[TriEmbed] Battery Connection for Atmel Micro

[kschilf at yahoo.com]

Hi Chip,

Paralleling batteries is a tricky business.  Batteries never have exactly the same 
voltage.  The highest voltage battery will discharge into the lower 
voltage cell.  If a battery in parallel experiences a shorted cell, that battery will rapidly discharge the other batteries.  In commercial 
installations, large batteries are only connected in parallel with 
protection (diode or more complex current sharing) to prevent power loss and/or fires.


I would suggest connecting the two Lithium batteries in series and then 
using a buck converter to regulate down to what you need for your 
project.  On-semi (was Motorola) and Texas Instruments both offer PTH 
controllers, but you have to add the other ingredients (output inductor 
and cap, input cap, catch diode, etc.).  It is also highly recommended 
that you lay these components out on a PCB instead of flywires.


The good news is that you can buy pre-made modules using TI's (formerly 
National Semiconductor's Simple Switchers) LM259x controller on Ebay for as little as $5 in single quantity or less if you buy 5 or 10 in a 
lot.  They are small, simple to use and have potentiometers built in to 
adjust the output voltage.  They are advertised as having 90+ percent 
efficiency but that figure is dependent on the load.  They are very 
handy for small projects and will get you running in less than five 
minutes.

Good luck,Kevin Schilf
Digital Telesis, Inc.
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