# Current output of a solar panel when connected to battery and load

I am trying to build a microgrid (final year project) where a 100-250 W 48V solar array will charge a 24V battery bank and a couple of 24V loads (40W lights, 60W fan).

Batteries used (7 of this in series) : http://industrial.panasonic.com/www-cgi/jvcr13pz.cgi?E+BA+3+ACA4001+NCR18650+7+WW

I will be using a buck converter to step down the voltage of the array to that of the battery.

Assuming the power of the solar array is 200W, 200/48 = 4A~ After step up by the buck converter, it should be around 8A?

Now I am trying to figure out what will happen if only the battery is connected across it. Will it only draw as required (what happens to the solar panel's excess current?) or will the 8A of the solar panel's output be forced into the battery? (and maybe spoiling it).

Also, if the battery is near fully charged, how do I know that? Is it by checking the voltage level? Do I have to turn off the mosfet switch then (cut the connection between the array and battery)? I will be using a microcontroller to regulate PV supply using the buck converter.

• You need a charging controller. Hooking up lithium ion batteries directly to a solar array is not going to work very well. Feb 20, 2015 at 3:22
• May I know why? Since this is a school graded project I can't very well buy a commercial one... There are certain chips out for this purpose right? I can also design and fabricate PCBs for this purpose. Feb 20, 2015 at 3:51
• You can design the necessary circuitry. You seem to lack a lot of the necessary knowledge, but you can fix that if you have time and internet access. I have a question: are you free to select different batteries? For example, could you use lead acid 12V batteries? If so, I recommend it as safer and a little bit easier. If not, then you will have to use lithium batteries. I think you need to read about two topics, mainly. One is charging methods for Lithium Ion (or lead acid) batteries. Second topic is maximum power point tracking (MPPT) solar charge control. Feb 20, 2015 at 5:46
• Yup I know about MPPT, spent a long time simulating them in Matlab. I think I am free to select different batteries, but I'd like to know how to control the way they are charged and would appreciate if you guys give me advice or point me to suitable literature. Thanks! Feb 20, 2015 at 8:59