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How much margin due to charging inefficiency should I add when calculating the size of batteries needed for a trip?

You need approximately 50% more due to inefficiencies while charging from a power bank.

How much energy you really need will depend a lot on the actual usage, so you'll have to make some experiments. Put your fully charged phone in airplane mode and use it as you would on your trip (navigation or whatever) and measure how much time the battery lasts until it gets to 90%. Multiply this by 10 to get the maximum battery life. Some phones also calculate this themselves and show "battery time remaining" somewhere in the settings.

Then look for the design capacity of your phone's battery to see how much energy (expressed in mAh) you need for this time and extrapolate for two weeks. Just to be sure I'd add a considerable overhead (perhaps another +50% or even more, depending on how bad a power outage would be).

A major contribution to energy consumption will be the screen, depending on the brightness, so that would be a good first place to look for possible savings.

It all depends on how (and how much) you use those devices.

Personally, I can say more about DSLRs than phones, and with 2 different DSLRs on 5 tours (of 2-3 weeks) there was never an identical usage so far. The "official" number of photos you can take with one battery can be exceeded by limiting every usage outside of taking the photos - those are included in the estimations. Videos and liveview are extremely draining (continuous image processing) and should be avoided if possible. Turning off the display for longer use, and taking an excessively large SD card help too (you won't waste battery deleting bad pics).

Also, with more planning and using the analog viewfinder before turning on the camera, you might save battery time while planning a shot and might skip it alltogether if it's not working out. I've noticed the amount of shots I take decreased with every tour, as I knew some shots (if taken) would get deleted anyway, which in turn reduced the number of batteries.

Always leaving the camera on would be detrimental - that might be useful for phones that need to start up a metric ton of programs each time, but cameras are rather simple in that regard. My cameras have default power saving modes that turn off the camera when not used for a minute, indicating that the manufacturer has done that math. Leaving them on for 8+ hours per day seems excessive, but you can just try that at home if you want.

Last but not least, as Jasper's answer explains, charging inefficiency wastes additional battery power. If you think your DSLR batteries are not enough, you should rather add DSLR batteries instead of carrying additional charger devices and powerbanks. Even the cheap chinese ones give you a better mileage per weight. Charging everything from the same source might be more flexible and convenient, but wastes battery (by charging inefficiency) and weight (by additional devices).

Consider buying a power bank with a solar charging panel; that lets you recover from an accident like leaving your phone on overnight with something consuming power.

There are two main types on the market:

1. Big panels you unfold and hang up at your campsite or on the back of your pack while walking. (I've never used these.)




2. Standard batteries with a small charging panel set into the side of your case; you can carry it in a mesh outer pocket or some models have a cutout you can clip a carabiner through. The technology on these is getting better but isn't quite mature. Expect a slow trickle charge and efficiency that starts to degrade after a year or two.

I've had four batteries with built-in solar panels. They were perhaps an ounce heavier than standard batteries of their generation, and about $5-10 more expensive than regular ones. (One degraded solar capability over time, one was recycled when the battery cell inside started to swell, and I have a pair in current use (no pun intended.)) They aren't great, but they do give me peace of mind as part of my earthquake kit.

Also consider a crank charged battery -- I don't have direct experience with those, but it's another way to acquire electric power on the trail.

Also consider taking your cost and weight budget and splitting it in half, and getting two smaller batteries, to eliminate a single point of failure.

It hopefully goes without saying that you must also carry good paper maps in case your phone breaks or gets wet; many phones don't handle high temperatures well, either. There is a big difference between not being able to get that perfect photo and not being able to find the trail (or the right trail from several choices) after detouring around an obstacle.

It depends on how many pictures you want to take and how much weight you want to carry. In your case it looks like two camera batteries is enough for 1600 pictures, that may or may not be enough for your purposes.

As far as your phone goes keeping it only in airplane mode, using the the low power setting and only turning it on when needed means that usage will be as minimal as possible. If you are going to use it as a GPS then the longer you can go without looking at it or using it for tracking the less battery you will use.

If you start getting low you can prioritize the phone over the camera.

As far as what sizes, I would suggest a 20,000 mAh power bank to start with, and then go to a 10,000 mAh size if that's too heavy and you don't need the extra size.

Personally I carried a 20,000 mAh power bank to charge cellphone, GPS and camera for trips lasting up to almost two weeks and that was a larger charger than I needed.

A couple of points that I think haven't been addressed:

• Charging efficiency is greatest when you are charging between around 20/30% and 80%. So try to not let it get below 20% and try to stop when it's at about 80%. This will help you charge more from the same charger. You probably can't do this with the camera batteries, but you can with the phone battery if you're able to charge during the day. Also, don't leave it plugged in; only plug it in to charge and then unplug, or it may discharge some while plugged in (called 'maintenance charge') even once full.


• Battery performance is much worse in cold weather. So if you're going hiking somewhere quite cold, below 10°C, expect much more difficulty keeping your phone charged than in moderate weather. Try to not use it during the night, and try to charge during the warmer daytime.


• Hot weather can be a problem also. If it's going to be over 30°C much of the day, expect that to mean both your phone might run out of charge faster, and that the charger might not work as well. Try to charge at night time in hotter weather.

Overall you should definitely test what the discharge rate of your phone is - I wouldn't test 100%->90%, but 60%->50% or so; if you're able to keep it in the 30%-80% range that will be more representative of your performance. I would expect that if you're cautious about what you use you should be able to be okay with around 25000 mAh of battery or so. But this depends in part on the phone; an iPhone XS Max will take a LOT more battery than a much smaller iPhone 5c, for example.

Considering I will always have my phone turned on while hiking

There's a lot of variability in this, even given that you've specified airplane mode. Why is it on?

• Track logging with the screen off? This won't use much power, but you could still get through a charge in a couple of days on many phones. If you ose your track logging you're still safe.


• Navigation with the screen on? This can eat battery especially if the sun is out (the backlight is a significant power drain). If this is your promary navigation, are you safe if your power lets you down (or, for that matter, if you break your phone).

(Navigation with only needing to look at the screen occasionally is somewhere between these two - if your app behaves with the screen off)

• Comms when you do get a signal? (i.e. take it out of airplane mode at the top of a hill to check in). You can turn it off for much of the hike, turning it on only as you climb.

Turning it right off overnight will save you battery, but not if you're using it for an alarm clock.

With the DSLR, the number of shots you take makes a big difference, but biggest of all is if you use the flash. On a trekking trip you shouldn't really need it; even in low light propping the camera on something solid and avoiding the flash is likely to give nicer results much of the time.

Although this has been mentioned in some comments, it needs to be stressed more strongly:

Do not rely exclusively on electronics for navigation, in particular not on a smartphone!

You must always bring a paper map¹. You must always know where you are on that map. I recommend to hang a good map case around your neck, so you can monitor your progress on the map continuously.

For me, a dedicated GNSS receiver serves several purposes, none of which are essential:

1. Route ideas — get tracks from wikiloc as inspiration on where to go. This is particularly useful in areas where topographic maps are poor, such as in the United States, Canada, or Spain. In principle you can use this functionality without any GNSS receiver, by printing the track on a suitable background (national topographic map if available, opentopomap otherwise).


2. Track logs — upload your tracks to wikiloc or combine them with your photos to find out where you are. A smartphone will not work for this purpose due to battery life, except perhaps for day hikes. A good GNSS receiver with good batteries can last for weeks on two AA batteries.



3. Saving time — if you are hiking on a trail that is on openstreetmap, or following a track or route from wikiloc, you may realise more quickly than otherwise if you have taken a wrong turn. But since you should always know where you are on the map, you should normally realise your trail isn't doing what you're expecting it to do anyway.




4. (really 3b): If you really do get lost, it could save you a lot of time, to the degree that it could in some scenarios save your life. My friend has a GPS receiver for this purpose alone. He has never used it.

A camera is of course a non-essential item, it's up to you how much you are willing to carry. I've found that bringing one extra battery is enough for a mirrorless camera for 2 weeks.

So, to conserve power?

• Switch off your smartphone. You do not need a smartphone in the wilderness. By switching off your smartphone, you will conserve its battery in case you need it (although I can't think of any scenario in which you will, unless you're hiking in a densely populated area with mobile phone coverage and are therefore not bringing a satellite phone or PLB or similar).


• Obviously, only switch on your camera when taking a photo, and switch it off again afterward.


• Use battery save mode in your GNSS receiver, or expedition mode if available.

¹Some people insist on navigating terrain without a map. You can do that if you are sure you know exactly what you are doing, but it takes a lot of confidence and experience, in particular in areas that are unfamiliar, unfrequented, unsignposted.

I run into similar problems on canoe trips.

• My camera is a through-the-lens viewfinder, not an electric screen. Doing an entire canoe trip (3 weeks) and taking ~500 frames is easy. In addition, for the Nikon you can get a cute carrier that allows you to use 3 CR123 lithium batteries instead of the rechargeable one. A set of the disposable lithium are equivalent to roughly 2 of the rechargeable ones. I have the screen set to the shortest period available for review.




• I carry an InReach satellite communicator. I turn it on for a few minutes in the morning when I get up, and again a few minutes at night. I exchange a few messages with home to show I'm all right.




• I have a Magellan PN-60 GPS. This has an LCD screen, not a touch screen. I turn it on in the morning and off at night, and have it set to go to sleep when stationary. I can usually get 2-3 days of usage from a single pair of AA batteries. By default I have WAAS switched off, and backlight for the screen set to off or the minimum value.




• I carry a set of paper maps, and a good compass too.




• I have a small 3000 mAhr battery that can recharge the InReach device.

Some general observations:

• Phones generally don't have good GPSs. 50 meter errors are routine.


• My iPhone will drain the battery in about 3-4 hours if I'm using an app with GPS. This, even if I'm only checking the screen for a few seconds every half our or so.


• Due to the way topo maps were generated, there is frequently a 50 to 100m horizontal error in the placement of contour lines. The error is usually N/S and is due to parallax errors from the point of view of the aerial camera. The errors are locally systematic. e.g. Within a region of about a km they will all be shifted the same way by the same amount. This usually affects E/W running contours on a steepish slope. I have found situations where the GPS location plotted me at the base of a 50 meter hill, while the topo map showed that location as the top of that same slope. DO NOT DEPEND ON PRECISE LOCATION.

Topo maps are not all inclusive. They are the Cole's Notes version of the landscape. On canoe trips I have found deadly rapids (Class VI) that were unmarked. I have found missing islands, and ones that were on the map twice.

The versions that are created to fit on a GPS/phone are even more summarized.