Li-ion battery charging termination current

Question

I have almost no experience with Li-Ion charging.

Documentation to battery is very poor, but there is an info about charging profile:

• Stage 1: Constant Current 0.82 Ampere (0.2C) until Voltage reach 4.2V
• Stage 2: Constant Voltage (4.2V) with decreasing current 0.82A -> 0.12A

I want to design charger for that battery. I have found several charging ICs (for instance LT3650) that fit my design pretty well, but this IC terminate charging process at 1/10 of programmed charge current. In my case: 0.82A/10 = 0.082A what is about 1.5 times lower than 0.120A in battery specification.

I confused a little with min charge current in spec, I supposed the lower current you can provide the more "fully" you charge battery.

• Is it safe to continue charge Li-ion after 0.12A threshold till 0.082A (and how it could affect battery), or
• is it better to use IC's with user programmed termination current (like LTC4068-4.2) to stop charging process exactly at 0.12A?

Answer 1

It would be helpful to have a data sheet of this battery, however bad it is.

0.2C charging appears excessively conservative. This would mean over 5 hours for a full charge (most likely 6-7 hours). I’m not familiar with every possible Lithium-Ion chemistry, but I hadn’t ever heard of a chemistry requiring only 0.2 C charging; note that it might be beneficial in minimizing battery wear, especially if such a long charging time is of no concern to you. Thus, it would also be helpful if you pointed out what you’re optimizing for: battery wear, charging time, running time to discharge the battery, etc.

Given that 0.2C is 0.82 A, then your battery is supposedly a 4.1 Ah battery. Typical guidelines indicate charge cutoff at 0.03C to 0.05C. Looks like the 0.03C figure was chosen to approximately reach the 0.12 A recommended by the manufacturer.

Assuming this is a run-of-the-mill chemistry like LiCoO2, and your charger cuts off at 10% of the programmed charge current, I would think you could go for a bit over 0.2C charge current, so that the cutoff falls in this 0.03C to 0.05C. Given that 0.5C is generally safe for chemistries I’m familiar with, I would personally go with somewhere from 0.3C to 0.5C so that charge is cut off at 0.03C to 0.05C. Thus, 1.2 A to 2 A would probably represent a good choice; choose from the lower end if you want to be conservative and minimize battery wear, and from the higher end if you can’t afford slow charging times.

Again, note that I’m assuming this is a run-of-the-mill chemistry and may not necessarily apply to your battery’s specific chemistry. Hence, this is why a datasheet would be most helpful to give a better answer.

Regarding a lower current cutoff “more fully charging a battery”: yes, you may get a couple % extra by stopping the charge later. However, this reflects negatively on battery wear. Batteries wear more the longer they stay on a high charge level. If it is easy and cheap to replace the battery, it may be a valid tradeoff. On the other hand, on my laptops, tablets, cell phones, I personally avoid charging them to 100% to try to extract some extra lifetime from their batteries.

Answer 2

Is it safe to continue charge Li-ion after 0.12A threshold till 0.082A

Probably. The amount of extra charge put in between 0.12 A and 0.082 A should be very small.

If you are worried about overcharging then lower the voltage to eg. 4.1 V instead of 4.2 V. This will slightly reduce capacity, but puts less stress on the battery than fully charging and letting it settle to a higher voltage.

is it better to use IC's with user programmed termination current (like LTC4068-4.2) to stop charging process exactly at 0.12A?

The exact termination current isn't critical, but voltage is. Usually the goal is to charge as quickly as possible, which requires a slightly higher voltage to overcome internal resistance. Then charge termination is required to avoid over-charging. But if you don't mind waiting you can charge to a slightly lower voltage and the current will automatically drop to zero at a safe voltage (< 4.2 V).

What reduces battery life the most is leaving it fully charged for long periods (especially at high temperature) and discharging to less than 20% capacity remaining (below 7.4 V 'open circuit'). At high voltage the cathode oxidizes faster, increasing resistance and reducing available power. Fully cycling the battery increases mechanical degradation of the active material on the plates.

How to Prolong Lithium-based Batteries