Li Ion Battery Float Charger

Background

I first wrote about my constant current constant voltage charger here. It certainly generated a lot of interest and I suspect that will increase after completing some tests which to date have not shown any ill-effects from float charging a Li Ion cell at 70% capacity.

With a plethora of wall wart type power supplies I wanted a charger that was voltage and polarity agnostic. The circuit used is an extension of that used in my backup led lighting. AC or DC, and therefore polarity is unimportant, between 8 and 20V. Just connect power and battery and walk away.

Circuit

Originally I had provision for charging three batteries. However, only two 18650 cell holders can be installed on the lid of the case. You can either mount the third holder on the side or, as I did, use an inline connector for a pouch style cell.

While I originally used a LM317 voltage regulator and resistors to set the voltage I found the variation in output voltages between each charging circuit frustrating. Far simpler to use a fixed 3.9V or 4.0V regulator.

Provision has been made for daisy chaining these together should you want to float charge more than 3 batteries. 

With all the SMD parts fitted prior to cleaning

A fair bit of additional componentry using TL431's gives visual indication for each battery of the state of charge. On one hand it is comforting to know that when the battery is connected that charging has commenced. However, it feels superfluous since for months at a time the battery just sits there.

Recently we did in fact have an extended power outage due to equipment failure, around 36 hours before a backup generator was used to power up the subdivision. I had many cells ready for backup lighting and keeping cell phones charged which confirmed my view that something like this is useful. I was fortunately at home and could disconnect each battery to stop it discharging back though the charging circuit. The version above avoids that with an isolating mosfet automatically disconnecting the battery when power goes out.

I'm no "prepper" but I can see that this project does help be prepared.