Many recommend just bringing along a portable power bank. Which is fine for short trips. However, in the spirit of being off the grid, I am also seeking a lightweight solar panel.

The solar panel can either connect to my phone, or a “power bank”. A power bank is convenient so your phone isn't leashed. Also because the power delivery from the solar panel isn't reliable.

I wish there was an app to keep the phone charging after moving into the shade. The phone requires replugging the cable to continue charging if the voltage drops. Or, the phone keeps beeping telling you it's been plugged in and turning on its screen, possibly using more energy than its getting from the solar panel.

There are pocket sized battery banks, but you can also use any rechargeable device that isn't fussy about a variable charging voltage, such as a headlamp. Something you will carry anyway, so you save the weight of carrying a separate battery bank. A headlamp's capacity for brightness largely depends on the amperage delivery from the battery, which seems to depend on size (see next section below).

The most common battery sizes are 18650, 21700, and 26500 invetispower.com. So you might choose 21700 or 26500 as a way to select the size of the headlamp/power-bank. A 21700 has 4000-5000 mah capacity, much like a cell phone battery.

Adapter for naked battery as power bank

I found a very powerful headlamp that also serves as a power bank. It was perfect except for the lack of a red light. It looks like a flashlight, but it has a right angle LED, and mounts into a head strap. This headlamp is great for night hiking, but at camp it converts to a lantern using an inflated balloon, and I use a small red headlamp to keep from blinding others.

When coming out of the wilderness, passing through a town, it's convenient to have a fast charger for multiple devices. See: https://atlasguides.com/choosing-the-right-charger-for-thru-hike

Another wiki: https://flashlightwiki.com/Main_Page

Hypothesis: If you assume the LED are all about equally efficient in producing light based on power delivery, then you can judge how bright a light will be based on the battery it uses. Most bright flashlights and headlamps use 18650 batteries. Although 21700 batteries, a bit larger at 21mm diameter and 70mm height, are being mass produced due to Tesla adoption.

So let's do a little digging to see if this may be true… apparently, the makers of Fenix lights think so:

source: fenix-store.com blog

From http://web.mit.edu/evt/summary_battery_specifications.pdf

• Cell, modules, and packs – Hybrid and electric vehicles have a high voltage battery pack that consists of individual modules and cells organized in series and parallel. A cell is the smallest, packaged form a battery can take and is generally on the order of one to six volts. A module consists of several cells generally connected in either series or parallel. A battery pack is then assembled by connecting modules together, again either in series or parallel.

• Battery Classifications – Not all batteries are created equal, even batteries of the same chemistry. The main trade-off in battery development is between power and energy: batteries can be either high-power or high-energy, but not both. Often manufacturers will classify batteries using these categories. Other common classifications are High Durability, meaning that the chemistry has been modified to provide higher battery life at the expense of power and energy.

• C- and E- rates – In describing batteries, discharge current is often expressed as a C-rate in order to normalize against battery capacity, which is often very different between batteries. A C-rate is a measure of the rate at which a battery is discharged relative to its maximum capacity. A 1C rate means that the discharge current will discharge the entire battery in 1 hour. For a battery with a capacity of 100 Amp-hrs, this equates to a discharge current of 100 Amps. A 5C rate for this battery would be 500 Amps, and a C/2 rate would be 50 Amps. Similarly, an E-rate describes the discharge power. A 1E rate is the discharge power to discharge the entire battery in 1 hour.

• Specific Energy (Wh/kg) – The nominal battery energy per unit mass, sometimes referred to as the gravimetric energy density. Specific energy is a characteristic of the battery chemistry and packaging. Along with the energy consumption of the vehicle, it determines the battery weight required to achieve a given electric range.

• Energy Density (Wh/L) – The nominal battery energy per unit volume, sometimes referred to as the volumetric energy density. Specific energy is a characteristic of the battery chemistry and packaging. Along with the energy consumption of the vehicle, it determines the battery size required to achieve a given electric range.

There are other useful definitions in the pdf. Not mentioned is that nominal capacity and nominal energy decrease with increasing c-rate, because the efficiency of LEDs decrease with increased current delivery, due to something called the Auger recombination.

“The theoretical limit for white LEDs is about 260-300 lm/W.” White LEDs with super-high luminous efficacy could satisfy all general lighting needs, phys.org 20100831

“Cree reports that the LED efficacy was measured at 303 lumens per watt, at a correlated color temperature of 5150 K and 350 mA. Standard room temperature was used to achieve the results.” cree.com 20140326

20210329: The highest lumens per watt currently available from CREE is the “XLamp XP-G3” at 190 lumens per watt. https://cree-led.com/products/xlamp-leds

For the Cree XP-L2, the maximum is 169 lm/W.

From https://en.wikipedia.org/wiki/LED_lamp#Efficiency_droop:

“The term “efficiency droop” refers to the decrease in luminous efficacy of LEDs as the electric current increases above tens of milliamps (mA). Instead of increasing current levels, luminance is usually increased by combining multiple LEDs in one lamp. Solving the problem of efficiency droop would mean that household LED lamps would require fewer LEDs, which would significantly reduce costs.

In addition to being less efficient, operating LEDs at higher electric currents produces high temperatures which compromise the lifetime of the LED. Because of this increased heating at higher currents, high-brightness LEDs have an industry standard of operating at only 350 mA, giving a good compromise between light output, efficiency, and longevity.[45][46][47][48]

Early suspicions were that the LED droop was caused by elevated temperatures. Scientists proved the opposite to be true – that, although the life of the LED would be shortened, elevated temperatures actually improved the efficiency of the LED.[49] The mechanism causing efficiency droop was identified in 2007 as Auger recombination, which was taken with mixed reaction.[48] A 2013 study conclusively identified Auger recombination as the cause of efficiency droop.[50]”

There are different LEDs capable of different lumens. The strange thing is, that for flashlights and headlamps, practically all I can find are LEDs made by the company Wolfspeed, makers of CREE LEDs.

Cree, Lumileds, and Nichia spar on LED future at The LED Show, 20130814

Analyses of Five Major LED Manufacturers Vertical Integration Strategies (Part 1), 20160112 “Cree is with no doubt the epitome of vertical integration in the LED industry, from SiC material advantages to vertical LED chips, to record holder of luminous efficacy. The component manufacturer has been the leading lighting brand in the U.S., and has an entirely different development path from sapphire substrate based LED companies.”

My conclusion from the above article from 2016, CREE is the only option for high performance flashlight and headlamp LEDs. However, Chinese make knock-off versions that are about 70% as bright, according to a youtube video guy.

More LED manufacturers here: https://flashlightwiki.com/Brightness_Bins

https://cree-led.com/products/xlamp-leds

My tests with 5 models of cheap china flashlights, some having multiple “T6” LEDs delivered the same output as those with single LEDs, all powered by dual 18650 batteries. Were the single LEDs not T6? I won't know because I don't have them anymore.

Check out:
https://flashlightwiki.com/Brightness_Bins
https://flashlightwiki.com/Cree

Notice for 1000mA, Cree LEDs max out at 530 lumens with the XM-L2-U3, and 550 lumens for the XP-L-V6.
Compare to the competition at 450 lumens or less.

For more current model versions, see the Cree website: https://cree-led.com/products/xlamp-leds.

https://cree-led.com/products/xlamp-leds
https://cree-led.com/media/documents/ds-XHP702.pdf

Note that the biggest and baddest XHP70.2 requires too much power for most any ultralight backpacking use, at least at its maximum rated wattage. Although, who says you have to run it at maximum power? You could use a lesser voltage or lesser amperage, while slightly increase the efficiency:

At 6 Watts, or 6 Volts x 1000mA, you would get 181 lm/w * 6w = 1086 lumens. Two 3.7 Volt 16850 batteries in series would work for that (7.4 Volts is close enough to 6 Volts, and battery voltage goes down with charge anyway).

For a headlamp, I think I want a flat LED, along with a sliding lens that can focus or diffuse. However, the XHP70.2 only comes as a bubble.

https://cree-led.com/products/xhp-family-leds/xhp-leds
https://cree-led.com/media/documents/datasheet-XHP35-2.pdf

The maximum drive current is more of an ultralight backpacking size, at 1050mA. At 13 Watts, it would produce 1284 lumens. It requires 12 Volts. An A23 battery is 12 volts and tiny, but only has about 60mah, so you would need about 50 of them for 3000mah. Scratch that. There are headlamps out there with the XHP35 and circuitry to bump up the 3.7 volts of a single 16850 battery to 12 volts.

Note that the lumens per watt is much less than the 70.2 model. However, I used the Max Efficiency at Binning Conditions for the 70.2, which I didn't find data for with the 35.2 model.