kept since 2019, Bishop CA

Gizmo Trail

Headlamps, satellite messengers, power banks, and the arithmetic of keeping them charged between resupply towns.

power budgets

11 min

Eight days on one 10,000 mAh bank: a power budget from Kennedy Meadows to Kearsarge Pass

Between Kennedy Meadows South and Onion Valley there is nowhere to plug in for about 94 miles. We logged every percentage point across it, and the power bank died two days before the trailhead.

By Tess Albrecht

the battery line, KMS to Onion Valley

100% 75% 50% 25% 0% day mile d0 d1 d2 d3 d4 d5 d6 d7 d8 702 716 731 745 759 766 774 787 Onion Valley cottonwood pass jct, 31°F — bank in quilt guitar lake: left bank in canister, read 2 LEDs low at dawn whitney summit, 1 bar LTE, out of airplane mode 40 min tyndall creek: bank empty forester pass 13,153 ft onion valley, phone 4% phone (Pixel 8a) bank (10,000 mAh) 100% 75% 50% 25% 0% day mile d1 d2 d3 d4 d5 d6 d7 d8 702 716 731 745 759 766 774 787 Onion Valley phone (Pixel 8a) bank (10,000 mAh)
  • d3 — cottonwood pass jct, 31°F — bank in quilt
  • d5 — guitar lake: left bank in canister, read 2 LEDs low at dawn
  • d6 — whitney summit, 1 bar LTE, out of airplane mode 40 min
  • d6 — tyndall creek: bank empty
  • d7 — forester pass 13,153 ft
  • d8 — onion valley, phone 4%

31°F, bank in quilt

phone 4% at trailhead

For a northbound hiker on the Pacific Crest Trail, the stretch from Kennedy Meadows South (mile 702) to the Kearsarge Pass junction (mile 789) is the first long carry in the Sierra and the first real test of an electronics kit. No road crosses it. Add the 7½ miles over Kearsarge Pass to the Onion Valley trailhead, and the side trip up Mount Whitney that most people take, and you have six to nine days with no outlet. I walked it in eight days in late August, and I wrote down the charge on every device each morning and each evening.

The kit was ordinary on purpose: a Pixel 8a (4,492 mAh, about 17 Wh) running FarOut and the camera; a Garmin inReach Mini 2 tracking at 10-minute intervals; a Nitecore NU25 UL headlamp with its built-in 650 mAh cell; and a Nitecore NB10000 Gen 2 power bank, rated at 36 Wh and weighing 150 g. The phone stayed in airplane mode with Bluetooth and Wi-Fi scanning off, and brightness was set manually to about a third. I used one USB-C to USB-C cable, 30 cm long, for everything.

The phone averaged 29% a day, but the average hides the shape. Ordinary walking days with a dozen map checks and forty-odd photos cost 26–28%. Whitney summit day cost 41%: I took the phone out of airplane mode for forty minutes on top to catch the single bar of LTE and send photos home, and the radio searching for a tower drained it faster than anything else on the trip. Forester Pass day cost 33%, nearly all of it camera. If you want one rule from this log, it is that the radio costs more than the screen.

Cold mattered less than I expected, except where I got lazy. At the Cottonwood Pass junction the overnight low was 31°F. I slept with the bank and phone in the footbox of my quilt and charged the phone there; it was fine. At Guitar Lake I left the bank in the bear canister, and at dawn its four-LED gauge showed two bars where it had shown four the night before. Once it was back in a pocket for an hour it read four again. Lithium-ion cells sag in voltage when cold, so the gauge under-reads. But a bank charging a phone at 30°F also delivers less, because more is lost as heat in its own conversion.

Here is the arithmetic. Over three overnight top-ups the bank put 137% of a phone battery back in, about 24 Wh. It also recharged the headlamp once (about 2.4 Wh) and topped the inReach from 35% to full (roughly 1 Wh). That totals about 27½ Wh delivered from a bank rated at 36 Wh. That 75% figure is normal and is explained in our piece on what the number on the label means. The bank’s last LED went out at Tyndall Creek on the night of day six, with the phone at 63%. I reached Onion Valley on day eight with the phone at 4%. I had turned off the camera entirely for the descent and kept the inReach for the hitch into Independence.

So a 10,000 mAh bank is roughly a day short for this stretch at Whitney-day habits, and about right if you skip the summit photos or keep the phone in airplane mode on top. Three changes would close the gap. Carry a 20,000 mAh bank instead, for about 160 g more and comfortable margin. Keep the 10,000 and leave the phone in airplane mode all the way up Whitney, which saves about 15%. Or charge the headlamp to full at Kennedy Meadows and run it on its 60-lumen setting, which would have saved the entire headlamp recharge. Whichever you choose, recharge the bank properly in Independence: at its 18 W input it needs a little under three hours on a wall charger, so ask for a room near the outlet.

Further field notes

earlier field tests

  • power budgets

    7 min

    Why a 20,000 mAh power bank delivers about 12,500 mAh to your phone

    Cell voltage, the boost to 5 volts, and the cable all take a share, and the label is not lying, just measuring something else.

    By Kenji Maruyama

    The mAh figure on a power bank describes its internal lithium-ion cells at their nominal 3.6–3.7 volts. A 20,000 mAh bank at 3.7 V holds about 74 Wh, comfortably under the 100 Wh airline limit, which is why that figure is so common. Your phone does not charge at 3.7 V. The bank has to boost its output to 5 V for USB, or to 9 V when it negotiates USB Power Delivery, and that conversion runs at roughly 85–90% efficiency in a decent bank.

    Do the sum: 74 Wh × 0.85 ≈ 63 Wh, and 63 Wh at 5 V is about 12,600 mAh. That is the honest number to plan with. Then the phone loses a little more converting that 5 V back down to charge its own cell. In our bench tests a further 5–10% disappears as warmth in the phone and the cable, more with a long thin cable and more in the cold.

    The practical rule we use: count on 60–65% of the label in phone-battery terms in summer, and 55% if you will be charging below freezing. A Pixel 8a or an iPhone 15 gets about two and a half full charges from a 20,000 mAh bank, not the four and a half that dividing the label suggests.

  • comms & satellite

    8 min

    How long an inReach Mini 2 really tracks at 10-minute intervals in a cold canyon

    Garmin quotes up to 14 days; nine nights in the Kern River canyon at 28°F gave us eight and a half.

    By Priya Venkataraman

    We started from a full charge at the Kern River trailhead and left every setting at default except the tracking interval, which we set to 10 minutes, the common choice among thru-hikers who want family to follow along. We sent one check-in preset each evening, a handful of two-way messages, and pulled a forecast on the third and seventh days.

    The unit hit 10% on the morning of the ninth day. Two things accounted for most of the gap from the 14-day figure. Canyon walls blocked part of the sky, so the device spent longer acquiring satellites for each track point. And nights at 28°F slowed the cell, so the battery percentage dropped in steps overnight and crept back up slightly once the sun reached it.

    Our advice is to budget ten days for open country and seven to eight for deep canyons or forest. If your carry is longer than that, a single top-up from your power bank costs about 1 Wh, a very small share of even a 10,000 mAh bank.

  • town charging

    9 min

    The outlet at the Kennedy Meadows General Store, and nine other places to charge between mile 700 and 900

    Where the sockets are, how many people are usually queued at them, and which stops have a USB-C charger you can borrow.

    By Lou Delacroix

  • headlamps

    6 min

    Red mode at camp: what 630 nm light does and doesn’t do for your night vision

    Why a red beam lets your eyes keep most of their dark adaptation, and why it makes red map lines disappear.

    By Harriet Mbeki

  • repairs in the field

    5 min

    Replacing a headlamp’s battery-door O-ring with a size 012 from a hardware store

    A 3⁄8-inch nitrile ring, a dab of silicone grease, and five minutes on a picnic table at a trail town restored the seal on a lamp we had written off.

    By Ruth Calloway

  • power budgets

    10 min

    Three weeks of readings from a 4-ounce solar panel strapped to a pack on the CDT in Colorado

    On the best day it put 6 Wh into a bank; under afternoon thunderstorms it averaged barely 2 Wh, less than a phone uses in a day.

    By Kenji Maruyama