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battery charging--can't charge the li-ion battery fully

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bean123

Newbie level 5
charging lithium ion battery

Hi,
My battery charging circult has only constant Current, no constant Voltage method,
and the constant current is 1A.
When the battery voltage arrive the cutting voltage 4.2V, the circult cut off. but the battery voltage descent from 4.2V to 4.11V within about 4 minute.
So the battery can't arrive the voltage (4.2V) I want to get.
And when the constant Current is smaller ,the battry voltage is higher. For instance, when the constant current is 400mA, the V get about 4.16V.
How can I make the Voltage arrive 4.2V?
Someone tell me that the li-ion battery has a long chemical response time. and this is the reason.
Is that right?
And please give me the sollution.
I need to design the canstant voltage charging???
Thanks

battery cant charge

You have to follow the basic charging algorithm, that is to charge at constant current (0.2 C to 0.7 C depending on manufacturer) until the battery reaches 4.2 Vpc (volts per cell), and hold the voltage at 4.2 volts until the charge current has dropped to 10% of the initial charge rate ..
The termination condition is the drop in charge current to 10% ..

In other word, your charger has to be constant current, then constant voltage with current sense and for safety reason you should also include a timer ..

Rgds,
IanP

is it bad to fully charge li-ion

your mean is that the constant voltage charging is necessary for li-ion battery charging?
I also trend to agree with the answer.

charge a lion battery

HI

Simply use a charger IC - since charging LI-ION battary might be denger - battery can explode

Also don't consume current from battery while charging it - use a switch to disconnect the battery from load when charging

All the best

Bobi

After the voltage comes to 4.2V after constant current charging, it takes almost equal time to charge with the constant voltage as it took for constant current.

4.2V doesn't necessarily means "fully charged". If you do constant current at 1C, you will get 4.2V but nearly 70% of charge only. When you change to constant voltage method, the voltage drop in the internal ESR of the battery lowers, and the actual battery voltage gets closer to 4.2. As it has been said before, is a good practice to cut the energy sourcing when is below 0.035-0.07C to achieve a nearly 99% state of charge.

However you might not want to get that closer to the 100% because Li-Ion batteries are highly sensitive to overvoltage problems, and don't allow trickle charge. Adittionally, that voltage drop is natural as there are differences between close and open circuit voltages. Moreover, Rincón-Mora battery model (which claims a 0.2% accuracy with real batteries) has an open circuit voltage for full-charge of 4.117V. I have developed the verilog-AMS version of the model and it works properly and the real behavior of the circuit is practically the same.

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