Balance charge/BMS circuit problem starting handtools

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I replaced an old 12v gel battery (2AH) in a hand vacuum unit - with a
set of 4 Li 18650 cells and a Chinese Battery Management circuit board:
https://www.ebay.co.uk/itm/1S-2S-3S-4S-5S-6S-PCB-BMS-Protection-Board-For-18650-Li-ion-Lithium-Battery-Cell/252994763157?ssPageName=STRK%3AMEBIDX%3AIT&var55%2038242317&_trksid=p2060353.m2749.l2649
(4S 30A version with balance).
The balance charge part of the board works a treat but if I try to run
the motor through the discharge part of the circuit the motor spins for
about 1/5 second kick then cuts.  
My guess is that the motor draws a monster current on startup (in normal
running it draws at about 12v 1.5A) and the BMS sees it as a short
circuit or a big spike and cuts out - but thats only my guess, anyone
know what is going on here really and how to correct the problem? I cant
find anything useful on this problem on search.  
I tried connecting a 350uf cap across the battery but this didnt sort
it..
There is no circuit diagram available for these boards, at the moment I
have connected it all as balanced charge and the motor running on a
direct switch.
Works fine but no discharge management of course so have to guess with
the motor slowing appreciably! C+

Re: Balance charge/BMS circuit problem starting handtools
On 13/05/2018 08:53, Charlie+ wrote:
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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

Re: Balance charge/BMS circuit problem starting handtools
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.

Re: Balance charge/BMS circuit problem starting handtools
On Sun, 13 May 2018 17:08:10 -0400, Bob Engelhardt

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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+

Re: Balance charge/BMS circuit problem starting handtools
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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.

Re: Balance charge/BMS circuit problem starting handtools
On 5/20/2018 11:56 AM, Charlie+ wrote:
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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.

4x 18650 is 16.8V max.  That will cause your motor to draw
more current than would a 12V Pb.

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:

https://www.lowes.com/pd/Kobalt-24-Volt-Max-1-5-Amp-Hours-Lithium-Power-Tool-Battery/1000090833

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?


Re: Balance charge/BMS circuit problem starting handtools
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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+

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