As also posted in my Hearoid showcase topic, but useful for anyone running LiPo batteries (could be adapted easily for 3S and 4S without too much of a problem).

Parts needed:
1 x Zener Diode 5.1v 500mA (Farnell ref 1861447)
3 x 100k ohm Resistors (Farnell ref 9339078)
1 x Small Piece of Strip Board (12 strips x 7 rows) - Maplin FL17T
3 x Pin Header (1x3) or 1 x Pin Header (1x3) and 1 x Pin Header (1x6) eBay Search
1 x servo Extensions (male to male) - made from 2 normal ones cut & soldered.
Solder
Soldering Iron
Cutters


The Schematic:







Method:
1. Solder R1 - 100k resistor from Row 2 Column 3 to Row 2 Column 11



2. Solder R2 - 100k resistor from Row 3 Column 5 to Row 3 Column 11



3. Solder R2 - 100k resistor from Row 4 Column 7 to Row 4 Column 11



4. Solder D1 - Zener Diode from Row 5 Column 5 to Row 5 Column 11 (Band to the right)



5. Solder the pin headers on Row 1 Columns 1, 2, 3, 5, 6, 7, 8, 9 & 10



6. Solder some black off cuts of wire from Row 6 Column 1 to Row 6 Column 5 and from Row 7 Column 5 to Row 6 Column 8





7. Solder an off cut of red wire from Row 7 Column 2 to Row 7 Column 10



8. You're all done. Plug the balance port connector in to the 3 pin header on Row 1 Columns 1 to 3 (Ground to the left) and plug in some servo extensions (male to male) or 3 pin JST to servo on the two other headers (Ground on left on both), connect the other ends to ADC0 and ADC1 (or any other two ports if those are taken but you will need to adjust the script below if you do).

Now, to use the information in EZ-Builder...

The EZ Script

Code:

# Read voltage level of 2S LiPo battery and report to software.
# Set variables
# Change $vundervoltage to the under voltage of your specific battery (leave at 3v if unsure)
# Change $vcritical to your lowest voltage you wish for the battery to get before human intervention
# Change $vmin to low level alert value
# Change $vmax to battery full charge
# Change $multiplyer if using voltage divider
# Factor is 5/255 for adc value conversion to volts
$vundervoltage = 3
$vcritical = 3.5
$vmin = 3.7
$vmax = 4.5
$multiplier = 2
$factor = 0.019607843

:ReadCells
# Get ADC values
$vc1 = GetADC(ADC0)
$vc2 = GetADC(ADC1)

# Convert values to voltage
$cell2 = $vc1 * $factor
$cell1 = $vc2 * $factor * $multiplier
$cell1 = $cell1 - $cell2
$batteryv = $cell1 + $cell2

# Check for errors on circuit
IF ($cell1 > $vmax or $cell2 > $vmax)
  Print("Battery Monitor Error")
  Print("Check Monitor Circuits")
  
  # Check for errors on battery connection
ELSEIF ($cell1 <= 0 or $cell2 <= 0)
  Print ("Battery Connection Error")
  Print("Check Battery Connection")

  # Check for under voltage
ELSEIF ($cell1 <= $vundervoltage or $cell2 <= $undervoltage)
  Print ("Battery Under Voltage")
  Print("Check Or Replace Battery")

  # Check if at critical levels
  # Cell 1
ELSEIF ($cell1 <= $vcritical)
  # Do sometihing if critical
  Print("Cell 1 Critical")
  # Shut down all power, switch to back up battery alarm?
  
  # Cell 2
ELSEIF ($cell2 <= $vcritical)
  # Do sometihing if critical
  Print("Cell 2 Critical")
  # Shut down all power, switch to back up battery alarm?
  
  # Check if below recommended levels
  # Cell 1
ELSEIF ($cell1 < $vmin)
  # Do sometihing if voltage low
  Print("Cell 1 Low")
  
  # Cell 2
ELSEIF ($cell2 < $vmin)
  # Do sometihing if voltage low
  Print("Cell 2 Low")

ELSE 
  # Output voltages
  Print("C1 Round($cell1,2)V")
  Print("C2 Round($cell2,2)V")
  Print("To Round($batteryv,2)V")
ENDIF 
# Wait 5 seconds
Sleep(5000)

# Go back to the start
Goto(ReadCells)