On Mon, May 30, 2011 at 7:16 AM, RussellMc wrote: > There are many many many LiIon "prismatic" (aka flat) cells around - > the smallest would likely tolerate this role well. > LiIon is very easy to charge reasonably well. Keeping Vmax charge > slightly lower than usual reduces cell capacity, removes any need for > temperature compensation on full charge, and vastly extends cycle > lifetimes. f you set Vmax slightly lwo - say 4.1 or even 4.05 V/cell > (4.0?) charging can be made 100% foolproof - plug it in for as long as > you like and it just assymptotes to Vmax at a safe value. > An competenyt charger can bew two R's in series (R1, R2) to the > battery from V+_charge_in with a zener clamp to ground beyween the two > ressistors and an eg TL431 clamp at the battery. > Zener set charge rate max via (Vz-Vcell)/R2. There are issues with > this if the cell goes very low but they can be essentially eliminated > with a v small amount of design. > TL431 needs two resistors to "program" it. R1 limits worst case > current for high Vin. Whole charger can be 4 x R, 1 x zener, 1 x > TL431. Compact and cheap. Slightly more complex can do better again, > but the basic version is fine. > > IF the usage % s low a PV panel may allow charging from roomlight or > distant daylight. > Probably acceptable size PV panel gives 50 mA at 1 sun =3D 0.5 mA at 1% s= un. > If you draw 25 mA operating you need 50:1 + duty cycle. > 1 hour of 1% sun per minutes of use. > A few minutes in OK daylight (say 25-50% sun) gives 1 minute use per 1 > to 2 minutes of sun. > Desklamp close can give > 10% sun (up to 1 un with abusive brightness > and heat). > > > Russell This sounds perfect. So at the very least, I can supply a (current limited) maximum 4 volts and let the charger sit at 4 volts indefinitely? I value battery cycle life and shelf life far more than capacity for an application like this. I'll probably even let the battery sit at 3.9V or something like that (maybe around 60% charge and not let it fall below 40% charge if it means the battery will last a few years. It's a guitar tuner, so run time i= s probably a couple of minutes max, then the user should put it back to charg= e and forget about it until the next time it is required. USB port charging i= s probably the most convenient since it's /everywhere/. > --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .