Solar Power Charger

[Tania Proksch]

I wanted to find a good solar power charger for my camera when I'm out in the field, and I think it would be fun to try and build one myself.

I'm trying to charge a Canon LP-E6N Lithium-Ion Battery Pack, rather at 7.2V, 1865mAh. I'm not sure what type of solar panel to buy, and what type. know that the voltage applied should be slighly above the battery voltage in order for the battery to charge. I'd also like to build something that charges my camera relatively quickly. (Although this is very subjective to the sun's presence). Any suggestions on the right panel I should invest in?

Ciao,
Tania

 

[MikeMl]

Get a panel which has an open-circuit voltage of at least 12Vdc. With 12-7.2V= 4.8V of headroom, the panel will be operating at near its Maximum Power Point. If all you can buy is a panel with Voc =~15V, then get that.

 

[Rich D.]

Be very careful. If you just hook up a large voltage to a Lithium-Ion type battery pack, you can over-charge and damage and even get the battery to self-destruct in a chemical fire that can't be put out with water or traditional methods, as the chemicals generate their own oxygen!

Mostly, commercial Li-I battery packs have built-in over-charge and discharge circuitry because of the danger, but it's best to find out for sure.
You will need a commercial type charger, and likely one can be found for 12-volt car battery applications. Find a solar cell that can reach 12 volts or a bit more, and depending on the recharge time, select a minimum current needed.
For example, you have a 1865mAh battery. If it charges in 1 hour, you should have at least 1.865 Amp capacity, if 1/2 hour, double that to 1.865*2=3.73 amps. 1/4 hour charge will need 4X or 7.46 amps.
Basically it's 1.865 * 60/charge time minutes. Add to that about 50 % more for energy loss in the charger and the efficiency of the battery charging process itself, so I would use these numbers based on the recharge time of the recharger you use.
60 minutes................ 2.8 Ahr (also 2800mAh)
30 minutes................ 5.6 Ahr
20 minutes................ 8.4 Ahr
15 minutes................. 11.2 Ahr
10 minutes................. 16.8 Ahr
Those are big numbers, requiring a big solar cell. But that's for optimal performance and charge times. You can always go less than that if you accept that you will not get a full-speed, full-capacity charge in the specified time.

There are several reasons why Lithium type rechargeable are not like other batteries.
1) if heavily discharged, a rapid charge can damage the battery permanently. Most intelligent chargers will trickle-charge (very low current) until the voltage is at a minimum threshold before more heavy charging can be done.
2) once charged, the current must be reduced again to a trickle-charge or the overcharge can permanently damage the battery. It must be shut off at a particular voltage threshold for the battery type.
3) Better battery chargers will also have temperature sensors, to cut-off charging if the battery is for some reason absorbing too much energy that is instead turning to heat instead of potential voltage energy.
There is a fair chance that the Canon battery pack would have the temperature sensor internal, but I don't know for sure.
4) Better Lithium chargers will also have a safety cutoff time interval, if for some unforeseen reason the charge takes an abnormally long time to complete.

I stress the problem of a damaged battery, because unlike other chemistries, Lithium can self-ignite and ruin your day. There's plenty of examples on you-tube and with those Samsung phones recently.

You must ignore battery charging ideas and circuits you might find for Lead-Acid, NiCd, or NiMH battery types - they are much more forgiving and the chemistry and use is much different. Those types of chargers made for the other types of batteries can be damaging and/or harmful to Lithium type batteries.

So again, your best bet is to find a decent commercial battery charger for a 12-volt car source, and use a 12 volt solar cell. I know I have a car-type charger for my Nikon, so I assume Canon makes one too, or any number of possibly a hundred third-party manufacturers that feed off of the big makers, Sony, Nikon, Canon, Olympus, JVC, ...

Of course if you are an electronic engineer, you can design a Lithium charger based on the charge controller chip like I have done: the Microchip MCP78341 / 2 / 3 / 4 series. It worked well!