# how to compute battery charging power

I have a very basic question about battery charging power. In a very simplified scenario, assume a battery with voltage 3.7 is connected to another rechargeable battery with voltage 3.2 which results in charge flow from the first battery to the second one with current 10mA. The question is, with how much power energy flows from the first battery to the second battery? I know Power=Current*Voltage but voltage of which battery?

• They will both have the same voltage when you connect them in parallel. Make sure you know what you're doing. This could be dangerous as very high currents will flow. – Transistor Dec 22 '18 at 15:27
• Battery charge/discharge becomes pretty complicated if you need accuracy. If you have .01A current flow resulting from your 0.5v difference, it means you connected the batteries with a 50 ohm resistance, so make sure you take the power wasted by the resistor into account. Lead acid and ni-cad batteries can be tolerant to direct charge balancing, but most types of batteries (particularly Lithium-Ion variants) should never be charged by anything other than a circuit designed for the purpose. – K H Dec 23 '18 at 1:32