Balance charge/BMS circuit problem starting handtools

How about trying one of those NTC thermistors for SMPS or torroidal transformers (find a scrapper), to avoid blowing fuses at start-up. Much lower voltage in your case , so may have to parallel a few

Will it run on the BMS after it's been started with a direct connect? If so, there are a variety of ways to connect it so that it's a direct connection to start & then the BMS to run.

On Sun, 13 May 2018 17:08:10 -0400, Bob Engelhardt wrote as underneath :

Thanks to you both - Im away for a few days now and will return to this problem with a reply to that question Bob. The NTC I think has a drawback in on-off-on switching time. Thanks for the idea NC! C+

On Mon, 14 May 2018 06:34:59 +0100, Charlie+ wrote as underneath :

Hi I'm back now!! Sorry about that.. Bob you are right - once running at a good speed (about 70%) it will continue quite happily through the BMS. I could put in a bypass switch or button to run the startup - but as this is a hand unit the use pattern is probably 20s - 1m on then off then on etc. until the battery is used up, so a bypass switch would be a bit of a nuisance, any simple better ideas in your drawer!? I remeasured the running amps and it is about 9.5A at full chat. The BMS is rated for 30A, so I can see the start Amps may be miles over that (I dont have an inrush max measuring meter).. BMS control is on the negative side.

As I recall, your original post linked to an ebay list. I had to guess which one, but it clearly said, "don't use these with power tools," or words to that effect.

Earlier, you said you put a cap on the BATTERY. You'd need to put it at the OUTPUT of the BMS. But it would have to be a BIG cap.

Best chance is to try to find a spec on the chip and put a cap on the current sense line...and maybe a resistor to get the time constant long enough to start the tool. I wouldn't recommend disconnecting the current sense. That would be unsafe.

If you bought cheap cells off Ebay. Or if you bought any cells with the name *fire*. Or if the cells don't actually SPECIFY maximum current. Or if you used a battery holder. Or if you soldered to the cells. You can be sure that the battery is crap and unsuited for the intended application. Get cells that are RATED for your application. Spot weld them together.

You can get six of them for $10 here:

The cells in mine are rated for 18A continuous discharge.

Cut off four cells and solder to the tabs, not the cells.

The pack doesn't have the classic BMS. Near as I can tell, they monitor the cells and send data to the drill to shut it off at maximum discharge. That's useless for your application.

Are we having fun yet?

On Sun, 20 May 2018 23:32:05 -0700, mike wrote as underneath :

part snipped

Mike Hi - thanks for post, Yep, happy with voltage, runs great but wont last as long! eBay - only some BMS are not for power tool use. The inrush/fan spoolup time is about 3 seconds so 70% of that is ~ 2 seconds, no lightweight say 30v capacitor could cope with that. C+

A brute force approach would be to use a time delay relay to connect the battery directly to the fan (through the switch, of course). NC at start & then opens a couple of seconds later. You'd have to test whether the relay in parallel with the BMS would be enough off-load.

Or, it could probably be done with a honking big BJT that has its base biased with a capacitor that's charged in the off time and then discharged through the base in a few seconds when on. But I'm much too transistor illiterate to be more specific. Try S.E.Basics or S.E.Design if you don't get help here.

I'm happy to prefer NiCd/NiMh when available. Seems the OP has caused his own problem.

A current limiting transistor stage could fix it, but going NiMH would surely be easier.

NT

On Mon, 21 May 2018 08:05:30 -0700 (PDT), snipped-for-privacy@gmail.com wrote as underneath :

Not a chance - we are replacing wet Lead acid type power! Original Pb. battery just wore out quite quickly .. C+

On Mon, 21 May 2018 10:30:39 -0400, Bob Engelhardt wrote as underneath :

OK Bob thanks for thaughts, think I might just have a start button and release once running. Thats cheap and cheerful, Ill report if it works ok. C+

On Mon, 21 May 2018 18:22:34 +0100, Charlie+ wrote as underneath :

Ok Done and dusted - instant-on press button works fine (just bypasses the BMS negative on startup) - time will tell if it keeps on happily trucking! Thanks for help everyone... C+

That's what I did when I converted a battery powered rotary tool from 3 x NiMH to an 18650 and the charge controller / battery protection module did the same as you're reporting. My system also worked fine. ;)