> I do use super caps for a couple of memory circuits and find them > invaluable, but they wouldn't do in this case. Powering the circuitry > is not a problem, but operating the solenoid (4R load for 1/2s every > 30 seconds) would quickly drain it. An SLA is not too expensive on > a cost/year basis This started as a simple suggestion, but has got me thinking about Alkaline battery shelf life: Alkaline batteries sound like a possible solution here! Shelf life unused is 4 to 6 years shelf life 5-7 according to http://www.zbattery.com/zbattery/chemistry101.html). Panasonic say "up to 7 years" Kodak state 80% capacity at 7 years for their AA & AAA Alkalines !!!!!!!!!!!!!!!!!!!!!!! http://www.techadvice.com/info/pil.asp?pid=1295&old=0&SortColumn=company_name Even lithium at 10 years shelf life may be attractive. It would be interesting to see how standard Alkaline batteries lasted under a minute trickle charge. Maybe a small Peltier effect cooler in a thermally insulated battery pack would give a far longer shelf life (reactions tend to halve in rate per 10C drop). Maybe a 10C target. Runs only when mains power available. RM -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.