# Relation between terminal voltage and charging speed in Lithium-Ion batteries?

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?

• 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. Oct 10, 2018 at 12:41
• The charging rate expressed in watts as from your example is: Voltage * Current. Oct 10, 2018 at 12:46
• @Bimpelrekkie Thank you for the link. This is the answer. Oct 10, 2018 at 13:06
• Converted that comment into an answer. Oct 10, 2018 at 13:08
• @Bimpelrekkie Great. I marked it as accepted. Oct 10, 2018 at 17:29 