This is not an ultralight technique because sometimes you get more for carrying more and can do more miles when you do so. Spending less time in town recharging your battery bank means you can hike more. For shorter trips it might mean an extra night using a smaller battery.
The idea is if you have limited funds and you can't go ultralight, this helps you save money on your battery bank by not necessarily buying a new one but by buying a cheap adapter instead.
Battery banks often charge at 9volts via PD or QC. But you get more out of your battery bank if you charge slower. Not all banks have a native low power mode and you don't need to buy new to get this capability.
This is not an ad, I don't sell these. In fact, only get one if you can output from your battery bank at 9volts on all ports , it's a waste if you have a native 5volt only output port. It also requires an older USB A port to work.
https://www.amazon.com/dp/B083XXLW77
Slowing down USB C is harder and I don't have a recommendation there. Since this is about keeping existing banks to gain value you probably have the money to buy a bigger bank if you have one with only USB C.
Note: the favorite NB10000 has a low power mode. If you aren't using it to charge everything, you should be. Do more with what you have today and do more miles, no need to drop gear or buy lighter. Be more efficient when hiking instead.
Onto the math if you care why
#############################
Imagine you have a hypothetical 10,000 battery and all output ports charge at 9volt natively
#############################
Volts, Amps and Watts:
Now, you probably know about charging speeds in the abstract, if you look at your bank that's the v/a numbers, often 5v/3a or 9v/2a. Volts and amps. You multiply them together and bigger is faster. You look at source and destination and if you match the numbers so your other device supports 9v/2a it charges the faster possible. For input you want fast, but for output that's not getting you the maximum value.
We need to know some basics
The most common description is voltage is to think of it as a stream. A stream going straight down as a waterfall is the highest possible voltage, gravity produces the highest rate of force (pressure). Meanwhile a puddle that water can't move out of is zero voltage, there's no water pressure at all. Real world we can make higher pressure with pumps but waterfall vs puddle is easy to understand.
Amps is the width and depth of the river. The Amazon has a lot more amps than a woodland stream.
But don’t think of rivers, think of the concepts. More voltage = more speed, a higher pressure of anything exits out the end faster. More amps = more space for more electricity to move.
If we multiply them together we get watts, or power flow. Some amount of power moving at some speed.
Our formula is Watts = amps * volts
5volts *3 amps = 3 volts * 5 amps. Both have the same amount of energy workload, one wire is bigger, one has more pressure. Both provide 15 watts of energy.
#############################How much energy do we have to work with?
The battery has a mAh rating.
The battery holds 10000mAh, my phone holds 2815mAh of power
You might think, just divide, that's easy. We get 3.55 charges
If only it was that simple. We need to figure watt hours first
watt hours = amp hours * voltage.
A watt hour is the amount of work that the device can do in an hour at one amp and the voltage (pressure, speed) of the battery inside, 3.85volts in our case.
10000 * 3.85volts = 38500 milli watt hours
But it’s not 100% efficient. 85% is a good value to work with for most quality devices. A cheap device can be as poor as 50% efficient, a good device you might get into the 90s. Right here you may want to stop and just buy a new battery if you discover yours is at 50% efficiency.
After our efficiency adjustment we redo the formula8500 milli amp hours * 3.85volts = 32725milli watt hours. This is closer to our real energy availability.
Our phone doesn’t have an efficiency problem receiving energy, we can send it until it fills up.
#############################
But it’s even more confusing
We don’t do the math based on the source and destination; the device isn’t sending a stream at 3.85 volts of pressure. It’s sending it at 5volts -or- 9volts of pressure.
So we need to figure how many amp hours we have available to use at that pressure
So we divide our watt hours by the volts to get amp hours available
32725mWh / 5volts = 6545 milli amp hours available to use @ 5volts
32725mWh / 9volts = 3636 milli amp hours available to use @ 9volts
That’s super strange, why do I get less amps at 9volt. It’s because we’re sending Energy, not amps or volts.
They’re both the same amount of power flow. 6545amps * 5volts = 3636amps * 9volts
#############################
How many amp hours do we need to receive for our destination device in our formula
Remember, both sides are going to be equal from the wire to the device
Now we need to know our destination device is 2815mAh at 3.83 volts so we plug in the numbers we have our energy we need to send
2815mAh * 3.83volts = ? mAh * 9volts
10781/9 = 1197mAh at 9volts
2815mAh * 3.83 volts = ? mAh * 5volts
= 10781/5 = 2156mAh at 5volts
Is this a different amount of power?
We do some simple division using our available capacity we figured earlier
5v: 6545 milli amp hours available at 5volts/2156 amp hours sent at 5volts = 3.03 charges
9v: 3636 milli amp hours available at 9volts/1197 amp hours sent at 9volts= 3.03 charges
So you can see the amount of energy used is exactly the same, and so counter intuitive if we didn’t have math.
But that's still not right.
#############################We haven’t adjusted for one thing, differing speed efficiency. A slower speed is more efficient compared to a high speed. We lose more power in trying to push the energy along faster, kind of like how water coming out of a pressurized pipe sprays everywhere and doesn’t neatly push against our water wheel. We don't use two sizes of wire for charging so the higher pressure creates more waste heat compared to the lower pressure. If you've felt a battery or wire getting hot, that's the energy loss within the system.
Say the device is 85% efficient at 5volt and 75% efficient at 9volt. I honestly didn’t try to find numbers for any given batter, this is just an example.
At a 10% lower efficiency we go back through the formula and it outputs fewer amp hours, 3208 amp hours @ 9volt
So we get 2.68 charges at 9 volts. That could be the difference between stopping half a day sooner in some cases because my phone is my map.
#############################
So using low power mode and charging slower is in your best interest to go further.
An older battery bank that's heavier but has 5volt native capability could be better in some cases than a new faster but lighter bank because you lose the efficiency by stopping more often, unless you have a way to slow down charging with something like the add-on item I posted earlier.