Recharging batteries with solar panel in winter?

Hi all,
I was wondering if one could hope to recharge a pair of AA batteries with a solar panel charger in winter? Since it'll be cold, I was thinking I might have to put the solar panel on my head and keep the batteries in my coat so they don't freeze

I also think it might be just be worth it to carry some spares for the duration of a trip instead (2-3 weeks).

Fish

Winter brings short days and weak sun. My guess is that it would be marginal. You might have a chance if you basecamp (or at least stay put for a day).

Charging a pair of AAs could easily take several days and determining when a cell is fully charged in the varing temps may be difficult.

Also the temp limits are narrower for charging than for use. IIRC the min charging temp might be ~40F. (Don't have time to research it right now--there might be something at http://www.batteryuniversity.com/index.htm.)

You might be better off (less weight and more convient) just bringing a bunch of lithiums and being careful to conserve power.

BTW, NiMH and NiCad cells do not freeze. Their available output decreases with decreasing temp.

Doug

DougPaul said:

Winter brings short days and weak sun. My guess is that it would be marginal. You might have a chance if you basecamp (or at least stay put for a day).

Charging a pair of AAs could easily take several days and determining when a cell is fully charged in the varing temps may be difficult.

Also the temp limits are narrower for charging than for use. IIRC the min charging temp might be ~40F. (Don't have time to research it right now--there might be something at http://www.batteryuniversity.com/index.htm.)

You might be better off (less weight and more convient) just bringing a bunch of lithiums and being careful to conserve power.

BTW, NiMH and NiCad cells do not freeze. Their available output decreases with decreasing temp.

Doug

Click to expand...

Blast! There goes a perfectly good excuse for buying myself a nifty little solar panel recharger. Oh well I had a feeling this would be the case though.

Cheers and thanks for the reply.

Fish

hikingfish said:

Blast! There goes a perfectly good excuse for buying myself a nifty little solar panel recharger. Oh well I had a feeling this would be the case though.

Click to expand...

You need a nifty big solar panel for practical charging. Summer hiking in the desert also helps...

There are small hand generators which might be practical. And the work will help keep you warm...

Doug

solar charger

I carried a AA battery solar charger on my AT thru-hike. It worked perfectly, even though everyone was doubtful. I figured I'd try it anyway since mine cost under $15 from Sundancesolar.com (they don't seem to carry the same model I have anymore). I would hang it on the back outside of my pack and since I started in winter (Feb.) the trees were leafless and it charged. Even after the leaves were out it worked too, though. I used it all the way until May, when I got to New England and my husband started meeting/resupplying me on the weekends. When I remembered I'd sit it out facing the sun during snack or lunch breaks. The temps got cold, down in the teens. My camera blows through disposables so fast (3-4 days), it would have been ridiculous to try to carry that many. I was really thrilled with not having to use disposable batteries at all. Two sets of recargables did it!

I looked into solar chargers a bit and had to wonder what the specs I was looking at meant. Soppose you need 2 x 2500 mAh worth of current to fully charge a pair of AA's. How do the following specs relate to that in terms of charging time, assuming "full sunlight": Output: 6.5 watts (15.6 volts, 433 mA)


My probably incorrect calculations of 1.2 volts x 2500 mAh gives me 3.1 watts for an hour. Therefore, the 6,5 watt charger would charge a pair of batteries in 4 hours under ideal conditions. Does that make sense? (there must be ONE person on the forum who can answer me ).

In my case I would be at 50 degrees north in feb-march under mostly sunny skies. Short days, low angled rays.

Neil said:

I looked into solar chargers a bit and had to wonder what the specs I was looking at meant. Soppose you need 2 x 2500 mAh worth of current to fully charge a pair of AA's. How do the following specs relate to that in terms of charging time, assuming "full sunlight": Output: 6.5 watts (15.6 volts, 433 mA)


My probably incorrect calculations of 1.2 volts x 2500 mAh gives me 3.1 watts for an hour. Therefore, the 6,5 watt charger would charge a pair of batteries in 4 hours under ideal conditions. Does that make sense? (there must be ONE person on the forum who can answer me ).

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1.2V x 2.5Ah x 2 = 6 wh (total energy capacity of 2 cells)

6wh/6.5w = ~1hr. (charging time would actually be longer because charging is not 100% efficient)

So you are wrong, it would be well under 4 hrs...


To take some data from a known charger: Brunton SolarPort 4.4 http://www.rei.com/product/759981 ($129 which buys lots of batteries and 19oz which is ~19 lithium cells*) produces up to 4.4 watts.

Take into accout inefficenencies, poor pointing at the sun, etc, maybe it would charge a pair of NiMHs in one or two days if the tree cover isn't too heavy.

OK, that might win on a trip that is long enough to use more than 20 lithium cells*.

* IIRC, a lithium AA cell weighs ~1oz, alkiline ~1.5oz, and NiMH ~2oz.

Hikingfish: maybe you have an excuse to spend the money. I'll wait for you to let me know how well it works before I spend anything...

Doug

One nice offset to cold weather for solar charging is that solar panels work better the colder they get.

With cold panels and the extra reflected light from the snow, they actually can exceed the manufacturers rating.

Dont know if this offsets the batterie's inefficiency when cold and how much it affects small chargers, but it is a factor in sizing solar arrays. I have personally witnessed it on a real cold sunny day on my home system.

peakbagger said:

One nice offset to cold weather for solar charging is that solar panels work better the colder they get.

With cold panels and the extra reflected light from the snow, they actually can exceed the manufacturers rating.

Dont know if this offsets the batterie's inefficiency when cold and how much it affects small chargers, but it is a factor in sizing solar arrays. I have personally witnessed it on a real cold sunny day on my home system.

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One thing I noticed when looking at two models of solar cells sold for hiking was low peak efficiency. Sunlight in a place like the Arizona desert peaks at about 1Kw/sq-m or ~93w/sq-ft.

One product advertises 4.4w from a size of 9.3 x 6in for an output of 11.4w/sq-ft or an efficiency of ~12% (actually not too bad for an "inexpensive" consumer device). The technology is polycrystaline (presumably silicon). (The panel is rigid.) The cost is $333/sq-ft or $29/w.

A second product advertises 14w from a size of 12 x 57in for an output of 2.9w/sq-ft or an efficiency of 3%. The technology is floropolymer. (The panel is flexible--it rolls up.) The cost is $94/sq-ft or $32/w.

Single crystal silicon is the most efficient, but also the most expensive and polycrystaline silicon is less efficient and less costly. Judging by the above data, the floropolymer is significantly less efficient than either of the silicon technologies. (I am not familiar with the floropolymer technology.)

I don't know which technology you are referring to in your comments about temperature effects (presumably one of the silicon technologies), but it may not apply to the other technologies.

Batteries are electrochemical devices and chemical reactions slow down when chilled. We all know this affects use--it probably also affects charging. And effects on the charger depend upon its design.

Doug

I want to second the lithium battery idea. They are a third lighter than alkalines. They last three to seven times as long. I have a Canon Powershot A610 camera that I changed the batteries in several weeks before a trip to London. I found I have to leave the camera on during the day because the on/off switch is failing and it can take 5 minutes of pushing the button to turn it from a cold start. The camera was on for most of the day for a week, hundreds of flash pictures, same set of batteries still in it. I found that they seem to last roughly 5x as long in my 60CSx GPS. Lithiums seem very expensive when looking at the package in WallyWorld, but I'll bet they are of similar value to alkalines, and they are lighter.

Paradox said:

I want to second the lithium battery idea. They are a third lighter than alkalines. They last three to seven times as long. I have a Canon Powershot A610 camera that I changed the batteries in several weeks before a trip to London. I found I have to leave the camera on during the day because the on/off switch is failing and it can take 5 minutes of pushing the button to turn it from a cold start. The camera was on for most of the day for a week, hundreds of flash pictures, same set of batteries still in it. I found that they seem to last roughly 5x as long in my 60CSx GPS. Lithiums seem very expensive when looking at the package in WallyWorld, but I'll bet they are of similar value to alkalines, and they are lighter.

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Actually, lithiums hold about the same amount of energy as alkalines (2500-3000 mAh for an AA cell). However, digital cameras are high drain for a short period devices and lithiums (and NiMHs and NiCads) are very good at this kind of service, alkalines are not. In medium drain service (eg a GPS or radio), one would expect similar lifetimes.

Also, my Canon A75 manual specifies alkaline or NiMH. The camera works if you use lithiums, but it tends to get hot. If I use lithiums for hiking, I turn it on, take the pic, and turn it off before it gets too hot. Obviously hard for you to do with the flaky switch. I don't know how risky this is for the camera, obviously it would be safer with NiMHs. Unfortunately, NiMHs are a bit on the heavy side.

Doug