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How to connect to a BMS with no P- pad?
#1
I've bought one of these to replace nicads in a cordless drill
 
It has no P- connection on the pcb.

this is the supplied diagram:
 

Can I simply connect the load [motor] across the battery pack + and - ?

any downside to that?
(I posted a longer, more detailed message before, containing a URl, and it just disappeared...)

any help gratefully received!
John
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#2
Downside is the BMS no longer protects the battery if you connect straight.

When a BMS doesn't have a label for one of the connects, then that connection is shared with the other portion. Yours doesn't have P-, but has C-. In this case, C- = Common Negative. So both Load and Charger connect to the Negative pad.

If you look, between B- and C- is all the protection circuitry is.


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#3
(09-05-2020, 02:13 PM)Korishan Wrote: Downside is the BMS no longer protects the battery if you connect straight.

When a BMS doesn't have a label for one of the connects, then that connection is shared with the other portion. Yours doesn't have P-, but has C-. In this case, C- = Common Negative. So both Load and Charger connect to the Negative pad.

If you look, between B- and C- is all the protection circuitry is.


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Thanks very much for putting me on the right track.
I have a 10Kohm NTC thermistor, is it worth adding that to the circuit [next to the cells, of course]?
Are the diode connections [top edge of the PCB] for indicating a fault, or 'pack fully charged' or what, do you think?
Thanks again.
John
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#4
NTC wouldn't hurt. However, you would need to use the one "specifically" they designed for (unless it can be programmed to accept different variances). Because they would have used a specific resistance for a matched temperature. If you don't use the one they intended, then you could be off on accuracy. In other words, it's possible the NTC could be reporting 140F but the BMS is reading 90F instead because the resistance values don't match the designed setup.
Best way is to test it.

As far as the diodes (I'm assuming you mean leds), you'd have to test it. Start off with low voltage packs, see what it does. Start charging and see what changes. When pack gets full, see what changes. Etc.
Proceed with caution. Knowledge is Power! Literally! Cool 
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Certified 18650 Cell Reclamation Technician

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#5
Aren't Most NTC either 10k or 100k?
later floyd
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#6
The thermistor I have to hand is a 10K one at 18-20 degrees C, and:
2k @ 65C
1.2K @ 80C
0.8K @ 90C
(The one Makita put in the Nicad pack is c.20K, but that's irrelevant.)
Yes, no doubt the connections marked with a diode are for a LED (but they actually used the symbol for a regular diode   Rolleyes )  I wondered if there was a 'normal' for what the diode indicated. As you say, I can try it and see.
(sorry for the delay in replying, my pc crashed Sad )
Thanks,
John
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#7
Can I please check that I'm not doing something silly?
I'm putting the BMS shown earlier in the battery pack and connecting all the balance wires as shown.
I'm also taking the battery pack positive, and the BMS C- connection, /and/ all the balance [inter-cell] connections out to a connector so that the pack can be charged in a balanced mode by an I-Max B6 charger.
Am I right in thinking that the I-Max charger and the BMS board won't 'fight' each other, and the BMS will prevent over discharging by shutting down the whole pack when one cell goes too low?
Right?
Sorry if any of this is FAQ. (And if all the answers are collected together elsewhere on this site, please tell me where.)
As you can tell, I'm new to this game!  Smile
Thanks
John
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