How to size a power bank?

I have a fenix tactical flashlight that uses a 5000mAh rechargeable battery.
I use the light as a bike light and will need to charge it during the day of a 24hr ride.
Trying to figure out how to size an Anker Power Bank to charge this fenix flashlight in a reasonable amount of time…Note: I will have two fenix rechargeable batteries…reasonable time is 6hrs for one battery.

Thanks in advance as mAh is new to me…

For the described problem I’d carry charged 18650 or 21700 charged batteries as the limiting factor is recharge times. You’ll end up carrying more weight using a battery to recharge a battery than simply carrying a replaceable.

2/3rds rules

When you use a battery to recharge a battery you suffer losses.

The process of converting stored chemical energy into electricity is fairly efficient, about 90% is good rule of thumb.

The process of converting electricity into stored chemical energy is much less efficient, about 70% is a good rule of thumb.

These combine to mean to use a battery to recharge a battery is about 60% efficient.


A Powercore is not just a battery, it is the charging electronics (two sets, to recharge itself and discharge itself) and the plastic protective case.


Anything can fail, the more complex, the more chance of failure. A 18650 cell has some risk, but put them in parallel and add electronics, the chance of fail is much higher. Not saying Anker is unreliable, simply saying if you have a more complex solution it will be more likely to fail.

Analogy: which is more reliable and lighter?

Use these:

image and image

or instead carry this


The simplest solution is the more reliable.

I recommend ditch your ideas of the flashlight, you are focused too much on a specific flashlight. Change the flashlight to one which takes a pre-charged 18650 cell and carry the light and pre-charged cells. It will be lighter, cheaper, and more reliable.

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You are correct…I have been told that…the problem is I will use the power bank again in life…but probably not a $28 extra battery…I’ll probably go with a 3rd battery…but if I were to charge a 5000mAH battery what size power bank would I be looking at to get it done in 6-8hrs or so? Just curious…

The short answer is use the 2/3rds rule. So to get 5000mAh into something you need 6700mAh out.

A 10000mAh Powercore will recharge 5000mAh roughly 1.3 times.

10000/5000/3*2 = 4/3 = 1.33

The greatest part of the power loss is the chemical process all recharging does. The only thing under Anker engineering control is the electronics and Anker usually is around 95% efficient.

Due to energy loss in recharging, it’s best to carry pre-charged cells which you swap. My Anker flashlight is the original LC40 which uses 18650 cells and I carry as many cells the trip needs.

It’s also more efficient to keep a phone charged and when you use it to connect power as the electrons go cells to CPU, not cells to cells to CPU. So for evening/overnight I keep my Powercore connected so the phone is always 100%.

tks Prof…any idea on “charge time”?

Recharge time of the Powercore is published by Anker, recharge time of your devices are stated by them, if you don’t know then do a test and measure. As a rough rule smaller cells ingest energy slower.

Half the recharge time is spent getting from 70% to 100% so if you halved the stated recharge time you’re 70% full, as the trickle charge to not overheat the cells slowly fills up.