Like most long-distance cyclists I need an external battery pack to power my navigation device for the duration of some of my rides. I recently lost mine and bought a new one on Amazon. Like an idiot I just went for the one with the largest capacity. When it arrived it was the size and weight of a brick (21.5oz) - much too heavy to lug around with me all day.
So I decided to actually think about it and determine how much capacity I really need. These are the numbers that are relevant.
Ci = Capacity of my navigation device's internal battery
Di = Duration of my navigation device using its internal battery
Dt = Total duration I need
My navigation device is a Galaxy Note 4 which, according to the specs, has a capacity of 3220 mAh
The last time I tested it the device ran the rwgps app for 6 hours from a full charge down to 10%
The longest I can conceive of riding between charge opportunities is 27 hours for the hypothetical 400 km from hell.
Therefore I need an external capacity of...
Ce = (Dt - Di) * Ci / Di = (27 - 6) * 3220 / 6 = 11270 mAh
So that brick, with it's capacity of 30,000 mAh, was serious overkill. It would power my Note 4 for another 55 hours for a total of 61 hours approximately between charges.
Anyone want to buy a brick?
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