First up was figuring out how I would recharge the battery and supply power to my BT beacon.  The only part that I had  were those 18650 lithium batteries  my son had given me. So I sat down and started brainstorming,  I decided to create the battery system first, because  while all four batteries were showing about a minimum of 3.8 volts, that would not last, I needed a charger!

I've been a fan of  Maxim Integrated  for years, so I headed over there to do some research.

After some time (about a week) of reading app notes, data sheets, and design notes, I came up with what I thought was a simple but smart combination of a Li+ charger and a beacon power supply.  During this period I also chose my BT beacon, the BL-600 made by Laird Technologies.  More about it later.  First the power system.

The Charger: 

The MAX 1811 was the winner of this contest.  It is a cheap (about $5.00), low parts count ( 2  capacitors) smart Li+ charger.  It takes anywhere from 4.35 to 6.50 Volts in(i.e. USB standard) at either 100 mA or 500 mA.; it outputs either 4.1 V to 4.2 and does smart charging of Lithium Ion (Li+) batteries. It has enable, which I tie to the input so it is only on when we are plugged in.

There is no heat sink, instead it dissipates heat via its' ground connections at pins 3 and 6.  It will cycle its charging to keep the temperature down. It can continue to work while in this thermal loop. 

It comes in a SOIC-8 format and requires two caps, a 4.7 uF on the input and a minimum of 2.2 uF on the output.  I'm going with 10 uF on the output because it will also be the input cap of the power regulator (This may change if I find that the reg is not happy. with the cap being on the other side of the battery)

Next, the Power Regulator!