nessence:
When building a wireless device, how do I determine what my power requirements will be, what type of batteries are best for what applications, and life span?
I ask this in a vague fashion because I’m hoping some of you can post your experiences and I can read specs and figure out details. I know the basics but I’m self taught so unfortunately I know very little about battery power supply.
My current design will be a keyfob that is activated by devices which have permanent power. The keyfob will receive data from one device, transmit it to another. I’d like the keyfob to be as small but last as long as possible, although more than 3 years probably isn’t necessary (depending on build costs). What RF I use would be based on power requirements and physical size.
Thanks!
It’s all about what strategy you implement for “sleeping” the microprocessor and radio. Example: in 802.15.4 radios, there is a specific protocol similar to 802.11 (WiFi) to allow YOU to arrange for a wakeup now and then and poll the coordinator to see if there is data waiting. Or you can choose not to do that, and just wakeup the microprocessor on some I/O event (button, sensor) and then wakeup the radio and transmit/get ACK and go back to sleep.
Battery life
worst case, with say, 802.15.4, is about 20-40mA constant if you leave the receiver on at all times and don’t sleep. Add the microprocessor, such as a low power Atmel AVR- a few mA.
Now reduce this by some duty cycle factor. Say you arrange to sleep 95% of the time. So the average battery consumption is 5% of what it was with no sleeping.
Don’t forget to allow time for the radio to power up and stabilize- this might be 10-20mSec.
Often, the radio’s transmitter consumes more than the receiver. But it’s normal that the transmitter-on time is very small compared to the receiver, and a sleeping receiver.
once you have the strategy and duty cycles, you can get to mA hours, average. And batteries are rated in ma hour for capacity. But beware specs: look at mAH before the voltage falls to x, where x is what your regulator needs to be reliable. Many radios and most microprocessors these says run on 3.3V and use “low dropout” regulators.