# Efficiency of charging a lithium ion battery with a lithium ion battery

Should I need to charge one lithium ion battery with another, what sort of loss would be typical?

If I had a 65wh battery, can I calculate how big a battery I need to charge it taking into account the loss?

• What interconnecting charger circuit would you use and how energy efficient is that circuit? – Andy aka Jun 6 '20 at 10:04
• I do not know, I am looking generally at so called "power banks" to charge an internal laptop battery. – seventeen Jun 6 '20 at 10:07
• This is rough - many variables: If you use a power-bank and then a charger you get efficiency of PB x efficiency of charger x efficiency of battery process. I'd guesstimate 90% for each, or less. So 0.9^# ~= 70%. || If you integrate the the first two in a battery to charger stage you can expect maybe 80%. | Note that LiIOn CURRENT charge efficincy is 99%++ but energy charge efficiency is less. look at Vcgg profile with time. Towards the end of CC mode Vghg rises at a higher rate for same current. Efficiency drops in this region. – Russell McMahon Jun 6 '20 at 10:28
• Vghg -> Vchg -> Vcharge – Russell McMahon Jun 6 '20 at 11:11
• Thanks, that helps alot. – seventeen Jun 6 '20 at 11:23

## 1 Answer

The ESR of each battery determines the fundamental "charge efficiency", e.g. no matter what you do on the outside, some energy will be lost to heat due to this,

Due to this, the lower the charge rate, the less total energy is lost to heating the ESR, this sets your maximum theoretical efficiency,

Next up you need some circuitry in between the 2 to both limit the charge rate, and convert between these 2 voltages with reasonable efficiency, the efficiency of this charge circuit will decrease you below that theoretical value.

You might notice a lot of these values need specifics to solve, e.g. charge rate, each batteries ESR curve, and the charge circuit's efficiency curve in relation to this. so an exact answer will take a bit of work to reach, a ballpark however, choose a charge rate, determine the ESR, and work off an efficiency between 80-90%, most switching charge devices should be able to handle this.