Thread: Low cost BMS
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Old 01-06-2013, 04:49 PM   #71 (permalink)
harlequin2
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Just happened to turn the computer on for the first time in several days and there was your message! (Its 10:00 am on Monday here).
The shunt resistors are 4 x 1 watt, 6R8 smds connected in series/parallel to provide a single 6R8 resistor capable of dissipating up to 4 watts. They are connected across the module supply terminals in series with a transistor that is turned on or off by the micro.
You would have to re-arrange the circuit so the voltage divider feeding the micro etc is separated from the shunt. Also, feeding say 15V into those shunt resistors would cause lots of melting! ie 33 watts.
The power dissipation of the shunts is something to be considered carefully and the Li cell circuit dissipates 1.9 watts when shunting at 3.6 V. This is about the max that can be tolerated in that small space. For the same dissipation from a 15V battery, you'd need to change the resistors to 120 ohms and then you'd only bypass .125 A which is not much to try and do balancing with. If you want a shunt current of 1/2 amp, then you need 30 ohms and dissipate 7.5 watts, so need 15 - 20 watt resistors! Or a heat sink. So that's what I mean when I say you need to change the shunting.

I use DesignSpark for schematic and pcb files. You can download it free from their web site.
The C code is all compiled with the CCS C compiler running under MPLAB.
I'll be sending Mark some updated files shortly.

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