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# a problem of li-ion battery charging

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#### bean123

##### Newbie level 5
charging lithium ion battery

Hi,
My battery charging circuit 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 circuit cut off， but the battery voltage descent from 4.2V to 4.11V within about 4 minute.
So the battery can't reach the voltage (4.2V) I want to get.
And when the constant Current is smaller, the battery voltage is higher. For instance, when the constant current is 400mA, the V gets 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 solution.
I need to design the constant voltage charging???
Thanks

Haven't you looked at the graph of the voltage of a charging lithium cell? The Battery University on the web has one.
It reaches 4.2V in maybe one hour but needs to remain charging at 4.2V for two more hours.

It is maybe 65% - 70% charged when its voltage reaches 4.2V. It is fully charged when its charging current reduces to 3% of its rated current.

thanks!

Hello!

Can you explain me how you would charge a battery (or capacitor) at
current voltage?
You cannot. At the beginning of the charge it will be 2.8 (or was it 2.9??)
and at the end you have to stop at 4.2 V. You cannot have instantly
4.2V or whatever other voltage.

Dora.

bean123 said:
Hi,
My battery charging circuit has only constant Current, no constant Voltage method,
[...]
I need to design the constant voltage charging???
Thanks

Hi!
Li-ion battery charger almost has trickle charging/constant-current charging/constant-voltage charging. And I agree with the words that "You cannot have instantly 4.2V or whatever other voltage" you mentioned.
My question is that my charger without constant-voltage can't reach 4.2V which I want to get. When the charger cuts off, the voltage descends quickly from 4.2V to maybe 4.14V or 4.13V in about ten or twenty seconds, and the voltage finally reaches maybe 4.11V in about five minutes.
Someone told me that the reason is the li-ion battery has a long chemical response time. So I can't cut off the current once the voltage reaches 4.12V. And maybe this is why needs the constant-voltage charging. At first, I want to ensure the answer, and wander other solution except adding constant-voltage charging.
And Audioguru also mentions these.

doraemon said:
Hello!

Can you explain me how you would charge a battery (or capacitor) at
current voltage?
You cannot. At the beginning of the charge it will be 2.8 (or was it 2.9??)
and at the end you have to stop at 4.2 V. You cannot have instantly
4.2V or whatever other voltage.

Dora.

bean123 said:
Hi,
My battery charging circuit has only constant Current, no constant Voltage method,
[...]
I need to design the constant voltage charging???
Thanks

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Not open for further replies.