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Getting the Most Out of Your Portable Solar Charger

Portable solar chargers are appearing in many different forms.

They range from mobile phone-sized units with a single solar panel and a socket to accept the charger cable to large portable folding solar panels that close like a notebook.

Some mobile phones manufacturers are already playing around with the idea of integrating a solar panel into the back panel of a mobile phone to enable solar recharging in addition to the usual charger, but realistically that will probably not be economically justifiable because mobile phones are consumables that eventually become obsolete or just out of fashion. It creates another component to dispose of. We already have considerable environmental damage caused by irresponsible dumping of old electronics and should not add to that tally.

That idea is doomed to the same fate as the TV/ VCR ( who even remembers those?) Far better to have a separate solar cell that has universal charging capabilities. Then the the solar charger can be used to charge several personal devices. But, who knows, I might be wrong! we may soon see notebooks and PDA's with built-in solar panels too!

Getting back to our main topic; chargers that use the sun to generate electricity that is stored in a battery, to be used when required, make a very useful and environmentally friendly alternative to AC powered chargers. You can use them when you are camping for example, no need for long extension chords or noisy generators!

Some solar chargers and portable solar panels can be connected directly to a device that requires power, for instance the Soldius solar chargers have inbuilt voltage regulators with MPPT ( maximum power point tracking) electronics to charge a mobile phone more efficiently. It is an elegant portable solar charger with just the one socket to accept the charging cable and a selection of adapter tips that connect to a mobile phone or other personal device.

The Soldius is a small and compact unit but its lack of an internal storage battery means it can't store power for later use as the other solar chargers can, so it is actually more accurately a mini solar power generator with an integrated voltage regulator. True environmentalists see this as a plus because batteries are one of the worst offenders of environmental contamination by heavy metals. In fact the eco advocates would be pleased to hear that most solar chargers use eco friendly Li- Ion batteries. Lithium Ion cells are more eco friendly because they do not use heavy metals.

We should differentiate between solar chargers and portable solar panels.

A solar charger is a single unit able to store solar power for later use because It has internal batteries for storing the charge. Portable solar panels are units made up of one or more solar panels and sometimes an attached solar regulator like the the folding solar panels from Ecopia.

A useful feature of most solar chargers is the ability to charge its internal battery using other means than solar power such as USB port power from a computer or 12 volt power from a vehicle's accessories power socket if immediate solar power is not available. The green advocates might see this as a sell out to the 'evil brown energy' industry, but this need not be so, you might be running your laptop on solar power and a vehicle's battery is recharged while driving and normally has excess power to spare.

Not all solar battery chargers are capable of fully charging your mobile phone.

Why? because they are using the solar charger battery to recharge another battery and these solar batteries are usually not big enough to provide a complete recharge.

A typical solar battery has a capacity of 1000mAh and a typical mobile phone battery is 750mAh.

Now, you might think a 1000mAh battery should fully recharge a 750mAh battery, after all its 25% larger.

Not so. As the mobile phone battery is recharged it builds up internal resistance and the voltage of the solar charger battery just can't overcome this internal resistance once the phone battery has reached a certain level of charge.

Voltage (V) is electrical pressure equal to current flow (I) multiplied by resistance (R) V= IR

. The internal resistance of the solar charger battery decreases as its charge decreases and therefore its voltage decreases, but the opposite is occurring in the phone battery, as it is being charged up its internal resistance is increasing to the point where it is greater than the solar charger battery voltage so current stops flowing into it and it is no longer being charged. The better solar chargers have clever electronics to help to get the most out of the charger but at the end of the day you can't cheat physics and the only way to improve on this situation is to increase the solar charger battery size.

This is why larger solar panels have charge controllers, so as the battery charge increases the charging voltage changes to ensure the battery is fully charged.

Always choose a solar battery charger with the largest solar battery.

Therefore when selecting a solar charger look at the capacity of the solar battery, it will show capacity in Milli-amp hours, for example 2000mAH ( 2000 Milli-amp hours) One thousand Milli-amps equals one amp. In terms of battery capacity it will provide 1 amp of current for two hours, 100 Milli-amps for 20 hours, 200 Milli-amps for 10 hours, etc.. before it runs flat. Well, that's not strictly true because the battery volts will drop at about 20% capacity and the battery might appear to be flat at that point, and that's another reason why recharging using one battery to charge another battery is not as good as a constant electrical supply. Solar chargers use Li-Ion ( lithium ion) batteries. Li-ion are the new type of light, flat battery and have advantages over conventional Ni Cads and the older lead acid cells,they are lighter and can be discharged more before needing a recharge, and most importantly they are eco friendly.

Always choose a solar charger with the largest solar panel

Once you have chosen from solar charger with a good-sized battery you have to use the solar cells to charge it. Choose the solar charger with larger solar cells ( we can call them solar panels, but strictly speaking a solar panel is usually a larger separate panel composed of several solar cells)

Sometimes solar cell size is not given as watts but instead the voltage and current (Volts and Amps) are given, so to find the watts we multiply volts by Amps, (W = IV) for instance a solar cell specifications might state: 5.5Volts / 330 mA. Multiply 5.5V by 0.33A to find watts, therefore output is about 1.8 watts.

Armed with these facts you can select a solar charger with some confidence that it will fulfill your needs.

About the Author

John is a technical consultant for Ecopia. He has a background is in engineering and an interest in solar power and wind energy.

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