In this article, I will concentrate on my personal experience, not on the physics of the process. If you would want to explore the subject further, please check the link at the end of this post.
1. Spare batteries
If you are only out for a little while, having one or more spare batteries for your cameras and other electronics may be the answer. The key question is how many would you need. My personal approach is double the number I expect to use on the trip. For example, if I am planning to shoot less than 800 images, which is doable with my current cameras using only one fully charged battery, I would take a second fully charged battery with me, just in case.
2. External battery as a charger
If you do not have enough spare batteries for your cameras and other electronics, the next option is to buy an external battery holding enough charge to recharge your camera batteries and other electronics. These batteries come in various shapes and sizes. If you are planning to fly, check with your airline before buying an external battery, to make sure it will be allowed on board.
3. Solar panel
If properly cared for and in the right conditions, a solar panel may keep charging your camera batteries almost indefinitely. If you are going on a long trip or suspect that the previous options may not cover your needs, a solar panel could be an answer, with some caveats.
As implied by the name, solar panels need sunlight to work. Depending on the size of the panel (and therefore its output power), the amount of light required to provide sufficient charge may range from a direct sunlight on a bright sunny day to a very dim light of a rainy day.
My experience has been with a 4.5 Watt panel. It will not charge on anything less than a sunny day or at least a very bright cloudy day, when the sun almost breaks through the clouds, and the panel has to be positioned perpendicular to the sun. On my expeditions in the Great Bear Rainforest, it has been an on-going challenge to get enough light. Sunny days there, as infrequent as they are, often mean rest days (or half-days), so I can stay ashore and do the charging.
Larger panels require less light, but they are significantly more expensive, heavier and take more space. One option, if you are considering both light weight solo trips and group expeditions, is to buy stackable panels, when two or more panels are connected together to provide more output power.
Buy waterproof panels for wilderness, but do not treat them as waterproof unless you are absolutely certain that they actually are. Also, while the solar panel might be waterproof, the charger is likely not.
Regarding the charging time, with my 4.5Watt solar panel in right conditions (sunshine or a very bright cloudy day, the panel positioned perpendicular to the sun), I have been able to charge a fully depleted 1500 mAh camera battery in around five hours.
4. Wind generator
This is a quite common device on sailboats; however, I have not been able to locate anything portable enough for a hike or a solo kayaking expedition that provides enough output power and is sold in stores. There are plenty of DIY options on the Internet. For larger wind generators, boating supply stores could be a good starting point. One of the advantages of having a wind generator on a wilderness expedition is that it complements a solar panel charging on stormy days when there is not enough light but plenty of wind.
5. Towed generator
This type of generator is towed behind a boat. I have no experience with these devices; however, I would not consider them for kayaking: too much drag. These would be more suitable for sailboats. Some models might work if placed in a fast stream.
6. Hand crank generator
They are often called emergency generators and for a good reason: only in emergency one would have enough will to keep rotating a handle for more than a few minutes. I have one that I am taking with me for an unlikely scenario when everything else fails.
In many cases, cameras are sold with chargers that only have 110/220V AC connectors. Portable external batteries and solar panels are made to charge through USB, 12V or similar low voltage adapters. Technically, it is possible to convert a low-voltage DC current to a high voltage 110/220V AC current. It is frequently done on boats with large on-board batteries providing only a low voltage output. I do not believe this would be practical with small hikable/kayakable power sources. The alternative option is to buy camera battery chargers with 12V car adapters. That is what I do. Fortunately, where are plenty of third-party chargers with 12V car adapters available on the market, some quite inexpensive.
Cameras from different manufacturers, and often different cameras from the same manufacturer, use different batteries requiring matching chargers. This greatly complicates both the wilderness photography and the charging. Some years ago, I had three different cameras using same batteries. I had five identical batteries that I could use in any of my cameras, two identical chargers (the main and the backup) and lived a happy live. Since then, while I have been trying to buy my cameras strategically, minimizing the range of batteries, I am now stuck with three different proprietary battery types. This means three different chargers. Plus backup chargers, in case the main ones are damaged.
At some point, I have realized that it just does not make sense to keep accumulating and carrying in the field so many charges, and I am currently gradually switching to universal chargers. I still have my three primary chargers for three battery types, but I only take one universal charger as a backup.
A few words about universal chargers. They come with either removable plates that accommodate different batteries, including AAs, or with movable contacts that slide to fit various battery types, with some restrictions. Going forward, I plan on switching to a couple of universal chargers for all my needs, once my existing primary chargers retire.
not USB chargers?
The camera batteries that I use require higher voltage than a USB
port can provide. There are likely devices available to increase the
voltage; however, this means an extra set of gadgets I could do
without. I have standardized all my chargers so that all my
electronics, not just camera batteries, are being charged through 12V
Prior to my very first multi-week solo kayaking expedition, I had checked my charging setup. Everything worked, or so it seemed. Camera batteries got a full charge from my solar panel, mobile phones were indicating that they were being charged. The revelation came when I was already in the wilderness. When charged with the solar panel, the mobile phones did not hold the charge.
The lesson is that, before hitting the road, make sure that all your camera batteries and other devices are actually being fully charged with the power source of your choice. It is not enough to connect your solar panel or external battery to a charger, see the charging light on and assume that everything works. Make sure that you go through the full charging cycle. Then disconnect the battery/electronic device and use it as if it was charged through conventional means. Keep an eye on how it performs. Any visible deviations from the normal, like a mobile phone dying only after a couple of hours in a stand-by mode?
If that is the case, something is not working. Some electronics require certain wattage that your setup might not be providing. It might be that the charger was actually at fault: a knowledgeable person at your local electronics store might help you with selecting an alternative. Or it might be that the actual source of charge was not powerful enough. Then it is either buying a more powerful external battery or, if you are using a solar panel, adding an adequate external battery to your setup. You first charge the external battery with your solar panel, then your device with the external battery.
If you are planning to spend some time in a car or on a bus, a driver might be willing to plug in your charger, especially if you have a multi-socket car adapter, allowing to charge more than one device from the car outlet. This way, the driver would not have to unplug his/her device to accommodate yours.
1. Bungee cords with hooks are great for attaching a solar panel almost anywhere and for adjusting its position to follow the sun.
2. Do not leave the solar panel unattended for long periods of time: a wind may blow it off, a sun position may change so that a branch in front of the panel initially causing no trouble would start casting a strong shadow reducing the output. Or an unexpected cloud may pass through depositing a fair amount of rain.
3. When charging, keep chargers away from the ground and protected, for example inside a plastic bag tied up with a rubber band.
4. If you are planning to use devices running on AA/AAA batteries, consider standardizing so that they all run on either AAs or AAAs. I prefer AAs, as it is somewhat easier to buy them, especially in out of the beaten path places. Often, batteries can be “recycled” through several types of devices before being fully depleted. Electronics seem to be most sensitive, reporting low charge while there is still lots of juice left. After being rejected by a GPS, the set of batteries may still be good to power an LED light for long time. Finally, if you use rechargeable AA/AAA batteries, consider buying the low self-discharge ones, often called "pre-charged", "hybrid", "ready-to-use" etc. While they are more expensive than traditional NiMH batteries, they hold charge for much longer, losing several percent a month versus several percent a day.
helpful link: some sound
advice from the Modern Outpost on selecting solar panels for your needs.