Some notes on Li-ion battery charging

I've used lithium ion batteries in many projects for several years without mishap. I never realised how much mis-information exists in relation to these batteries. Claims such as "You have to cycle the battery 5 times before it reaches it's rated capacity" do nothing but demonstrate how stupid the person is making such a claim.

Many years ago I sighted research which showed that if you can keep a lithium ion battery between 80% and 20% of it's capacity it will last essentially forever. Now I question if that was real research or folklore. Consider that one of my laptops a decade ago actually had a bios setting that allowed you to only charge the battery to 80%. Like all "information" in this field I don't know if this is still, or even was, valid.

Guidelines

However, I have a few guidelines that have not let me down:

• Never fully charge 
• Never discharge below 3.1V (revised from 3.0V after measurement)
  
• charge at C/10 or less.
  

BRCL4054CME

When I have to package the battery and charging circuit into a small enclosure I reach for the BRCL4054CME. Simple, reliable and effective. However, like every charging IC I have looked at it is cumbersome to terminate the charging voltage at less than the designed limit of 4.2V. So when I use this IC I generally don't satisfy the first guideline. 

The second guideline is satisfied by shutting down the circuit using voltage detection and third by selecting the appropriate resistor.  Simple but not quite the complete solution. And it limits you to charging from a 5V source.

Preferred Solution

Where space permits the following circuit is the basis of my preferred approach.

 

Any DC source from 8V to 30V can be used and it has survived mishaps. In my multiple single cell charger I now include a mosfet to isolate each battery during a power outage. That stops discharge through the series resistors that set the voltage. It also means where fitted I can simply switch off the power when leaving the workshop.

My longer term goal is to have enough batteries sitting charged that I only need to switch the chargers on when a cell has been used in a torch or project and needs recharging. I plan to write up the finished project once my battery testing is finished. Preliminary findings will be blogged in a few days or so.

I never had to consider extended power outages before but on reflection of the increasing natural disasters in the world it seems prudent to keep as much charge in the battery  as possible without doing harm until it is needed.