Electronic – How to test a Lithium-Ion charging IC

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.

I am not aware of how often this happens, but it shouldn't.

One possible cause is that the open circuit voltage of the battery is enough to hold the charger in the "fully charged - wait from battery voltage droop" state. This should not be allowed to occur and is easily overcome in a good design - see below.

A more likely reason is that the charger IC or charger is poorly conceived or designed and that a startup transient condition occurs which discombobulates it. An alternative is that this is a purposeful design choice aimed at increasing notional safety and perhaps avoiding liability claims.

Of interest in researching what is wrong would be knowing the voltage Vbat just before power on, and the effect of loading the battery with a bleed resistor sos that it trickles down. A resistor of substantial draw could be added to 'hurry things up'. IF this works a larger resistor could be added long term.

Longer:

The situation that you describe is easy to detect and manage and any charger circuit and IC worth its salt should handle it OK.
Unlike some battery chemistries whose charge characteristics need to be determined by observing dynamic behaviour, a Lithium Ion cell's charging requirement can be determined by it's behaviour either statically or after a very small test time period.

While super arcane chargers (which may or may not produce super arcane results) will build on the following, a LiIon charger is reasonably fully defined by the steps below.
LiIon and LiPo have !~~~ the following values. Will be fine tuned in practice EXCEPT Venough > 4.2V is VERY unwise. Vfatal ~ 2V
Vtoolow ~= 2.5V
Venough ~+ 4.2V. (4.1V for longer life, 4.3V for vent with flame.)
K - 0.5 to 0.05. Smaller = more capacity and shorter life. K=0.25 probably about right.
Vtopup ~= Venough - 0.1V

• i Vbat < Vfatal - walk away

• ii Vbat < Vtoo low. Charge at low rate )(say C/100 until Vbat rises to >= Vtoolow)

• iii Vtoolow < Vbat < Venough. Charge at C/1 rate.

• iv Vbat = Venough.
  While not letting Vbat rise above Venough
  charge while Ichg >= (C1 rate) x K
  where K is a predecided fraction of C/1. eg 50% or 25% or 10% or whatever.

• v When Vbat = Venough and Ichg drops to 1/K x full charge rate, terminate charging COMPLETELY
  Remove Vchg from battery.
  DO NOT float battery at Vchg even if Ichg is very low.

• vi Monitor Vbat and when Vbat falls to say Vtopup restart step v, charging at up to C/1 if needed while holding at <= Venough until Ichg again falls to C/1 x K

IF the battery is in state Vbat = Venough when power is returned the charger MAY start in step vi and hand there until Vbat drops. For this to happen the battery would need to be at about 66%+ capacity.