Hi, I need to know the battery life for a project. The circuit sleeps for 8 seconds, then wakes up for 2 seconds and goes back to sleep. During sleep mode it consumes 0.03mA and when it is working it uses 40mA. I am confused about how to calculate the consumption in 1 hour so that I can use it to calculate the battery life of a battery rated 2500 mAh. Any advise is greatly appreciated, thanks!

 

Your average current draw is (8s*0,03mA+2s*40mA)/10s=8.024mA. 2500mAh/8.024mA=311 hours, or close to 13 days.

 

I get the same answer as kubeek.

 

Another way to come at it is to calculate the charge draw during one cycle. First, as noted in your thread title, "milliamperes per hour" is meaningless. What you likely meant is "milliampere-hours"; this is milliamps times hours and is the amount of charge consumed when one milliamp flows for one hour (or anything that works out to the same product).

So during one cycle your system consumes

When asleep: 0.03 mA * 8 s = 0.24 mAs
When awake: 40 mA * 2 s = 80 mAs

So you consume 80.24 mAs/cycle.

FYI: 1 mAs = 1 mC (1/1000 of a coulomb)

The number of cycles you can support is

# cycles = [2500 mAh / (80.24 mAs/cycle)] * (3600 s/h) = 112164 cycles

The amount of time is then

time = # cycles * (10 s / cycle) * (1 h / 3600 s) * (1 day / 24 h) = 12.98 days

Which, reassuringly, matches what the other two got.

 

Thank you a lot guys for your answers , I understand it now

 

Oh, one thing that should be mentioned is that the capacity of a battery, 2500 mAh in this case, is not as fixed a number as most people would like to believe. As a rule, the higher the current draw, the less the capacity and, for some battery types, the difference can be significant. So that 2500 mAh rating is for a particular discharge profile. My guess is that your draw is sufficiently low that you might even get more capacity than the nominal rating.

 

The rated mAh may also assume you run the battery completely flat: which is not a good idea if you want the battery to have a long and happy life .