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

Its charging level at the beginning was 20%. The terminal voltage was 3.68V. Now, I applied 10W of charging rate to the battery. The terminal voltage has then risen to 3.94V, so risen by 0.26V.

Let's increase the charging rate until the terminal voltage increases by 0.26V again. Now, we have 4.2V of terminal voltage.

Is the charging rate now 20W 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?

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  • \$\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\$ – Bimpelrekkie Oct 10 '18 at 12:41
  • \$\begingroup\$ The charging rate expressed in watts as from your example is: Voltage * Current. \$\endgroup\$ – Marko Buršič Oct 10 '18 at 12:46
  • \$\begingroup\$ @Bimpelrekkie Thank you for the link. This is the answer. \$\endgroup\$ – neverMind9 Oct 10 '18 at 13:06
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    \$\begingroup\$ Converted that comment into an answer. \$\endgroup\$ – Bimpelrekkie Oct 10 '18 at 13:08
  • \$\begingroup\$ @Bimpelrekkie Great. I marked it as accepted. \$\endgroup\$ – neverMind9 Oct 10 '18 at 17:29
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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

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