- Rechargeable or non-rechargeable.
- CR2032 (~220mAh) or 3xAA (800 - 3000+mAh depending on price).
- Battery brand - there are so many, argh!
It seems likely that for battery life reasons I will go with the 3xAA approach. What kind of battery life will this give me? This requires that I work out the current use of my planned circuit. Time to consult some datasheets!
- ATTiny85 - 0.3mA
- TMP36 - 0.05mA
- RF Transmitter - 8mA
If all of these components were active I would need 8.35mA. Lets round that up to 9mA. Lets also assume that I use a 2500mAh battery. This gives the following battery life.
- 2500 / 9 / 24 = 11.5 days
I don't want to change batteries every 11 days - I can do better by putting the components to sleep when they aren't being used.
Let's assume that I only wake up the circuit to transmit a reading every 5 minutes and that the wakeup lasts for 30 seconds - I will need to transmit the reading 3 times to try and ensure that it gets through. This means every 60 minutes the circuit is at full current 12 times for a total of 6 minutes. The remaining 54 minutes the power usage will be as follows.
- ATTiny85 - 0.0001mA (sleep mode)
- TMP36 - 0mA - assume I can disconnect from PWR e.g. with a transistor
- RF Transmitter - 0mA - assume I can disconnect from PWR e.g. with a transistor
This gives an idle power usage of 0.0001mA. The overall mA/hour requirement is as follows.
- (54/60) * 0.0001 + (6/60) * 8.35 = 0.84mA
- 2500 / 0.84 / 24 / 7 = 17 weeks = 4+ months
Much better!
