Measuring current draw from the battery

[CMatic]

I have a pic16F877A board running with 9V battery. How do I measure the current draw from the battery. When I use a multimeter to measure the current drawn from the battery, the board stops running the program. I am trying to figure out how I can measure the battery health when the circuit is operating.
Thanks in advance.

[temtronic]

One method is to insert a very low ohm resistor and measure the voltage before and after it. Obviously you need some kind of resistive dividers and feed the signals into two analog channels.
Then there's a bunch of math, as YOU have to decide based on type of battery what is an acceptable 'low battery' flag since we have no idea what battery you're using, current draw, etc.
There are commerically available 'current meter' modules, fairly cheap that work as above.
Another method is to take a 'reasonable' load resistor and place it in parallel with the battery every so often and read the battery voltage. Again, you need to decide on voltage dividers and what is a 'low battery' condition.
I've used this method for the past 20+ years.

hth
jay

[gpsmikey]

As Temtronic says, you typically measure the voltage across a load resistor (which is what most multimeters do also - it is just internal to the meter). Depending on what your software is doing you may get odd readings - it may be jumping around depending on what the chip is driving (or if it is in low power mode part of the time). If it is doing the same thing, you may find the reading steady, but if it is doing different things, then you may have to add some filtering to your measurement to get the average (or look at the voltage across the load resistor with an oscilloscope). Ohms law will give you the current if you know the voltage and resistance. It also depends if you are trying to measure the current after your regulator or before it how it will act.

mikey
_________________
mikey
-- you can't have too many gadgets or too much disk space !
old engineering saying: 1+1 = 3 for sufficiently large values of 1 or small values of 3

[PCM programmer]

[CMatic]

Great discussion, thank you for your replies. Temtronic your two methods to measure the battery voltage were awesome. I will try these over the next few days. PCMProgrammer, as always your answers are so insightful and filled with details....I am so thankful. Gpsmikey thanks for your answers I will be using them also in the next day or so.