Lithium Ion Cell Precharge

Question

The IC I am using is: STNS01. As I read, charging Lithium ion cells consists of three phases:

Precharge ("conditioning" on other articles) phase -> Approximately 10% or 20% of fast charge current (Constant current phase). A Precharge timer is used here apparently. When this timer times out,(30 mins for STNS01) charging is stopped.

Constant current phase -> Happens when Vbat is between 3v and 4.2v. Constant current is applied while voltage rises until ~4.2v. Apparently, this charges the cell to approximate 70% full.

Constant voltage phase -> Constant (4.2v) voltage is maintained while charging current dies down (phase terminated when charging current reaches some low threshold).

1. When this precharge timer times out, does this indicate a faulty lithium ion cell?
2. I understand that in the first place, lithium ion cells shouldn't be discharged to below cutoff to prevent damage (2.6-3.0v for most Li-ion). Is this correct?
3. If statement 2. is correct, does this mean that "deeply" discharged batteries (Li-ion cells discharged all the way to 0v) are not always successfully charged by STNS01 (or similar charging ICs)?
4. If this timer timed-out, is better to assume the battery is damaged or can we restart the entire charging cycle to try to recharge the battery again (ie., restart the 30 min timer)?

Answer 1

The charger will progress from pre-charge to CC phase when the voltage reaches a certain threshold.

If the cell fails to reach that threshold in 30 minutes, it is very likely that the cell is unusable. So you would not normally restart pre-charge again and again. That would be pretty much the same thing as having an infinite timer.

If you have a deeply discharged cell and it doesn't recover after 30 minutes, it might be reasonable to try it a second time, but that is different than implementing a charger that automatically restarts it over and over every 30 minutes.

One question is what is it safe to do when designing a product with lithium ion batteries that will be mass produced and sent out into the hands of consumers. Another question is, what is safe for me to do at home when I am keeping an eye on things. I think it is OK to experiment at home with going through pre-charge several times to see if the battery recovers. But it would not be reasonable to design a product that does that. So you have to decide which case applies to you.

Statement 2 is correct. Some products have a BMS that will permanently lock out charging if a battery cell goes below (pick a number: 2.6 V, for example). But some battery protection circuits and BMS's will allow pre-charge. I think it depends on the type of battery and type of use. For low-discharge rate products (things that run for hours on a charge) it might be OK to pre-charge. For power tools and other high-load applications, it might be best to permanently lock the battery out. The over-discharge may permanently reduce cell capacity and increase series resistance. So a high load application may no longer be safe.