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In order to charge my electrical equipment while backpacking (GPS, camera for timelapses, phone, etc) I bring along a large battery. To calculate the size of the battery you can figure out how many recharges each device will need and how big their batteries are.

However, charging one battery with another is not 100% efficient, a 20,000 mAh battery won't fully charge two 10,000 mAh batteries.

How much margin for the charging inefficiency should I add?

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The overall efficiency from a power bank to a smartphone battery is about 2/3, according to this source:

https://www.powerbankguide.com/powerbank-capacity-explained/

The amount of energy a battery can provide depends on its voltage (nominal 5V for a USB power bank), the size of the supplied current in Amperes and how long the current can be kept flowing. Since the voltage is fixed (5V), current and time are sufficient to express the capacity of a power bank. The units of current and time are then used together: 10,000 mAh means that a current of 10,000 mA (milli Ampere, so actually 10 A) could be provided for 1 h, or 5 A for 2 h, or, since common charging currents for mobile phones are around 2 A, this current could be provided for 5 h.

To get 10,000 mAh on the smartphone side, about 15,000 mAh will be drawn from the power bank. In other words, the required margin is about 50% of the desired capacity.

  • Calculate in energy (Ws or Wh) if you have different battery voltages. An important question is if the batteries even have the advertised capacity. – Michael Jun 20 at 6:29

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