Solar cell recharging of 18650 cells

[xevious]

Can it be done?

My idea is to connect a solar cell array to a 18650 charger... But I'm not sure if 18650 cells requires a certain minimum input to charge properly. Can they be trickle charged? If so are there any issues to consider, such as impact to cell life and memory?

 

[regulator]

You cannot directly connect the solar panel to the cells for charging since there is no "smart" electronics to monitor for proper charging voltage, current, and overcharge. If you had a charging unit that could work off of 12 volts then you could possibly use a panel to power that. However, larger solar panels needed to adequetly power the charger are higher than 12 volts and require a intermediate regulator.

In short it can be done but the equipment needed will be rather expensive. A simple example: solar panel, solar charge regulator, 12 volt capable lithium charger.

You would probably want at least a 20 watt panel.

 

[Fallingwater]

Good news: it can be done.
Bad news: it's going to be slow.
Worse news: it's going to be alarmingly expensive.

 

[EXPY]

I haven't done it but have what is necessary in my preps.

Start with a decent sized 18VDC nominal panel, as already mentioned, 20 watts or more should do the trick in good sun condx.

To 'regulate' the output of the panel. connect an appropriately sized 12VDC gelcell battery to it. [Other kinds of battery chemistries can be used based on your resources].

Use a simple cutoff type charge regulator if you aren't able to monitor the panel/battery charge condx in real time. Harbor Freight has them.

The battery has the added benefit of accepting a high charge rate from the panel in peak sun conditions and storing the energy so you can charge in darkness at your leasure.

But due to charging inefficiencies, be aware each time you charge any battery some energy is lost as heat so there are significant losses that should be minimized with proper planning. Obviously a larger panel helps offset this.

To charge your lithium battery, a least efficient method would be to connect a good, small 12VDC inverter to obtain 120VAC and power a standard charger directly. Or, use a charger having a 12VDC input directly.

Alternatively if you understand the issues involved, is to use a variable series resistance [if necessary, it could even be an LED providing light or other device] with multiple batteries in series, while monitoring charge current and terminal voltage of each cell. My plan to save energy and improve efficiency is to charge more batteries to less than a full charge.

Hope this gives you some ideas.

 

[PhantomPhoton]

Well it isn't too hard really, you need a 12V input at around 450 mA for the common Ultrafire 139 charger. Easily provided by a good solar cell setup. Luckily they make a lot of solar stuff for around 12V. There will be cost in getting decent solar cells but beyond that step it's easy. Unless of course you live in a place where there isn't much sun like me, then you need a lot more solar cells .

 

[Curious_character]

Solar panels act pretty much like constant current sources. A typical "12 volt" solar panel has an open circuit voltage of around 20 volts, and the current at 12 volts isn't much less than the short circuit current. So if you try connecting one directly to a charger, the solar panel voltage can be quite a bit more than 12 volts if the charger requires less current than the panel can provide, and the voltage can be much less if the charger draws more current. A lot of devices like chargers designed for operation from a more-or-less constant voltage source get unhappy when supplied by such a different source.

Directly connecting a solar panel to a lead-acid battery to stabilize the voltage will make your charger a lot happier, but the battery can be permanently damaged unless it's big enough to tolerate a continuous constant current charge equal to the solar panel's output. This means a big battery or a small solar panel.

The best idea is to get a voltage regulator specially designed for use with a solar panel. Connect the solar panel to the regulator, the regulator to the charger, and the charger to the battery you want to charge.

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