Lithium Ion Battery Charging Practices

I'm curious as to what recommendations can be made concerning charging routines for a typical lithium ion battery found in a device like a cell phone. The primary goal I have is to maintain the usable life of the battery for as long as possible. In my mind a battery is still usable if it can hold a charge for at least a day under minimal/mild use, such as a few short phone calls and/or text message each day. For the confines of this question I'm sticking with the cell phone's default charger and default hardware/software.

Things I'm looking to be answered would be:

1. Should I try to keep the battery at "full charge" (as determined by the manufacturer) as much as possible? Or should I try to cycle the power from full to nearly empty before recharging?


2. Should charging be performed routinely (i.e. every night before going to bed), or should I let the battery charge indicator determine my charging patterns?


3. My phone in particular has a message pop-up saying that I should disconnect my phone from the charger to "conserve energy". Should I follow that advice and allow the phone to use the battery, or should I try to leave the phone plugged in until I want to leave for somewhere?


4. What's the reasoning behind your answer? And I suppose most importantly how much does it matter with Li-ion batteries (i.e. how much more life can I expect using a good practice vs. a bad one)?

Answer

LiIon is usually charged at constant current until a max allowed voltage is reached and then is held at that voltage while current tails off under "control" of chemistry of battery until Ichg = k% of Imax where k% is chosen according to longevity or max energy concerns.50% or 25% of Imax gives longer life. 10% or 5% tail gives max capacity but lower life.

Lowering Vpedesatl by 0.1V greatly assists battery life.

Discharging to higher cutoff voltage aids cycle life.

LiIon also has calendar life and starts self destructing from day one so a lightly used battery still dies.

Best cycle life is achieved by stopping charge when Vpedestal is reached and systen changes from CC to Cv. By monitoring voltage this point can be observed. You could even do a "dumb" system that simply watched delta Vbat and declared constant V when delta fell to zero. Only slightly more than a comparator and an RC delay in one input would achieve that.(While Vin is ramping a delayed vin is lower. When Vin pedestals the delayed Vin almost catches up. An offset voltage is needed to allow comparator towork).

LiIon cells mechanically flex the cell as metallic Lithium is "plated" in and out of the cell*. Cycle life is in large part due to battery beating itself to death mechanically.(This is why LiFePO4 lasts much longer and has lower capacity - the material is held in an Olivine matrix that maintains constant shape as active material is moved in/out BUT it takes up some space. )

*Note: Bill Dubuque has suggested that this sentence would be better replaced with " 'LiIon cells mechanically flex the cell as Lithium ions are intercalated".
The distinction is a finer one than may be apparent. However, it is true to say that if you cut open a LiIon cell you would not usually find metallic Lithium in it. Bill notes that this makes primary Lithium cells, which do contain metallic Lithium, a greater fire hazard than LiIon cells.
If you charge a LiIon cell with excessive voltage metallic Lithium will be 'plated out' and "vent with flames" mode usually occurs at about the same time.

Charge to CV level as often and as soon as possible.

If charging all the way their "disconnect message" is a sign of bad ethos. They are probably trying to minimise the risk of fire without telling you.

For longest storage life (as opposed to long life in regular use) storing at a lower voltage than Vmax is in order. Probably at about 3.6 V and only about 30% state of charge. The various Mars Rovers use LiIon batteries and have a design life of about 8000 cycles - but charge to about 3.6 - 3.7 V maximum.

8000 / 365(~=) ~= 22 Terran years.