I tested charging a 5200 mAh (20 Wh) battery at 10W.

Its State of Charge level at the beginning was 20%. The terminal voltage was 3.68 V. Now, I applied 10 W of charging power to the battery. After some time, the terminal voltage rises to 3.94 V, which means that it rose by 0.26 V.

Let's increase the charging power until the terminal voltage increases by 0.26 V again, to 4.2 V of terminal voltage.

Is the charging power now 20 Wh or higher?

  • 00 Watts at 3.68 Volts
  • 10 Watts at 3.94 Volts
  • ?? Watts at 4.20 Volts ?

Is there a formula for that?

  • 1
    \$\begingroup\$ Now, I applied 10W of charging rate to the battery. Watch your units! Do you mean charging the battery at a current such that voltage times current equals 10 W? If so, then for how long? Watt is energy per second so we need to know the seconds. \$\endgroup\$ Oct 10, 2018 at 12:41
  • \$\begingroup\$ The charging rate expressed in watts as from your example is: Voltage * Current. \$\endgroup\$ Oct 10, 2018 at 12:46
  • \$\begingroup\$ @Bimpelrekkie Thank you for the link. This is the answer. \$\endgroup\$
    – neverMind9
    Oct 10, 2018 at 13:06
  • 1
    \$\begingroup\$ Converted that comment into an answer. \$\endgroup\$ Oct 10, 2018 at 13:08
  • \$\begingroup\$ @Bimpelrekkie Great. I marked it as accepted. \$\endgroup\$
    – neverMind9
    Oct 10, 2018 at 17:29

2 Answers 2


Have you read: https://batteryuniversity.com/learn/article/charging_lithium_ion_batteries ?

Look at a typical Lithium based cell charging curve:

enter image description here

Note how the charge level to voltage relation is very non-linear. When the battery is at a low charge the voltage increases more quickly with charge level compared to an almost full battery. So to get from 3.94 V to 4.20 V more energy is needed than going from 3.68 V to 3.94 V. So charging with another 10 W during one hour (1 Whr) will result in a lower voltage than 4.20 V


Is the charging power now 20 Wh or higher?

It can be anything. You can continue charging at 0.1 W, 1 W, 10 W, or whatever power you wish. It just affects how long it takes to charge from 3.94 V to 4.20 V.

I think you are confusing "power" (energy per time) and "energy". You can achieve a given amount of energy stored with any amount of power, as long as you do it long enough.


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