Batteries are heavy so dialing in the proper battery size is an easy way to drop a few ounces off your pack weight.

Step 1: List Device Battery Capacities

First, start off by listing your devices and their battery capacities (units here are milli-amp hours or mAH):

  • Pixel 1 phone: 2,770
  • inReach Mini satellite communicator: 1,250
  • Nitecore NU25 headlamp: 610
  • Fuji X-T20 camera: 1,260

That’s 5,890 mAH total.

Step 2: List Device Expected Lifetimes

Most devices provide some sort of estimate of lifetime based on typical usage. Let’s list those out:

  • Pixel 1: 19 days (standby)
  • inReach Mini: 20 days (extended tracking mode)
  • Nitecore NU25: 8 hours (medium setting)
  • Fuji X-T20: 350 exposures

Step 3: Calibrate Expected Lifetimes

Of course, these estimates are usually wildly inaccurate. Let’s go through the devices one-by-one and see if we can recalibrate:

Pixel 1

For the phone I’m not buying the specced standby time one bit – that would imply that just sitting around (not even on airplane or battery saver modes) that it’s losing a little over 5% charge per day. To check this I left my phone on airplane and battery saver mode overnight and found that it lost 5% of its charge in 9 hours or roughly 13% per day, yielding an expected lifetime of 7.5 days.

inReach Mini

I haven’t paid enough attention to the inReach’s battery consumption but I’m pretty sure 20 days is pushing it. Let’s cap it at five days.

Nitecore NU25

Night hiking aside, let’s say that I’ll use the headlamp at most an hour per day at the medium setting. That’ll bump up the expected lifetime to eight days.

Fuji X-T20

As for the camera, I ran some trials to figure out how much use I could get off a full charge:

  • Video: 90 to 105 minutes (it died somewhere in between)
  • Bracketed HDR shots (1/2000s, f1.4): 2,754 shots or 8,262 exposures
  • Long exposures (30s, f1.4): 187 exposures or 5,610 seconds or 93.5 minutes
Note that during the bracketed shot test the card filled up after 2,189 shots or 6,567 exposures.

I don’t really care about video so let’s just ignore that (or, pretend it’s the same as long exposures). Since the battery has 1,260 mAH of capacity we can estimate the consumption as follows:

  • Bracketed HDR shots: 0.458 mAH per shot, but let’s round up to 0.5 mAH
  • Long exposures: 0.225 mAH per second, but let’s round up to 0.25 mAH

If I typically take at most 100 bracketed shots per day and for astrophotography I’ll usually do 3-5 40 second foreground exposures and 3-5 13 second sky exposures. Combining those together gives us around 116 mAH per day as a worst-case scenario whereas just the bracketed shots gives us 50 mAH. Dividing 1,260 mAH by 116 mAH gives us around 11 days of use.

Calibration Results

So, our calibrated lifetimes based on real-world experience and usage are:

  • Pixel 1: 7.5 days
  • inReach Mini: 5 days
  • Nitecore NU25: 8 days
  • Fuji X-T20: 11 days

4. Calculate Daily Power Consumption

All we need to do here is take the battery capacities from step 1 and divide them by the corresponding lifetime from step 2:

  • Pixel 1: 2,770 mAh ÷ 7.5 days = 370 mAh per day
  • inReach Mini: 1,250 mAH ÷ 5 days = 250 mAh per day
  • Nitecore NU25: 610 mAH ÷ 8 days = 75 mAh per day
  • Fuji X-T20: 1,260 mAH ÷ 11 days = 115 mAh per day

5. Subtract Expected Lifetime From Trip Duration

Let’s say I’m going on a 10-day trip. If a device’s fully-charged battery is expected to last 10 days or longer then I don’t need to worry about charging it. On the other hand if it doesn’t last as long then I’ll need to extend the lifetime by whatever the difference is. Let’s list out the differences now:

  • Pixel 1: 10 day trip – 7.5 day expected lifetime = 2.5 day gap
  • inReach Mini: 10 day trip – 5 day expected lifetime = 5 day gap
  • Nitecore NU25: 10 day trip – 8 day expected lifetime = 2 day gap
  • Fuji X-T20: 11 day trip – 10 day expected lifetime = 0 day gap

6. Calculate Power Deficit

By multiplying the lifetime gaps by the daily power consumption for each device we can determine how much additional power we’ll need to bring:

  • Pixel 1: 2.5 day gap x 370 mAh per day = 925 mAh
  • inReach Mini: 5 day gap x 250 mAh per day = 1,250 mAh
  • Nitecore NU25: 2 day gap x 75 mAh per day = 150 mAh
  • Fuji X-T20: No gap = 0 mAh

Total power deficit: 925 + 1,250 + 150 + 0 = 2,325 mAh

Conclusion

A tiny 3,350 mAH battery is all the capacity I need for a 10-day trip. For anything more than a few days longer (or for a trip where my power consumption was above normal) I’d have to bring something bigger. To make this example more concrete, the weight difference between the RAVPower 3,350 mAH and 6700 mAH power banks is 53 grams or just under 2 ounces. I’ll leave it up to the reader (if any) to decide if the weight savings are worth all the fuss.

Further Reading

History

  • 2019-12-19: Original version
  • 2019-12-20: I put my Pixel in airplane and battery saver mode (the same configuration I use when backpacking) overnight and it lost 5% of its charge in 9 hours or a shade over 13% per day. The article has been updated to reflect this more-realistic data.
  • 2020-03-20: I got a little crazy and actually clicked the shutter button on my camera a couple thousand times over the course of a day in order to get a realistic estimate of how much use I could get out of a charge. In the course of doing this I noticed I made a logic error in the third method (extending all device lifetimes by that of the shortest device rather than calculating the shortcoming for each device individually) so I’ve updated it. As a nice side-effect it drastically reduced the amount of power required.
  • 2020-03-28: Ditched the three-method format in favor of a step-by-step walkthrough of the best method for capacity planning.