Prevent BMS tripping in modified drill - Page 1
Forum section: electronics / beginners

author: PhilLG
author_rank: Newbie
timestamp: March 29, 2015, 12:58:56 am
content: Hi,I've recently tried to modify a DC battery drill for a friend. It originally used 6 NiCd sub-C cells in series for a 7.2v nominal supply voltage. Since these cells are getting harder and more expensive to get and because I can get him more capacity from LiIon i decided to modify the packs to take 2x18650 cells in series for about 7.4-7.6v nominal. Since the higher current rated 18650s don't have a protection circuit built in I added one in the bottom of the pack. It's rated for 20A peak current (cells are rated for 30A) and has under/over voltage protection as well as short circuit protection.The problem I've run into is on start-up of the drill. It's a very basic style drill with a switch based trigger and no speed control. On start-up I'm either the spike in current draw drops the voltage below the protection limit or the short circuit or over current protection is triggered and the battery BMS shuts off. I've discovered if I spin the chuck of the drill by hand before pulling the trigger the drill runs well including with maximum load applied to the chuck. So the BMS seems adequate for operation of the drill but it has a problem with start-up.Do you think adding a capacitor in parallel with the output to the motor would help limit the current spike and/or prevent the short circuit triggering the BMS? I thought of putting a DC solid state relay in series held open by the output of the BMS but then I realised (after wasting $28 and a couple of hours modifying the relays to fit.....) that this wouldn't work as it would break the circuit when the battery was discharged and prevent the charger from recharging it.I've already modified his charger to accept the plug from a hobby smart charger to charge the new LiIon cells safely.Thanks very much for your time.Phil

author: mrkev
author_rank: Regular Contributor
timestamp: March 29, 2015, 01:14:52 am
content: The first thing you can do is to measure resistance of the motor coil. Because that's gonna give you the start-up current.Stopped motor is in fact just a wire with finite resistance...Thing I would do would be to use power mosfet based current limiter instead of current shutdown. Even power resistor (that would limit the current) in series with coil may help and you can just short it with the low RDSONmosfet after few seconds (MOSfets are usually better than relays at low voltages and those currents).

author: mrkev
author_rank: Regular Contributor
timestamp: March 29, 2015, 01:37:45 am
content: Btw. If you have 7.2V battery, something like 1ohm would be sufficient to limit the current below 7A.On the other hand, when you drill and the motor gets (almost) stuck, it could be useful to hold on those 20A for a few seconds instead of just shutting down whole thing. That's why I would rather use current limiter.And for that cap question. Just a little math: CU=It (simplifying with constant current)So if you need 20A for 10ms with voltage drop of 1V, you would need >0.2F cap (that can withstand 20 A)... And even worse is that you would have to charge it first, probably with even bigger current...

author: PhilLG
author_rank: Newbie
timestamp: March 29, 2015, 11:15:39 am
content: Quote from: mrkev on March 29, 2015, 01:14:52 amHi,Thanks very much for your help so far. The resistance is around 4ohms (I had to clip a lead onto the inside of the battery to measure it so it might be a little bit off but should be close).Do you mind elaborating a bit on the arrangement for the power mosfet and resistor? I'm a very much a beginner at this.... Also is this arrangement likely to cause any issues when current is flowing back through it while charging the battery? If so i could try pulling the drill apart and seeing if I could fit it in somewhere between the drill and motor.Thanks again.

author: PhilLG
author_rank: Newbie
timestamp: March 31, 2015, 08:34:19 am
content: Any chance of a few more details on your suggestion mrkev or anyone else?Thanks,Phil

author: mrkev
author_rank: Regular Contributor
timestamp: April 01, 2015, 01:34:31 am
content: Hi,sorry i wasn't near PC for a few days. Resistance of the coil is IMHO less than 4 ohms, because that would give you max. current about 2A. I think that you won't be able to measure the correct resistance just by basic resistance measurement on your multimeter. It's gonna be something like 0,1 - 0,3 ohms.I would use 0,5A current source for a short time (if you have one, if you don't, use 5V voltage source and 10 ohm power resistor in series).So connect your current source (or voltage source with that resistor) directly to the drill and measure voltage drop over it. Even better would be to use two MMs, one for current and one for voltage. Drill shouldn't be spinning and you should measure for less than few sec.F.e. 0,5A over 0,2ohms will give you 100mV.Btw. What is the method you are currently using for current shutdown? Did you use some dedicated chip?