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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.

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  • \$\begingroup\$ You need a charging controller. Hooking up lithium ion batteries directly to a solar array is not going to work very well. \$\endgroup\$
    – darron
    Commented Feb 20, 2015 at 3:22
  • \$\begingroup\$ 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. \$\endgroup\$
    – Mittens
    Commented Feb 20, 2015 at 3:51
  • \$\begingroup\$ 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. \$\endgroup\$
    – user57037
    Commented Feb 20, 2015 at 5:46
  • \$\begingroup\$ 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! \$\endgroup\$
    – Mittens
    Commented Feb 20, 2015 at 8:59

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First of all you need to be very careful when designing with Lithium-ion batteries. If you exceed their specified charge or discharge rates they can leak or explode, and nobody wants that! You definitely need a charge controller. This will regulate the battery charging to safe levels, and follow a recommended charge profile, which is typically to charge first at a constant current until the cells reach a certain voltage, and then to top off at a constant voltage. This is called "CCCV" charging. You also need something to limit the discharge rate of the batteries. This can be a very simple circuit, but for safety you need something. Your buck converter can supply both the load and the charger. Any excess capacity of the solar panels will just not be used. The panel output voltage will probably rise above 48V at lower loads. For 7 cells, a linear charge controller is not good as at low battery voltages the power dissipated will cause overheating. You probably need a switcher there too, either buck or SEPIC. If you can design a pcb for this, so much the better. I appreciate that your school would probably not really appreciate it if you went and bought this on Amazon!

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  • \$\begingroup\$ Thanks for your answer! I'd like to know if I need to regulate the current between the buck converter and the battery, or will the battery draw only the current it needs (at the empty state, half charged state, and nearly charged state). From what I read, all I have to do is just make sure the voltage of the battery doesn't exceed a certain limit, and then cut off the supply using a shunt. mpoweruk.com/chargers.htm Charger Types Shunt Regulator Buck Regulator \$\endgroup\$
    – Mittens
    Commented Feb 20, 2015 at 8:55
  • \$\begingroup\$ You absolutely must regulate the charge current going to the batteries, otherwise they can overheat when charging from a discharged state, and possibly explode. The battery makers specify a maximum allowable charge current, as well as discharge current. If you like I will send you a little block diagram of a proposed topology. I suggest you look at Linear Tech and TI's products. You have too many batteries in series to use a linear regulator for charging. Lead-acid is an option, but they have their own drawbacks too. The basic topoloby would be the same. \$\endgroup\$
    – charliemcandrew
    Commented Feb 21, 2015 at 18:58
  • \$\begingroup\$ The cheat's way of ensuring that you don't overcurrent the batteries is to overprovision them: arrange enough batteries in parallel (careful about balancing them first!) so that the "C" rated charge max current of the pack is greater than the maximum possible output from the solar panels. \$\endgroup\$
    – pjc50
    Commented Jun 1, 2015 at 14:14

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