Measuring a single cell in a multi cell battery pack

[JohnKennedy]

Hi

I need to be able to measure the individual voltage of each cell in a multi cell lithium battery pack. The standard availble IC's for doing this only go to a maximum of 4 cells and I need to measure 9.

Is there any way I can use a PIC for this?

The problem I can't seem to get my head around is that the PIC measures it's voltage on the A/D referenced to ground and I need to measure each cell voltage Individually not cumatively to perform cell balancing.

Anyone got any ideas

TIA

JFK

[asmallri]

Are the batteries wired in series?
_________________
Regards, Andrew

http://www.brushelectronics.com/software
Home of Ethernet, SD card and Encrypted Serial Bootloaders for PICs!!

[JohnKennedy]

[asmallri]

If you have plenty of PIC pins, wire each battery junction, via a suitable (preferably constant) resistor divider to an A/D input. Assume we are measuring battery x in the string. Battery voltage of X = voltage of junction x.y - voltage of junction w.x
_________________
Regards, Andrew

http://www.brushelectronics.com/software
Home of Ethernet, SD card and Encrypted Serial Bootloaders for PICs!!

[Humberto]

I would use:

1) Resistor divider coming from each cell junction. ( @ 1% tolerance)
The key is get 4.00 V max in the AD front end.
2) Analog multiplexer to collect all 9 resistor divider�s into one AD converter.
3) 12 Bit AD Converter. With 1mV resolution it�s a matter of single math to know the
voltage in each individual cell.


Humberto

[JohnKennedy]

Thanks Andrew new there had to be a relatively simple answer but I was damned if I could think of it.

thanks again

JFK

[guest]

The best way would be to use multiple level shifting differential op-amps, with unity gain, feeding separate adc inputs.

Just using a resisive divider, and subtracting one cell's voltage from the next will decrease the resolution of your measurements.

[kender]