Battery voltage measurement
I have been thinking about the battery voltage measurement. There are many trade offs in the different methods to accomplish this. The voltage divider I have now works fine. In a voltage divider, the lower the resistance values, the better resolution you have, but the more current you draw. I use 100k ohm resistors so the total current draw (continuous draw) is 20 micro amps. This doesn't feel really good as the Atmel mega1284 can power down to about 1 microamp. The alternative is to add a switching circuit like a FET to switch on the power to the voltage divider. This would make the current draw almost zero when not in use, but adds to the complexity of the board layout. I think for the current version I am going to go with the simple voltage divider and suck up the current draw. After all, the lcd is drawing about 250 microamps anyway and my goal is really to stay below half a milliamp in idle mode. For later models I will consider some of the new battery monitors/manager chips out there. They are good at not only reporting voltage but tracking battery capacity to give you a better idea of remaining battery life. Parts arrived from mouser (particularly the small switches I need) so I will be finalizing the design of the next version. As much as I hate it, I will probably leave out the Bluetooth at this point because I really want this to fit in a good looking case. Stay tuned for updates.
Hi Dan, thanks for sharing all this information on your project, it is very cool. In case it's a help, there are some posts on Jeelabs for measuring battery voltage that you might be interested in. For my projects I have been using a very simple circuit composed of only two 10M ohm resistors and 0.1uF capacitor, resulting in about a 0.2uA current draw @ lipoly battery voltages. The circuit and testing is well described here http://jeelabs.org/2013/05/16/measuring-the-battery-without-draining-it/. The capacitor stores a little juice so the ADC can get a good reading even with the very high resistances, and this circuit only needs this one additional component to significantly drop the power draw. I also made a little video showing how I measured my project's battery as its Vcc was changing https://www.youtube.com/watch?v=HXA_sv2mDdU
ReplyDeleteThanks. That is great info. When I tried using really high resistance values, I lost all sensitivity to the measurement. I can see that the capacitor could help average out small fluctuations and give you a more reliable reading with minimal current draw. Thanks so much.
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