Solar charge single 18650: how?

[finn]

Tried going to the "EE university" faq/sticky but the links are broken.

I would like to be able to solar charge a single 18650. I own a dsd charger that takes 5.5v @450mA input. It is easy to find a small pv panel that puts out 5.5v, but at much lower current. If charging time isn't an issue could a low-current panel possibly work to power the charger i own?
Any suggestions would be great. I don't care if it takes 2 days of daylight for a full charge. Has to be portable though.

 

[mdocod]

for a small solar panel your most efficient option would be to use a ~6V panel directly wired to the cell (gut a DSD or other charger and use it as just a cradle for the cell)... Then install a small voltage meter with a push-button switch on the back of the cradle. Manually monitor the charging by checking voltage every few hours by pushing the button to check voltage. Terminate manually when you reach 4.10V, use protected cells as a line of defense against accidental severe overcharge or over-discharge, but be pro-active in trying to keep that cell in the ~3.6-4.1V range.

This is not a perfectly ideal setup, but it would be the setup that gets the most juice from the panel into the cell without any unnecessary losses in efficiency. Ordinarily I would never suggest a charge method like this, but in most cases when people are trying to do this, it's with the intention of back-packing or other survival related gear-configurations that require maximum utility from minimum of weight, in which case we make some sacrifices.

Eric

 

[mwaldron]

My choice for solar charging in general is to use a solar panel in conjunction with a small SLA battery, then connect a conventional DC-powered charger to that system.

It's not very dangerous to you or harmful to the battery to let a SLA sit with a solar panel all day (essentially trickle charging). Also, the SLA can charge during the day and then you can use it to recharge your 18650 (or AA's, etc) at night.

I propose this since you didn't mention a need for light weight, I wouldn't suggest using this for hiking as even a 3AH SLA is fairly heavy even thought they can be fairly compact. I have mine built as more of an emergency preparedness item.

 

[finn]

I should have been specific light weight, at least backpack portable. Thanks for the suggestions though, that manual cutoff one sounds feasible.

 

[TigerhawkT3]

You could get an ordinary Li-Ion charger like the WF-139 and power it with a 12V, 5W panel.

 

[PhantomPhoton]

I use a WF 139 with the car adapter and plug it into a Brunton Solarport 4.4 Or you can put together a solar panel and solar charge regulator and wire up a 12v socket yourself if you're up to it.

 

[mdocod]

will the 139 continue running when the input voltage sags? If you loaded a cell into it, and it tried to maintain ~450mA into the cell, assuming 80% efficiency, you need to make sure 2 watts are supplied continuously to hold up around 12V on the inlet side. Any time you walk into a shadow I'd be concerned about the charger kicking off from low input voltage or something.

 

[PhantomPhoton]

The 139 charger simply stops charging when it receives too little current from the solar charger. It appears (I haven't taken it apart and stood around with the multimeter to test this completely) that it either gets enoughcurrent and works, or it doesn't get enough and thus doesn't work. I do have to watch it when I'm around shadows of trees and such.
I like the Brunton because it has a selectable 12V and 6V output and a USB plug, and thus far it has kept the above voltages within acceptable levels. It doesn;t ue the latest and greatest (most efficient) solar cells but it has gotten the job done for me so far.

 

[nightstalker101]

The 139 charger simply stops charging when it receives too little current from the solar charger. It appears (I haven't taken it apart and stood around with the multimeter to test this completely) that it either gets enoughcurrent and works, or it doesn't get enough and thus doesn't work. I do have to watch it when I'm around shadows of trees and such.
I like the Brunton because it has a selectable 12V and 6V output and a USB plug, and thus far it has kept the above voltages within acceptable levels. It doesn;t ue the latest and greatest (most efficient) solar cells but it has gotten the job done for me so far.

This sounds like your best bet. No matter what, I would say you don't want to directly wire the battery to the panel, especially without a diode (otherwise you run the risk of discharging your battery through the panel.) I have 3 15watt solar panels that I have used to charge my batteries, but I run it through a regulator that brings the voltage to a constant 14.5v, then I convert it to 120v AC and then I charge my batteries (lots of conversion loss, however I have more energy then needed)

I'll I can think is that li-ion should be treated carefully, so think through everything before you hook things up.