Solar Based Battery Charger.....

Hi Mujahid,

Don't forget that the charger itself is consuming energy and that it takes extra energy to store energy into a battery. Your overall system efficiency will be signifant lower than 100%. I would estimate, 90% for the charger and 85% for the battery, resulting in 0.9 x 0.85 = 76% overall efficiency (1W should than be at least 1.31W). I think that the maximum panel output power will not be reached because of the fluctuating light intensity. Try to estimate this!

Best regards,

Enrico Migchels

Charge the device instead of the battery directly.  Measure the output voltage of the charger and use an off the shelf dc-dc converter with the output set to match the chargers voltage and a solar panel that matches the input requirements of the converter.

Hi, Enrinco:

I agree with your point, the conversion efficiency from the ouput of solar cell to Charge IC, the power consumpation of Charge circuit, Charge efficiency on Batttery, the vary of the light intensity should be considered when we calculate the power rating of solar cell. Do you know if there are any common formula for this calculation in the industry? Or any solar cell suppliers can provide the application notes about this kind of design?

I am also curious about how to test the power conversion  from light to electric power, any testing equipment can help to get this data. I know that is very important parameter for this solar charge system. Thanks.

Hi Austinsu,

I'm not an expert on this but i would assume that you use the specification of the solar cell provider to work with. You need to estimate the average power coming out of the cell in 6 hours time. This is pretty much based on the field of use. (place of use, time of day, etc). You need to come up with statistical data that you estimation is that in .. % of the chargecycles the output power of the energy cell is .. W. You make your design based in those figures. Try is simulate this by using an articifial light source (which matches the spectrum of the sun) and measure the light intensity with a lux meter. (lacie sells those devices). Use MPPT for the charger unit and the efficiency of the whole chain is measured by 6 hours of xx light intensity on the cell and after that; discharge the battery on a electronic load (current constant) and trace the voltage-curve. This will give to the amount of watt.hours available for the application.

Best regards,

Enrico Migchels

You must consider that the nominal power of the solar panel is referred at full  and perfectly perpendicular sunlight!

using it for portable device I think that is impossible to have this condition for 6 hour!

I think you can multiply the time of charge for 2 or 3 times....or more, or use a bigger solar cell.

franco

We are designing the circuit for solar charger; we have a small problem in selecting the Inductor , even though there is a Equation given for Selecting the Inductor, I tried for the Application Information given in the datasheet they say Delta Imax to be between 0.25 to 0.35 of the max. charging current. But When I do reverse calculation for Delta Imax, for the example below its no where matching. Please can anyone help me in giving the solution for the same?

Hi Mujahid,

As per the Discussion we had with one of the Support Engineer from Linear Tech. , VIN(max) = 32 V(Maximum input Voltage to the Charger IC) and not peak power voltage from the  panel. Also ΔImax is not 30% of charging current, its value should be between 0.25 to 0.30. By using these values in equation for Selecting the Inductor, we get the Inductor Value.

Regards,

Chethan R.S.

Hi All,

Please check with the schemaitcs that I have designed around LT3652 for the solar based battery charger.

This schematics is for solar input of 7.5Vs to charger to charge a Li-ion battery of 3.7V 740mAh.