"Ammeters are cool and all, but a lot of people are hesitant to cut into a perfectly good charging cord."

Nor would there be any reason to do so.

--B.G.--

A = W / V

So if the panel is truly putting out 5 watts/5 volts then you will get 1 amp at the USB port. You will need perfect orientation to the sun for approximately 1.5 hours to charge a dead iPhone, assuming there is 100% efficiency.

Around Father's Day this year I posted a mini-review on the Goal Zero Nomad 7.

After that discussion, I did test it and output was just under 1 full amp. I have not taken it backpacking due to a "philosphical" problem with the concept. Bob and I have had several discussions about solar and we both agree it is generally not practical for the typical UL backpacker.

I do have a fair level of experience and understanding of solar systems. I installed a 250 watt system on my tent trailer. We have been using solar on our tent trailers since 2003, which includes over 1,000 camping days; if anyone wants to continue the discussion. If I were to integrate solar into a multi-week trip I would probably want an aux battery back so I could collect reserve capacity for those times my phone was fully charged.

If this post sounds dis-jointed it is because I am using my iPhone which is difficult to type on.

Bob, I knew you would come back with something -- something I can't handle! Where do you live?! I'm sure you have a super-duper multimeter and you can help me!

David, I think I can handle your suggestion. The charging times are very comparable to a wall unit, I would not consider it trickle charging. I kept 3 iphones charged on my trip -- only charging when we set up camp. Granted, only my phone was used for GPS, but the others were ipoding away!

I'll see if I can put something together this weekend. Thx.

> But are there situations where you know the battery can take,
> say, 100 mA for 10 hours. Then could you give it an unregulated
> 100 mA for 6 hours only, disconnecting after that time?

For NiMH, sure. A AAA NiMH will not be seriously damaged by a few hours of 100mA even after it has been fully charged; an AA will tolerate 200mA for a while even after being completely topped off.

I'm not brave enough to try it with Li-ion, though. You'd really need a multimeter to determine the starting state of charge and then do some calculations to see how long at what mA charge rate it could accept. And you'd need to recheck voltage frequently during charging since a solar cell's output is not constant nor fully predicable. Too much hassle and still a bit risky IMHO, best to stick with Li-ion devices that have integrated chargers.

I would think that if you are charging a battery that is inside a phone or other device that has a USB charge port you would not need to worry about a charge controller, it is probably built into the device. Charging batteries directly is another matter. For wilderness travel (if I were so inclined to use solar), I would want a larger panel say 7 watts with a charge controller. This way in less than optimal conditions you have more solar collecting capability.

To measure amps with a digital multi-meter (DMM), it has to be capable of measuring amps, or you will need a special amp probe, which are expensive. If your DMM can measure amps you need access to two points in the charge line, which you do not have with a USB charging cord.

Here is a simple solution. Buy a very short USB extension cord with the heaviest gauge wire you can find. Now strip two access points in the charge line. Connect your probes to the points with the device connected (make sure the device is less than 1/2 charged).

If the solar panel is rated at 1 ampere, 5 volts, then you can just put a power resistor across the panel output as a test load. The power resistor should be 5 ohms, 5 watts minimum. Then, with the solar panel lit in direct sun, a simple voltmeter should read 5 volts across the resistor. If it doesn't read 5 volts, and if it is much less, like 4 volts, then that tells you that the full rated 1 ampere is not there. Then it gets slightly more complicated to test. But this much you can do with a simple DC voltmeter.

--B.G.--

Bob's method is really good. Very simple if you can connect the resistor. Just use Ohms law to calculate the amps, since you know the voltage and resistance (5 ohms).

"For wilderness travel (if I were so inclined to use solar), I would want a larger panel say 7 watts with a charge controller."

But when you do the calculation, you have to use the solar panel many days before you can charge batterie(s) enough to offset the weight of the solar panel. Better to just bring extra batteries.

If you take a larger solar panel than necesary, then it'll be heavier and it will make even less sense.

You want an undersized panel, don't use any reverse blocking diode or regulator that throws some of your solar panel power away. As long as it doesn't destroy your battery or device.

Really, solar panel only makes sense if you're, for example, at Everest base camp where you're going to be there for a month and you can leave the solar panel pointing at the sun all day.

Jerry,

I agree, I would bring extra batteries.

However, lets consider what appears to be the single most popular brand of electronic devices here on BPL; the iPhone. I understand the allure of the iPhone. I have one. You can utilize an excellent GPS/Map program called Topo (very impressive). You can use it as an iPod or eReader and it has a Kindle application. It has a camera of so-so resolution. Also has a pretty good Web browser. You can text with it. You can even call someone with it :)

The battery is not replaceable, so you cannot bring a spare. You can purchase a battery pack with 4 AA cells to recharge the iPhone. But you still have the same problem, what do you do when you use up your battery pack.

There a 3 strategies with solar.

#1 - extend the battery life. That is, if it is going to go dead in lets say 4 days, you can use solar to extend it for 7 days.

#2 - replenish the amps you use each day to get it to a full charge by the end of the day.

#3 - reserve battery bank. That is charging an external battery pack so that if you run into a week of solid rain, you can use your reserve battery pack.

Under optimum conditions a Goal Zero Nomad 7 will recharge an iPhone in about 2 hours. It will also recharge the battery pack in about two hours. The charge controller has a USB port for your iPhone (5 volts) and a different port for the battery pack (6+ volts). Charging while hiking presents other problems such as orientation and shaded areas. But lets say under these conditions you can charge either in 4 times as long, which would be 8 hours.

So lets consider a thru hiker who may carry a camera, GPS, and a cell phone. Average weight of each is usually around 154 grams or 5.4 ounces. Add spare batteries for each; lets just say another 151 grams total (about what my Garmin and Canon camera weight - actually they are a little more). Now we have a total of 613 grams or 21.6 ounces. If you are thru hiking you may need a charger for your camera batteries and cell phone, lets say 68 grams each for the plug in the wall chargers X 2 = 136 grams or 4.8 ounces. Total weight of this thru hike kit is 751 grams or 26.5 ounces. Plus you now need to go into towns and find access to the grid to charge your stuff up. Okay there might be more efficient ways, but maybe this is what a lot of thru hikers use.

Now lets consider a Goal Zero set-up with an iPhone.

Solar Panel (quality noncrystalline) = 379 g
Battery Pack with cable = 181 g
iPhone with cable (remember enhanced capabilities) = 181 g

Total = 741 g or 26.1 oz

Solar net savings = 10 g or .4 ounce.

Also if you head light uses AA batteries, then you have spares from the battery pack. The battery back also has a pretty bright LED flashlight.

Will I ever do this? Probably not. But for some people it might be a viable option. If I were to do the PCT (which I may when I retire), I would take a serious look at this set-up.

Plus we are not buying throwing away AA batteries for some of our devices, which is an environmental issue, as is using the grid to recharge our batteries on zero days -- if that is a consideration.

Food for thought.