On Fri, Jun 17, 2011 at 1:44 PM, RussellMc wrote: > > How large have you made the R1/R1 combination and still gotten decent > > voltage tolerance over "room" temperature range? > > > > The input bias current is rated at 2 to 4 uA +- 0.8 to 2.5 uA over > > temperature range 0 - 70. > > > > Lots better than a zener but maybe would drain the battery fairly > > quickly unless the R1/R2 pair was very high and then input bias current > > shift eats away at accuracy. > > His app is a charger. > Power will be on most of the time and even if off is not going to > cause vast problems over charger residency type periods. > > 18650 LiIon are about 2000 mAh + . > > 1 mA drain at 24 hours =3D a bit over 1% capacity. > 100 UA - ~0.1% > 10 uA ... > > In order to avoid thinking about temperature compensation I suggested > he move the voltage back another 0.1 V to 4V. > This reduces capacity but (probably) allows simple float and greatly > improves cycle life. > Also avoids having to do current tail level triggered charge termination. > > Okay so I figured out another mechanism to charge. Uses a few transistors, but simplifies the charging itself. I'm guessing many will find this design messy and unnecessary. Please comment. Here's my circuit: http://solarwind.byethost7.com/pic2.png The left side MOSFETs, when enabled, connect the batteries in series, allowing regular discharge. The right side MOSFETs and BJTs, when enabled, do a parallel current limite= d charge up to 4V. As far as I know, the BJTs won't allow the batteries to charge each other when the BJTs are disabled. When they are enabled, the batteries are charging anyway. My only concern with this design is reverse biasing the BJTs and the batteries charging each other when in parallel charge mode. I plan to use SOT-23 transistors to keep it small. Battery discharge I'm guessing will be maximum 1A (to charge my iPhone). Battery charge will be around 2A per cell. --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .