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 $ :-)

5v supply so 6v battery OK as long as it has minimal droop across 
discharge cycle.
Peak current maybe 400 mA for milliseconds and 250 mA for seconds.
150 - 200 mA for minutes.
Used for up to 5 minutes then it is either charged for 5 to 30 minutes 
(possibly more) or discharged at under 200 microamps.
200 uA discharge may continue for up to a year. (1.75 AH of delivered 
capacity).

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.

Battery can be treated somewhat roughly as long as it retains adequate 
capacity and rechargeability capability for say 3 years. Longer is 
good.
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.

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.



            Russell McMahon



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