I've used solar chargers and power packs to charge things for a while now whilst possuming. They came up in a separate thread, so here's my experiences.
Summary
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Powerpacks alone:
You can use a powerpack to charge most things there days - phone, gps, etc. This seems handy as it avoids carrying all shapes & sizes of spare batteries.
However, powerpacks alone, plus assorted cabling to connect devices to them, were much bulkier and heavier than just carrying spare phone/gps lithium ion batteries. They were, however, effective at doing their job, but delivered 1/2 of their nominal charge into the battery they were charging. They were also near-useless for charging AA/AAA batteries.
Solar charging:
Solar->USB chargers were great for some devices (qstarz gps, degan radio) which would trickle charge off them in any light conditions.
However, for phones the only way to charge the phone in anything other than direct sunlight was to go solar -> powerpack -> phone, as the current requirements of the phones charging exceeded that generated by both solar panels tested.
For extended stays in one place the power packs have the advantage that you can leave the powerpack charging off solar panels at camp/hut all day, whilst you use your devices in the field, then can charge from the power pack to the device at night.
Details
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Powerpacks
Power ratings:
The process of charging a lithium ion battery seems about 60% efficient - i.e. the power required to do:
charger -> powerpack -> phone
was approx 2x the amount of power required to do:
charger -> phone
So if the power pack says 9000mAh, you get closer to 5000mAh into your phone batteries
Weight worth it?
My goal zero 8700mAh power pack (~5000mAh to phone) weighs 250g. Spare phone batteries weigh 45g. So on a weight-for-weight basis it's not worth using the powerpack. Plus, to charge camera, gps, phone I required 3 different USB cables to got from the powerpack to the device.
Solar charging
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I've bought a couple of USB-output solar panel sets. One from goal zero for $150 including above power pack, another online, unnamed one from dx.com for $30.
Both weigh 250g and are 140x150x20mm, and 120x120x30mm respectively. Both give pretty-much identical output under load, despite the dx.com one being rated 5W output and the goalzero one being 3.5W.
Charging devices:
QStarz GPS, Degan radio
Both of these devices will charge off the panel direct in most light conditions, only cutting off when it gets dark.
Samsung galaxy phone
This only charges directly in bright sunlight. In other conditions (under cloud) it shuts-down the charging process
Charging powerpacks
My 8700mAh powerpack took about 2 days to charge completely in normal conditions (sun with occasional cloud). This could be improved if you're around to reorientate the panels to the sum at all times.
Charging AA/AAA batteries.
I tried 2 different USB AA/AAA chargers from dicksmith and dx.com. To charge a AA/AAA you need a voltage somewhere just over 1.2V The powerpacks produce approx 5v. Both chargers I tried appeared to drop the 5v out of the powerpack down to the 1.2v+ required to charge the battery by disapating the remainder as heat. This was hugely inefficient. A fully charged 8700 mAh (holding about 40 watt-hours of power) power pack could only just charge two 2200mAh 1.2v AA batteries simultaneously (holding 5.2 watt-hours) before going flat.
Both chargers would just run direct off the solar panels in full sun conditions, but shut down if cloud was present, of the sun panel was not pointing directly at the sun.
More efficient USB->AA/AAA chargers may exist, but I failed to find any for sale with enough details in their technical specifications to convince me to try them.
This post has been edited by the author on 15 May 2015 at 10:46.