the original Eureka battery contained only five sub-C cells nothing else at all — and it only has two connections/terminals

If the motor draws 5A while running, it probably draws more than 15A at startup. Starting current is the same as stall current, which can be many times the run current.

DIWdiver said:

If the motor draws 5A while running, it probably draws more than 15A at startup. Starting current is the same as stall current, which can be many times the run current.

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Thank you so much! It appears my meter wasn't catching the peak startup current correctly. I confirmed this by attempting to power the vacuum with my variable power supply (that I found after a little research is rated to supply 15A with peaks of 18A) and found it cannot start the vacuum either! I was able to use the new battery pack and my power supply together to get the vaccum started, then confirmed just the new battery runs it just fine. So I think I just need a much, much larger BMS circuit.

Yeah, it's kind of surprising what it takes to start a DC motor. Incandescent lamps do the same thing, but not quite as bad.

If anyone is interested, I did some research on DC motors and found the inrush current that occurs at start-up because when the motor is at rest its resistance is very low. So I got to thinking the BMS circuit might be thinking it is detecting a short -- so it might not be a current issue. With some research and testing, I found a couple ways to make my existing BMS circuit work. One way is to put a DC-DC converter between the BMS output and the motor. The converters that I had on hand only handle 3A so I wired two in parallel. Their small LED lights dim significantly when I switch on the vacuum, but they recover quickly and the vac run just fine. The second way was to add a 4ohm power resistor is series between the BMS output and the vac. It limits the vacuum to only pull about 2A but once the motor is running I can short/remove the resistor then the motor gets up to full speed. This vacuum actually has two modes; one runs just the suction and the other also turns on a small motor that turns a beater brush. I always run it in the latter mode, so I could install the resistor on only the first mode then it would become just for start-up. I have also read about using a NTC thermistor that start with some resistance that then goes away as the temperature rises so that might an option too.

I haven't decided what to go with yet, but I am leaning towards the DC-DC converter route because I can drop the voltage output of the battery down to 7.5V which is the maximum voltage that could be seen from the original NiMH battery. The lower voltage should also yield a lower current pull too -- which means less stress on the vacuum and less stress on the battery.

All sounds pretty reasonable to me. I almost suggested the resistor approach, but didn't think of shorting it out after starting!

I ordered a larger BMS circuit so the above ideas are really just back-up plans should it not work. I will probably order some 2.5ohm NTC thermistors just in case. I decided that having a DC-DC converter and a BMS is just overkill, and any consumption savings I'd get from running the vacuum at a slightly lower voltage (and thus lower current) would likely mostly get eaten-up by the inefficiency of the converter anyway. I ran the vacuum this weekend directly off the 21700 batteries (but was sure they were fully charged and only ran them for about 20 minutes) and it worked just fine.