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  1. Why are constant current mode and constant voltage mode needed when charging Li-ion batteries?

  2. Why do we apply saturation charge even after the cell voltage reaches 4.2V?

  3. What exactly is 'I-sense' in Li-ion Battery charging?

It would be better if anyone can explain the whole charging process in detail.

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    $\begingroup$ You will probably get better answers if you ask one specific question instead of three questions plus a request for a complete explanation of the charging process. Also, I would recommend reading this note about titles. $\endgroup$ – DanielSank Jan 9 '15 at 6:08
  • $\begingroup$ I think this might be better on the Electrical Engineering SE. $\endgroup$ – John Rennie Jan 9 '15 at 8:03
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What happens during the charging process is this:

During charging, an external electrical power source (the charging circuit) applies an over-voltage (a higher voltage than the battery produces, of the same polarity), forcing a charging current to flow within the battery from the positive to the negative electrode, i.e. in the reverse direction of a discharge current under normal conditions.

What that means is that when you charge a lithium-ion battery the charger forces charge to flow from the positive to the negative electrode, reversing the process of discharge. Once that happens, your battery is charged. A diagram below shows the discharge process, which is the reverse of charging:

discharge process

Now for your more specific questions:

  1. The constant current is to keep the battery's current at the proper level. As the battery's voltage is increasing from charging its current would decrease without constant current.
  2. Saturation charge is basically adjusts the battery's current to the level the battery is rated for.
  3. I-sense is current sense.

Hope this helps!

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