Charging Rechargable Batteries With Solar Panel

Hello everyone!

Time for my second question post of the day :sigh:... Would it be possible for me to charge this battery (http://www.dealextreme.com/details.dx/sku.1283) by feeding it less than or equal to a constant 4v 80ma using this solar panel (http://www.dealextreme.com/details.dx/sku.5649)? Should I include a diode from the power source when charging the battery? A better suggestion for batteries and solar panels (that are sold on deal extreme)?

Any information could be of help. Thanks!

-Green Lantern

That's not a good plan. Firstly the cell is a lithium ion, and they should only be charged with properly designed and matched chargers that provide carefully controlled and regulated charging conditions. Ad-hoc charging arrangements should never be used.

Secondly, that solar panel at maximum output produces 80 mA. With a 2400 mAh battery it would take over 30 hours of bright sunlight to get a full charge. With anything less than bright sunlight the output will be less than 80 mA and the charging time would be longer.

I'd recommend you consider using NiCd instead of Li-ion. NiCd batteries are robust to uncontrolled charging and have a capacity that is more in keeping with the available charging current. A really convenient way to get solar panels for charging batteries is to find garden solar lights, especially ones on clearance in discount stores and garden centers. You will get the same kind of solar panel as the one from DX for the same or lower price. Plus you will get the charging circuit, battery holders and some NiCd cells into the bargain.

I've wondered this myself and though I don't have a good answer I'll share some thoughts and see if they can amount to anything.

DX also sells a number of USB solar battery packs such as this:
http://www.dealextreme.com/details.dx/sku.8222

The ones I've looked at come with a lithium rechargeable battery inside which are generally a flat type battery similar to those used in cell phones, so if there is circuitry needed to charge them via a solar panel then these units should have those circuits built in. These units are just about twice the cost of the solar panel you listed and I'm guessing they could be taken apart to provide the panel and circuits to charge any 3.6 lithium type battery even like the cell you listed.

However, even with these units I doubt the designers were super concerned with getting the most capacity out of the batteries they used so it's likely that they just matched the solar panel's output to be a few volts above 3.6 volts and let the battery take in what power it could at the trickle provided. With the small charge currents being outputed by the panel there is likely no danger of ever over charging the battery or even worrying about the controls and regulations any descent charger would deal with. But as Happy pointed out it could take a few days of charging for one of these little solar panels to charge anything close to a 2400 mah battery.

A note about the solar panel you mentioned. It lists the output current as 60-80mA ISC. ISC is the short circuit current which will be much higher than the actual current outputed under load. Expect the actual current that would be outputed for charging puposes to be around 50-60ma. You would need ~8 of these little panels in parrellel to get close to currents that would approach charging a single 2400 mah battery in one day of sun.

Where you have to be careful is that the "4 volt" output from a solar panel is a nominal rating. It is not the exact or maximum voltage produced by the panel. You might find a 4 volt panel in some conditions producing nearly 6 volts (depending on the particular panel type, the brightness and the ambient temperature).

So if you hooked this panel directly to a lithium ion cell and left it running once the cell was charged there is nothing to stop it bumping the cell voltage up above 4.3 volts. At this point the cell will start to be irreparably damaged.

This is why proper lithium ion charging circuits cut off completely when the cell voltage rises above 4.2 volts and never allow the battery to be trickle charged beyond that.

Well it's not just irreparably damaged- overcharging may result in fire.

Even clamping at 4.2V is not really "correct". At 4.2V it will still consume overcharge current if completely charged. The recommended termination was to hold at 4.2v, wait for the current to drop below x mA, then cut off charge completely.

The problem there is that solar can't guarantee it can perform this algorithm even if you had the ideal circuitry. It might be cloudy and thus unable to supply x mA at 4.2v for the duration of this phase. At that point the designer has to start guessing at all the possible scenarios and it gets messy.

Also after the charge has been cut off completely, the algorithm is somewhat unclear on when to restart it. I mean if you drain 10mAH off the batt then do we restart and wait for that termination process again? Because that's gonna deliver more than 10mAH for sure and that's an overcharge. I think once it's done a charge termination you need to wait for the voltage to drop below a certain threshold before restarting the charging phase to ensure it does not overcharge.

Oznog said:

The problem there is that solar can't guarantee it can perform this algorithm even if you had the ideal circuitry. It might be cloudy and thus unable to supply x mA at 4.2v for the duration of this phase. At that point the designer has to start guessing at all the possible scenarios and it gets messy.

Click to expand...

This is the main reason why you need some sort of "buffer" between the solar cell and the charger. To do it safely you pretty much have to have something that evens out the power supplied from the sun, I use a small SLA battery but as mentioned in a similar thread on solar charging, that's impractical when weight is an issue.

Thanks for all the help everyone. I guess maybe I will have to put off this project for later. I'll have to find something else to satisfy my flashlight obsession... Oh well, the TK11 will have to do.