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  Monitor for Li-Ion / Li-Poly Battery Packs 

with cell balance connector 

BM3 for 3S battery pack

BM4 for 3S / 4S battery pack


BM4  12F675   (  2005/12/17 )

A small piece of plastic been cut from the connector (red arrow) to fit both 4 pins and 5 pins balance connector

This device designed to be a "Battery Monitor", which will neither not cut off power nor control your model, but just a alarm. 

It has two function, monitor the "unbalance ", and the "over discharge " of each cell, suppose you use 3S1P or 4S1P packs.

1.  Unbalance: 

When the "voltage difference" between cells > 0.2~0.25 V ( nominal 0.3V ), BM beep a low tone for 1 second and repeat. 

2. Over discharge :

When any cell over discharge and it's voltage down below 3.2V,  BM beep shortly to indicate which cell is down.

If over discharge been detected, the unbalance detecting function will be disable.  


User Manual :

Plug the BM to  balance connector of battery pack, 

If the battery pack is 14.8V ( 4S ) BM will beep Do~Mi~So~Do to indicate it's OK. if the battery pack is 11.1V ( 3S ) , another two Do~Mi~So~Do sound, to indicate it's OK

To stop the alarm, disconnect the BM from battery pack

You may remark the ASM code to change the default value.

The HEX code in this page (see below) have three different file :

BM3_675_32_03 : define < 3.2 V is over discharge, > 0.3 V (nominal) is unbalance. ( suggested )

BM3_675_34_03 : define < 3.4 V is over discharge, > 0.3 V (nominal) is unbalance. 

BM3_675_30_05 : define < 3.0 V is over discharge, > 0.5 V (nominal) is unbalance. 

Choose one for yourself!

The sound samples : WAV


cell 2 and cell 4 over discharge

unbalance and cell 1 and cell 2 over discharge


BM3 V2.0  12F675   (  2005/12/12 )    *** BM3 won't update anymore !!!

Just plug the device into "balance connector" of battery pack, it well beep Do~Mi~So~Do to indicate it's OK.

If the voltage difference > 0.3 V ( nominal, actually 0.2~0.25V, adjustable within the ASM code), the LED flash and beep 5 sound at lower tone than the sound that indicate battery-low. 

When cells run below 3.0V, the LED on and sound beep to indicate which cell is low.

Bi~~~~Bi~~~~ indicate the first cell (B) is low

Bi~Bi~~~~Bi~Bi~~~~ indicate the second cell (B2) is low

Bi~Bi~Bi~~~~Bi~Bi~Bi~~~~ indicate the third cell (B3) is low

So, if you hear Bi~~~~Bi~Bi~Bi~~~~Bi~~~~Bi~Bi~Bi~~~~, it means B & B3 are low

It well continue beep sound until you unplug the battery pack

Here, How it work

ExpressSCH BM3 schema download

ExpressSCH BM4 schema download

PIC12F675 ASM BM3 V2.0                             PIC12F675 HEX BM3 V2.0

PIC12F675 ASM BM4 V1.0                             PIC12F675 HEX BM4 V1.0



You MAY duplicate, modify, give away or sell the original files in the web site or anything you derive from it.

ATTENTION : It is your own responsibility, if any damage cause by using the web site.

Philip Sun / TAIWAN

copyright 2004