On 8/1/05, Russell McMahon wrote: > Here we go again ... > > I need to provide a battery and charger in a piece of consumer > equipment. > Battery needs to be rechargeable. > Target cost for battery and charger is zero $ :-) OK, let's not dramatical the whole situation. A SLA charger discharger is not such a big deal. > > 5v supply so 6v battery OK as long as it has minimal droop across > discharge cycle. Perhaps then a minimum 7V dc on the bottom riple for the input voltage at -20% of the main supply ? Better 9V to avoid saturation of the charging transistor. > Peak current maybe 400 mA for milliseconds and 250 mA for seconds. > 150 - 200 mA for minutes. Reasonable, I suggest a long time charge with a current 5 to 10% bigger than the one sourced by the application. It will works only if the application source a relative constant current. ie sourcing 250mA, charging + sourcing 275mA, efective charge 25mA. Will request a simple low drop voltage to current conveter. A LM317 will do the job. Need to switch to another charging current ? change some resistors. > Used for up to 5 minutes then it is either charged for 5 to 30 minutes > (possibly more) or discharged at under 200 microamps. I think controlled discharge current it's too low even for a small capacity SLA. However this is an easy job, one resitor and one transistor. If discharge by the load itself then nothing. > 200 uA discharge may continue for up to a year. (1.75 AH of delivered > capacity). Only if you've charge it from time to time... :) > > Sound inexplicable? > It's exercise equipment. > It powers down in under 5 minutes of non use. > User action provides charging energy. > It can be woken up from standby by operation or a button press. > > Battery would ideally have a 1 year life to say 50% self discharge > point. > Available charging current can be sensibly as high as desired although > C1 rate (1 hour charge) is probably sensible max and would be hard on > some battery types. C1 (as one hour charge) on SLA is not applicable. > > Battery can be treated somewhat roughly as long as it retains adequate > capacity and rechargeability capability for say 3 years. Longer is > good. I never seen a sealed lead acid (Toshiba) with full capacity alive more than one year. > Weight and size (within reason) are not important. Actual battery > capacity unimportant as long as delivered capacity is OK. > > At present I'm favouring a 6V sealed lead acid battery solution with > an extremely simple charger. This will limit peak charge current to > some selected value and clamp peak terminal voltage to a float value. > This is less than ideal as it reduces available capacity, but this is > not a major issue here. It's true, in time (a half year or so) the internal impedance will increase so a charged SLA will have more than 7 or 8V. Decreasing the self impedance by charging with current pulses with reverse polarity. > NiCd, NimH have far too short a self discharge period. > LiIon tends to be dear and potentially tricky to charge (although > simple scheme that don't develop full capacity may be OK). > LiIon would be acceptable performance wise if cost was acceptable wrt > Lead Acid. > > Any comments most welcome. Any suggested ultra simple/cheap charger > circuits also welcome. I have ideas already (of course) but there may > well be some brilliant ideas out there. For SLA there is no briliant ideea better than a resistor.:) cheers, Vasile -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist