lipo battery charging directly with solar panel

Question

Hello, 
 I am building a solar battery module for LED lighting application. Task here was to use least hardware possible to reduce the cost per board. I read online that a solar panel with less than 5 watts can be used without a charger circuit. my battery is 
 also a smaller one with 7.4 volts. panels outs 12 volts, < 5watts. can I keep a resistor in between them and a diode in series for unidirectional current flow? will it be sufficient for general purpose use?

MK.Mcleary · Accepted Answer

A lithium ion battery should never be changed without overcharge protection. Some batteries have a built in protection board. This prevents overcharge and overdischarge. 
 In case there is such a built in protection, it's fine to just use a diode and resistor. 
 (If it is a 7.4V battery, it is very likely that it already has such a protection and balancing circuit) 
 How big is your battery?

vishnu_n93 · Answer

A diode would only help you to control the direction of current flow, and ensure that your batteries do not get drained through the solar panel. However It does not provide overvoltage protection for your batteries. Depending on the voltage and curret profile of your solar panel, the battery may initially charge without any issues as long as the voltage stays below 7.4V as you draw current. But as your battery voltage approaches 7.4, and the solar panel is capable of providing upto 12V (or actually more depending on sunlight), the voltage would now rise, thereby damaging your batteries. The cheapest solution would be to put a zener of 7.4V and high enough power (>5W in this case) parallel to your solar panel, and then continue with the series diode and resistor approach. Even though this solution is not perfect, it should solve your problem.

kkazem · Answer

Even though only 5W, I would recommend to charge the battery thru a 3-terminal linear regulator if you want maximum simplicity. You'll need to know the fully charged voltage and can use either an adjustable regulator with a calculated voltage divider, or a pot and adjust it once for no-load maximum output. This will protect the battery from charging at a 12v level. Or even simpler, for the adjust pin (or ground pin), use an LM7805 or equiv 5V output 3-terminal regulator and put a 2.4V zener diode between the gnd or adj pin and battery ground with the cathode toward the IC's gnd (or adj) pin. That will bring up the regulator's output form 5V to 7.4V. If 7.4V isn't the battery's fullly charged Vout, then the zener value might need to be adjusted. But this technique works with most any 3-terminal regulator. So you can use a 3.3V regulator and a 5V zener for an 8.3VDC output. Just sum the regulator's Vout with the zener voltage and that will be the circuit's regulated output voltage. 
 But caution, you may need to heat sink the 3-terminal regulator (but not the zener diode), depending on the max charging current. You could have about 7watts dissipation in the IC, so use a TO-220 package and heat sink it. Even if you don't it won't go bad as they typically have overcurrent protection and overtemperature protection, but if that kicks-in, you won't be charging till the IC cools off again. 
 The bottom-line is that an IC 3-terminal regulator like a TO-220 version of an LM7805, is low cost, low parts count for an increased Vout (2-parts, the IC and the zener diode), and gives several modes of protection for your battery.

lipo battery charging directly with solar panel

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