I'm doing some research on solar-energy battery charging. I have read the following article: http://www.windsun.com/ChargeControls/MPPT.htm (An article about Max Power Point Trackers) that regulates the current input so that the battery always is charged with the highest power, hence the highest current.

What is not mentioned though, is how the battery can handle this current..? Can any 12 V battery handle these charging speeds applied by the solar panel/MPPT's? What happens after the max current is adjusted by the MPPT, is it just applied to the battery?

 

AFIK, MPPT chargers are all about operating the panel itself at its maximum power delivery sweet spot, assuming that the battery is capable of absorbing all available charge. If the current must be limited by the battery's charge acceptance rate, or because the battery is charged, then the MPPT operating point goes out the window...

 

EDN published an interesting article about a solar MPPT charger earlier this year, including a schematic and the formulas used to design the charger. Can't remember who wrote it though.

You really need to get an idea of the battery AH capacity before you start charging it. Like I said in the other thread, if you charge it too quickly you risk excessive internal temp rise and resultant short battery life. Once batteries get heated up, it takes a very long time for them to cool down.

 

Solar tracking to keep electricity production at a maximum in a solar panel is a desirable goal. Now the hard part...Any battery requires a specific voltage level for charging, too little, and you don't charge the battery very much...too much and you start to boil or overheat the cells. For charging a lead-acid cell, a regulated level of 13.8 vdc is typically used. This can vary with temperature. The amount of current that the battery requires is determined by the battery chemistry, not the solar array!
If the battery is calling for current and the sun goes down or behind a cloud, you are not going to charge your battery until the sun is back. Hopefully, all charging is complete before the sun goes down...but I'll bet it isn't!

Best wishes, DPW

 

Think of MPPT as "impedance matching" between a solar panel and a load (the battery). The battery sets its own terminal voltage during charge. The MPPT is basically a switching step-down regulator which tunes itself such that the panel delivers maximum power (volts X amps).

 

From V-I plots on a small solar panel, it seems that if the panel is loaded down to where the V is 66% of the open ckt V is at the max power point. The operating V only shifts one or two V for obscurations of 36%, 68% ref to full sun, but the peak is fairly broad. With this 18V, one W panel, 920 mW covers 8 to 15V