You remind me of the babe.
February 11, 2014 10:36 AM   Subscribe

Or what do I have to look for on a portable generator to recharge my camera and tablet.

As I´m planning to be far from any electric outlet for long periods of time, but still would like to be able to take photos and read e-books, I´m been looking at hand cranked dynamos and portable solar panels, but I´ve realized I don´t remember almost anything of what I learned about electricity back in school.

Looking at the stats on the batteries I need to charge, and the ones on the generators, how do I figure out how long it would take to charge a specific battery? I´m lost with the numbers and I can´t seem to understand it any better after some googling.

I´m bringing a couple chargers with car lighter adapters too, but that won´t always be an option.
I may get a battery pack, but that will only take me so far between charges, and I´m backpacking light, so carrying a car battery with me is not something I would do. If I do bring a battery it will be something like the Astro Pro2.

Also, if you have any specific recommendation for a dynamo, solar panel or other solution
it will be greatly appreciated.

posted by Fermin to Technology (11 answers total) 2 users marked this as a favorite
You could get one of those USB chargers that run off AA batteries. And then get rechargeable AAs and a car charger for the AAs?
posted by ian1977 at 10:43 AM on February 11, 2014 [1 favorite]

The Wirecutter has two guides that should point you in the right direction and give you an overview of the technology.

The Best USB battery pack for travel

Best Solar Battery Pack

I own the Solar JOOS Orange that they recommend and it's worked great for me.
posted by Nonsteroidal Anti-Inflammatory Drug at 10:50 AM on February 11, 2014

At least one 2.1A port is key on a charger. That's about the maximum power anything requires for charging. Guessing everything you have is micro-USB, so I would simply get this and be done with it. It's smart, so it will dynamically change output based on the device.

I did a ton of testing and shopping around. Nothing is as good all-around as the Anker. 10,000 mAh is an insane amount of power. It would easily re-charge a Kindle Paperwhite 10x.

They also make a batshit crazy, 15,000 mAh version.
posted by lattiboy at 11:26 AM on February 11, 2014 [1 favorite]

Get the highest capacity mini battery pack you can. Like 15,000mah or greater. I was looking for ones in the 40k mah range but didn't really get any reliable hits.

What tablet are you charging? An iPad(non-mini) is going to take a lot more than say, a nexus 7. A 15k mah pack would charge say, an iPhone like 13 times, and most cameras probably close to 20-25 or even more if it's a tiny point and shoot or some mirror-less cams. My iPad 3 however would get charged like... Once, maybe.

Solar chargers suck and are annoying to deal with, as do dynamos and such. Just get a high capacity battery. 25k mah ones are out there, and I think I've seen even higher.
posted by emptythought at 11:27 AM on February 11, 2014

How long are you away for? External battery packs are great because they've really come down in price and weight but gone up in capacity in the past five years. You can buy one for around $50 these days that will charge a smartphone 3-4 times.

You can get a solar charger but they do take a while to charge and typically charge only one device at a time. Some don't hold a charge while some charge an external battery that you charge your devices from. Something to consider when the weather isn't consistent. They aren't totally reliable when it's partially cloudy so for the price of one solar charger I'd get a few external battery packs with at least 10,000 mAh. The hand crank ones are only really good for emergencies, when you only need a few minutes of talk time on your phone (a couple hundred mAh). Most device batteries range in the thousands so that's a lot of cranking.

Regarding camera batteries, you most likely have a point and shoot? As someone who hates charging the camera via usb, I would just buy a few extra camera batteries and charge them before you go. They weigh only a few ounces each. With point and shoots lasting for a couple hundred shots and assuming you only take a few dozen a day, you can estime about 2-3 days per battery, probably more (I'm guessing conservatively here).

What kind of tablet do you have? Depending on what kind, the battery capacity ranges from 1,500 mAh (Kindle) to 12,000 mAh (iPad). Using an external pack, most usb outlets are rated at 2A. For a Kindle it would take about an hour to charge, an iPad about 6. Do this at night or leave it in the tent during the day, otherwise ports can get ruined getting jostled around while hiking. I wouldn't really bring an iPad backpacking though.
posted by mlo at 11:34 AM on February 11, 2014

Response by poster: I was expecting answers with more math-fu actually, just to be able to shop around and see what is better for me, that´s why I didn´t give more specific details, but thanks.

The camera I have is a Fuji X100s, which uses NP-95 batteries. I don´t have an original battery with me right now, but there are some third party at 3.7V and 2200mAh (whatever that means, hehe, I at least know that the great the mAh figure the longer it lasts, and probably takes to load, but don´t know how to find out exactly how long) that are probably not enormously different (and I will bring some third party spares), so that would be a good approximation.

My tablet is a 2012 Nexus 7. Amazon has a replacement battery with 3.75V and 4270mAH.

The Astro Pro2 I´m looking at has 20000mAH (yep, four zeroes), but using about three NP 95 batteries per day even that will run out.

The nexus charges via micro USB (or mini, I never remember which one is which), and the camera batteries through the car adapter or regular wall plug.
posted by Fermin at 11:43 AM on February 11, 2014

Forgot to mention that most external battery packs run under a pound these days while the Joos is about two.

The Fuji X100s is good for 150-200 shots depending on EVF usage, shooting in RAW, using a wifi SD card. Figure out how many photos you take in a day from past trips and use that as a frame of reference. NP-95 batteries are less than 2 oz each, so just bring extra batteries.

If you're only using the Nexus 7 for reading a couple hours a day, it should last you about 4-5 days if you keep the wifi off etc. Using a 20,000 mAH external pack for charging a tablet only, you could get about 4 charges out of it. Depending on usage around 2 weeks off the grid.
posted by mlo at 11:53 AM on February 11, 2014

Nth'ing getting a solar charger and an external-battery-pack charger. The Astro Pro2 is the perfect sort of thing. Use the battery-pack to charge the devices at night, and leave the solar panel charging the external-battery-pack during the day.

Don't bother with hand cranks.

The Astro Pro2 probably charges at 18-20V, not 12V. Most solar panels are intended to charge 12V batteries (lead acid). Be sure to get a panel that outputs at something around 19V. A panel with a 18V output will probably be aimed at laptops, so it will probably be larger than you need (eg this) and so heavier and more costly. You might want to look at something other than the Astro Pro2, something that can be charged at 12V or even 5V.

The Astro Pro2 is 20Ah at 3.7V, so it holds about 75 Watt-hours. That's plenty to charge your two devices, you probably only need to charge it once every few days. A 10 watt panel over 4-6 hours of sun will probably mostly recharge it (40ish watt-hours) and so should be enough for a tablet and camera if you use the panel each day or most days. A 5-watt panel would probably work, but you'd need to diligently set it out every day to charge all day, more risk of running out of power. 15+ watt panel would probably keep you in business despite poor sun. 20+ watt and you're set up to throw a laptop into the mix if at some point you want to.

Have the Astro Pro2 in the shade while it's charging and in general - storing/using them in a cool environment reduces the oxidation degradation, so it can slightly prolong how many months it will work before the battery starts to degrade.
posted by anonymisc at 11:56 AM on February 11, 2014

I once hiked the Appalachian Trail (2,159 miles at the time) with a laptop, cellphone, and a cellular modem, so I've got a little experience in this realm. Then, as now, solar chargers are useless unless you are going to consistently be in places where you can rely on bright, sunny weather, and you will be able to have that solar panel in the sun.

If I were you, I'd do the math on how long a NP-95 is going to last under your use scenario, and carry enough to make it between charges. (At just 1.6 ounces apiece, the weight won't be an issue, although cost might be.) Don't even try to rig up a method of charging those batteries in the field. Then I'd do the same math on use of that Nexus 7, and size your portable USB charger accordingly.
posted by waldo at 4:32 PM on February 11, 2014

Response by poster: Heh, I just happened to do a quick search for Astro Pro2 on Amazon and found there are two packs with the same name, from different manufacturers, not just the one I was thinking about.

Introcircuit makes a 26000mAh beast slightly cheaper than Ankers.

Anonymisc, that panel is tempting me, even if it´s large it may be manageable if I rethink my packing strategy a bit. I´m going to the rainforest, where there tends to be a lot of rain, and a lot of forest, so going a bit overkill with the solar panel can be a good way to maximize the hours of bright sun when I get them. Am I wrong in thinking being larger it would charge any battery faster?

I still don´t know how to determine how long it would take to charge any battery with any given generator, at least in ideal conditions. How did you get to 4-6 hours for charging the Astro Pro2 with a 10 watt panel?
From here it looks like you multiplied the Astro´s Ah by its V to reach Watt-hour, and then the W on a panel is also per hour, so 10W for 4-6 hours should be 40-60 and you rounded it down to 40ish to take clouds and just loss into account. Did I get that right?

I´ll be away for weeks at a time, and I know I can easily eat through an NP95 per day, possibly 2, and that´s in city use (I used film on my last trip, so close to zero battery usage). I can extend that some if I minimize use of the EFV, but it´s still a digital camera.

I hear you don´t like hand cranks, but if I got your math right, using one of these I could charge an NP-95 with about an hour of cranking, right? And a full charge of the Nexus 7 (which wouldn´t be every day, since I would only use it to read) would take about two hours?
That sounds doable if I can maximize the NP-95 and don´t charge the Nexus fully at once.

Waldo, I have to take into account not just the cost of the NP-95, but making sure that I will be able to take a lot of them across borders without a brazilian customs official thinking I´m selling them, and see if the local Fuji store sells them (I can only order so much from the US per year until I have to start paying sky high import rates). But I´ll study the possibility of bringing only backup batteries, maybe two Astros and a bunch of NP-95...

You all gave me a lot to work with to find the most efficient approach, thank you. If you think of something else, I´m all eyes.
posted by Fermin at 5:06 AM on February 12, 2014

Best answer: I multiplied the Ah by the voltage of a lithium cell (3.7), because even though the minimum output is 5V, when they advertise 20,000mAh they're actually talking about the output of the cells, ie prior to voltage conversion.

The four hours around noon are the best for solar energy, the output drops after that, and you shouldn't plan on getting more than 6 useful hours in a day. There will always be substantial losses (eg the panel won't ever be facing the sun quite perpendicular without a heliostat, battery charging loses 10-20%, some panels have their wattage labelled... optimistically... etc etc.), so I'm figuring 6 hours at 10W = 60Wh, I cut that down to 40Wh (by guesstimate) because of losses, which is a bit over half a recharge of the 75Wh, and I would think, a little bit more power than your devices would use in a day. But that's under ideal conditions (a sunny day, all day in the sun, facing the sun). Under poorer conditions, 10W isn't going to cut it.

When unlimited power is available, lithium batteries fully charge in about 3-4 hours. About 40% of that charging will happen in the first 30 minutes (sucking down large amounts of power) with the charge-curve gradually tapering off as they get closer to fully charged (the final 30 minutes is probably only adding about 5%). Because of this, a big panel can be beneficial if you only have a short time in which to use it, if the cells are completely depleted.

Yeah, if you're in a rainforest, solar isn't going to be very good. A bigger panel (and bigger battery pack) to take better advantage of fewer charging opportunities seems like a good idea. Also, getting the angle as close to face-on to the sun really makes a difference - if possible you should shift the panel every hour or two to track the moving sun. With a fold-out panel like the one I linked, you'll get better results if you can find something to prop it up on an angle with, or some string/sticks to tent it, or some other way to get it directly facing the sun.
Regardless of how well the panels are facing the sun, all panels should be facing exactly the same direction. If you drape it over a boulder so that most of the panels are directly facing the sun, but some aren't, the ones that aren't will underperform and act as resistors that drag down the power coming from the overperforming panels resulting in a decrease in useable energy. (A bit like how a chain is only as good as its weakest link). Before you travel, it would be a good idea to start using a panel at home so you get a feel for what conditions produce what kind of charging, and when it's better to leave it flat on the ground vs propped up to sun if you can't prop it up without losing some panel alignment, etc.

using one of these I could charge an NP-95 with about an hour of cranking, right? And a full charge of the Nexus 7 (which wouldn´t be every day, since I would only use it to read) would take about two hours?

That sounds about right, but 1. You'll be dead sick of cranking before you hit the third minute. Cranking for an hour is setting up your own personal hell. It isn't an exercise bike that you can sit on and pedal, it's not ergonomic, it's just annoying and tiresome and boring and you have better things to do. 2. 10W strikes me as optimistic - yeah, you'll be able to get 10W out of it, in short bursts of frantic work, but your average output will be much lower. 3. I'm not sure I would even trust the thing to survive 2 hours at 10W cranking. The gears will be plastic. 4. It will be noisy and annoying to people around you.

The crank could be a good fallback solution for when you've had no chance to charge for days, and you just need a few minutes on the tablet or a couple of photos (no flash) on the camera, but I don't think you will want to rely on it or make it a necessary activity.

If you haven't already, start using the optical viewfinder on the camera and learning how to use it leaving the screen turned off. In my experience, by far most of the power a camera sucks out of the batteries is simply to power the screen&backlight. (Well, that and the flash, but you won't be needing the flash). If you're using neither screen nor flash, you get a lot more photos per battery.
posted by anonymisc at 11:19 AM on February 13, 2014

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