What is the optimal battery charging speed?

Question

I have 3.7V 4000mAh Li-ion battery.
I also have 0-30V/6A power supply which can limit output current. Provided that I charge the battery with voltage limited to 4.2V, I can change the maximum current output of the power supply.

Let's say that have set these parameters on the PSU
-voltage limit = 4.2V
-current limit = 500mA

On the powerIf current is higher than 500mA, then the voltage will drop to a value where the current is 500mA.
Assuming the battery is @5%, I can set the current limit to 4A and charge the battery less than 1 hour.

My questions are >

What is the optimal charging speed at which the battery will be safely charged?
What is the fastest charging speed (safe charge)?
What is the fastest charging speed (unsafe charge)?
unsafe= can damage battery life or charging cycles
Temperature may and may not be considered in the answer.

Answer 1

Li-Ion batteries are designed to be charged via a battery management IC, such as this one. They charge the battery using the following charging profile:

which is constant current (CC) followed by constant voltage (CV).

That said, it is possible to charge a Li-Ion battery with a good lab bench supply, which apparently you have. It is important that the power supply have a meter that can monitor the current being drawn.

The charging voltage should be set to 4.2V or slightly below. The current limit should be set to C/2 or C/3 for manual charging (2A or 1.33A in your case).

Charge the battery until the measured current draw drops down to 0.03C (120 mA in your case). If you let your power supply current go down to 0 mA, this "trickle charging" phase will oxidize the electrolyte inside the cell which is not good. Li-Ion batteries don't like to be trickle charged.

Periodically check the temperature of the battery with your finger to make sure it is not getting hot.

Answer 2

Lithium Ion batteries have an upper charge limit of 1C. This means once its capacity, or in your case 4000mA.

This does not mean that is the best, though. It is good to keep a margin. But the margin doesn't have to be large, as healthy cells of the LiIon type do not have a chemical recombination and produce nearly no heat while charging at an appropriate current.

Usually I take 0.5C, or in your case 2000mA. It'll get the job done in about 2 hours and your cell suffer nearly as little as when you charge it with 400mA or less.

Of course, if you want to squeeze ten more charge/discharge cycles out of it, you can limit your charging and discharging to 1/20th C, i.e. 200mA and your battery will love you for a few days more.

Other than that there's some guidelines to the best charging curves (which are a bit more complex than what a PSU does limited to current and peak voltage), but they will again only add a few percent at best to the lifetime. The best advise is, charge them to 4.2V or even 4.15V instead of 4.25V or more and don't keep them connected to the power too long after they are full and to never discharge them below 10%, or more conservatively 20%. Or put in a voltage: Shut off at about 2.85V instead of 2.55V

Answer 3

It depends on battery, the exact rules change all the time. Remember, that once you had to charge your cell phone for 24 hours first time? Now you don't have to.

Still, i would say, based on my experience with various cell phones qnd chargers, that i wouldn't exceed 2A.