Is fast charging a Lithium-Ion battery from 0% to 30% better or worse for the battery health than fast charging at 30% to 70%?

Question

According to this answer, fast charging is only recommended for the battery from 30% to 70% instead of 0% to 70%, while I thought that the emptier the battery is, the higher of a C rate it can handle.

So, is what he mentions in his answer correct?

Answer 1

This BU article indicates that recommended method is to fast charge from 0% to (in the case illustrated by the 2nd graph) about 60%. This is a typical charging cycle.

However, Li chemistries can be overdischarged by continuing to draw current after reaching the "dead battery" threshold. Many have protective circuitry that prevents charging if overdischarged (since self-discharge is one common source of overdischarge). Some of these cells may recover if trickle charged to 2.5-3V, after which they can accept full charging current.

Answer 2

Is fast charging a Lithium-Ion battery from 0% to 30% better

No, industry experience, as it is concentrated in charging products from, say, Texas Instruments, employs a "pre-charge stage" when the initial state of battery charge is low. Here is a typical charging cycle employed in BQ25896 charger:

The pre-charge current is usually set to about 10% of the fast charging current. So the fast charging from 0% is NOT OK.

CLARIFICATION: The above answer assumes that the battery is discharged to zero voltage. This battery state is designated as "dead" battery. In the above illustrative representation of Li-Ion charging phases, the charger starts with "trickle charging". After the voltage of the cell goes above 2 V (if), the battery is called "weak battery", and the charger proceeds with "pre-charge" current until the voltage rises above 2.8 V. Only then the charger proceeds with "fast Charge". The matter of confusion is that the above chart is for illustration only, and the time scale is highy distorted. Below is the charging chart for a real phone battery (2600mAh 3.8v Samsung Battery EB-BG530CBU) in a good SOH (State of Health) using BQ25896 charger:

The battery was discharged slightly below 2.5 V. The charger was configured for pre-charge level 3.0V and fast charge from 3.5V. The trickle stage would last about 10 seconds, and the pre-charge stage appears to be fairly short too, about 20-25 seconds, after which the charger enters the Fast Charge mode.

Note that the fast charge state lasts about 45 minutes before the cell begins its current drop. The charger is in CC mode, and the voltage is slowly creeping up from about 4.1 to the "regulation voltage" of 4.35 V. At this point the battery is charged to about 60% of its capacity. The rest of capacity gets acquired during CV stage, when the battery drops its consumption. Then the current reaches "I(termination)" threshold, the charger disconnects the voltage supply to prevent battery "overcharging".

The greenish line shows the accumulated SOC - state of charge. It is apparent that the charge amount during the trickle and pre-charge states is negligible and less than 1%.

The resolution of the discrepancy between the theory of Li-Ion charging and user experience is that all normal smartphones with good battery don't discharge the actual cell to 0% capacity. They never discharge the cell below 2.8 V. So in practice all smartphones would start the fast charge almost instantly, even before any application can access its fuel gauge and report percentage of battery charge. From the application standpoint the 2.8V discharge point is designated as 0% starting point, and that's why user experience is that the fast charge occurs from "0" and is "good".