Norman Tomlins wrote:
> 
> I want to be able to read a voltage from a battery and the amount of current
> during charging and discharging.   This will be used to my 90 watt solar
> panel.  (More panels to come later)
> 
> I have decided to use a Current Sensor from
> http://www.west.net/~escor/XAE25XL60.pdf and have chosen to use the XL60
> amp.
> 
> Okay now the question.
> 
> To Measure the Voltage:
>         I want to be able to read up to 25.6 volts using a 8 bit a/d convert (25.6
> / 256 is .1 res.)  I though about using 2 resistors to bring the voltage
> down to 5 volts and then measure it.   (A voltage Divider?)

That's ok. You can use a single 8 bits ADC with two channels, one for
voltage another for current.
Use 1% or better > 10k film resistors for the division.

> 
> For the Current:
>         The voltage is 2.5 volts at 0 amp and 21 mv per amp after that.   So at one
> amp the voltage output would be 2.539 volts for a current draw and 2.461
> volts when charging.    I was wondering if I would have to use a 12 bit A/D
> converter to detect the changes in voltage so I could get a decent reading.
> I would like to be able to read about 100ma resolution or better if
> possible.  Is this a good Idea or can someone due better?  Keep is simple...

The XL60 generates 22mV (not 21mV) per Ampere sensed.  Suppose you
connect the output of the XL60 to the + input of an op-amp, while the -
lead goes to an adjustable voltage divider at 2.45V, so the op-amp will
amplify from 11 mV up to 89 mV. If this op-amp gain is 20, your output
signal will be from 220mV up to 1.78V. Feeding this signal to a 12 bits
ADC (unipolar), with a reference voltage at 2.5Vdc you will read values
from 360 up to 2916, being your zero current equal to (1V) 1638, so your
resolution will be ADC output 2916-360=2556 for the XL60 output delta of
2.539-2.461=78 / 22 = 3.545A, so 3.545 / 2566 = 0.00138 (1.38mA) per ADC
step.  

A 8 bits ADC would give you a resolution of 16 x 1.38mA = 22mA, zero
current would be a count of 102. It is not enough?

Adjusting the ADC reference voltage to around 2.262V you can get a
resolution around 20mA, easy for your PIC math.

Remember that instead of everything above (for current), you can use a
shunt resistor of 0.1 Ohm connected directly to the inputs of the 8 bits
bipolar ADC, with a ref voltage of 200mV...  This shunt will steal only
(0.1 Ohms x 1.77A^2) = 313 mW from your system. So, your device will be
made with only few resistors, an ADC with two inputs, a PIC and the LCD
display.   Note: If using 200mV Vref at your ADC, you need to step down
the 25.6V to 200mV, use 1% commercial film resistors 64.9 kOhms and 511
Ohms, output is 199.99mV when input is 25.6V, @ 391.3ľA. 

Wagner.