If your power consumption is low enough, and you are willing to use Lithium manganese dioxide primary cells (non-rechargeable), then for less than the price of a Supercap you could have a battery with a shelf life of more than 10 years and a rather large capacity. About US $2 will buy you a 123A cell with a 1.5 Amp Hour capacity at 3V. If your datalogger draws 10mA @ 3V while running then this would power it for 150 hours. Those 150 hours could be spread out over 10 or maybe even 20 years without any need for a recharging circuit. On Sat, Jan 25, 2014 at 10:26 PM, Jason White < whitewaterssoftwareinfo@gmail.com> wrote: > On Sat, Jan 25, 2014 at 8:39 PM, Charles Craft > wrote: > > I'm building a PIC data logger that will run from a wall wart. > > During a power outage I would like it to stay alive for an hour or mayb= e > two. > > > > It's a one off for me at this time so cost and board space are not an > issue. > > I'm looking for simple and safe. Thoughts? > > > > thanks > > chuckc > > I think a beefy super cap would be the way to go. The only thing to be > aware of is the momentary inrush current required to charge it may be > quite high, perhaps heating the voltage regulator or causing excessive > turn on times for the MCU (their is a datasheet spec which requires > the voltage to raise faster than a certain rate for proper operation). > However, those are pretty trivial issues to deal with. > > The big deciding factor is the current consumption of your circuit. If > you haven't already, I would suggest working to optimize it for power > consumption. Operating buzzers, LEDs, Displays, and not going putting > the MCU in sleep mode when it's idle will burn through your backup > supply quickly. > > Super Cap: > Pros: Simple to implement and usually durable if used within specs > Cons: potentially high inrush currents, somewhat limited storage > capacity for cost (likely not a problem for your application's spec) > > Batteries: > Pro: Cheap for Large storage capacity (easily more than a just a few > hours of runtime depending upon current usage) > Con: Batteries require properly implemented recharging circuits to be > safe and will wear out requiring replacement after a limited number of > cycles. > > USD $3.60, 1.5F 5.5V Supercap > http://www.digikey.com/product-detail/en/DB-5R5D155T/604-1151-ND/3280644 > datasheet > http://www.elna.co.jp/en/capacitor/double_layer/catalog/pdf/dbn_e.pdf > > -- > Jason White > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .