On 15 March 2011 08:07, Bob Axtell wrote: > While the ideas presented here are good, I'd have absolutely NO > confidence in any > calculation. The only time I'd feel confident is to:... All points well taken. BUT sometimes you don'y have the luxury of eg 15 to 30 year testing. I'd say that given any system of *well defined* battery, load and overall environment a competent engineer should be able to get the uncertainty down to a factor of say less than 2:1, and in many cases much better than that. It's the engineers job to ensure that "well defined" is what it says and is well enough known. In most cases, the accuracy of the assumption set and match of theory to reality are the limiting factors. Otherwise known as applied engineering :-). Even believing the manufacturer depends greatly on who the manufacturer is and which product you are dealing with. Bob and I are generally in agreement though - if the volume is large enough and the answer important enough then a substantial amount of work may be required to get a half good answer for an apparently simple requirement. As an aside - where power levels get so low that "many years" of backup or operation are aimed at, use of "energy harvesting" may well provide an extremely valuable contribution. eg a 2000 mAh energy store operated over 5 years provides about 50 microamps continuous current. At say 3V that's about 150 uW power. 2 cm^2 of modern silicon PV material exposed to 0.01 sun (200 mm^2 and 10 millisun if Olin is asking) continually. Or* 250 millisun (25% of full sun for people who want the figure to leap out at a glance) for 1 hour per day would provide this power level . E&OE. A few cm^2 of PV panel storing to a supercap MAY provide long term powering needs relatively easily and cheaply in many such applications. Russell * Beware people who start sentences with "Or ..." :-). --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .