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Hello! I have made two small solar panels each measures 4.3V 1A with no load. I am going to connect them in series for charging 4xAA NiMH and separate for 2xAA NiMH. But I am afraid that they have too much voltage for that. When I connect 4xAA voltage drops to 5V but rises up rapidly. Will it rise up to 8.6V? How can I limit voltage to 6v (for 4x1.5V NiMH) on solar panel? Sorry for such a noobish question.
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Lm317
Here are a few links for you to check out. I've used one of these successfully.
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Link to my thread for similar problem -
Use a voltage divider circuit
Well connect a 100 ohm resistor with a 1K variable resistor called a preset. This is called a voltage divider circuit. Connect both resistor in series. Use voltage from the middle point and unused wire of preset. Or else search internet for voltage divider circuit.Comment
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Thank you for answers. But I have found only one method to do this - using diodes connected in series.Comment
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There are other ways, but that is the simplest passive system. You will lose out some on current capability if you operate the panel too close to its open circuit voltage (Voc). An actual DC voltage/current regulator circuit (either linear since it is such low power, or switched like a PWM CC) is better.SunnyBoy 3000 US, 18 BP Solar 175B panels.Comment
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Not sure I follow your logic here. If talking about a battery system, a panel can be directly connected to the battery, not a good idea, but can be done. In effect it is just like using a PWM controller operating at 100% duty cycle.
That is fine until the battery reaches 100% SOC. A panel directly connected to a battery, assuming the battery is less than 100% is just as efficient as if using a PWM controller because the panel is a current source from Isc to Vmp right? But once you hit atound 13.8 volts on th ebattery it becomes fully charged. However at 13.8 volts a standard battery panel Vmp is 18 volts, so the panel is still operating as a current source until it reaches Vmp of roughly 18 volts. At that point the battery is severly over charge and th eplates are corroading away into the battery jar. Once the panel reaches Vmp it is a voltage source up to Voc of roughly 22 volts right. If the battery ever reached Voc, safe to say the battery is toast and open circuit. A boat anchor.MSEE, PEComment
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Allow me to state my concern a little more clearly, since in hindsight I doubt that the OP would get it from my shorthand either:
Since the OP is not in a position to disconnect his 8.6 volt panel when he reaches full charge on his NiMH battery pack, he will need to do something to cut off the current from the panels at that point.
There are at least three basic ways to do that:
1. The ideal would be a voltage regulator which will do what a PWM controller does and cut off the charge when the battery reaches Vmax. Simple to do with available components, and the best solution.
2. Use a zener (or zener in series with a diode or two) in parallel with the batteries to limit the applied voltage. At this point the zener will be taking the full output of the panels at that voltage and will be getting warm. Needs to be sized for ~10 watts. No losses until the battery reaches Vmax.
3. Put a zener and/or rectifier diodes in series with the panels so that the maximum voltage (Voc - Vdiode) is no higher then Vmax of the battery pack. The good side of this is that the diodes will not have to handle as much power. One bad side is that they will be getting warm while the battery is charging. The other, which I was referring to perhaps without enough detail is that the panels will be operating at Vbat + Vdiode at all times, which may be above Vmp and so may be producing a lot less current than Imp, even when the batteries are low.
The last two ways are what I refer to as passive, since there are no active electronic components. Number 1 is active regulation.SunnyBoy 3000 US, 18 BP Solar 175B panels.Comment
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OK I got it. That changes things a bit. Nice thing about NiCd & MiMh is if you can limit the charge current to C/20 or less, you do not have to terminate the charge. With a panel that is pretty easy to do by selecting a panel Imp of the batteries C/20 or less current. Might take several days to fully recharge a discharge battery, but no real reason to terminate the charge. You could have a single cell NiMh 1.2 volt 100 AH, and connect say a Grid Tied 200 watt panel to it with a Vmp of 40 volts and an Imp of 5 amps and never stress the battery. No matter what the panel voltage would never exceed 1.4 to 1.6 volts. Now it would be a Hell of a waste of money turning a 200 watt panel into 8 watt panel, but other than that, no problem.MSEE, PEComment
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Ok, I understand that it will be better to use the PWM controller. I have a DC-DC step down converter based on LM2596 (bought it on ebay, looks like this one http://www.ebay.com/itm/NEW-LM2596-D...item460bf403f9), but it needs the difference Vin-Vout not less then 1.5V, so when I connect it to solar panel I can get only 0.4-0.5A with 5.8V on out. When I connect panel directly to batteries I get 0.77A @ 6.2V. May the problem be in a converter?Comment
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Not likely, the problem is with your opening statement of
The voltage you measured is Voc, not Vmp. I have no idea how you measured the current but I am certain it was not a real value because no current flows with no load or in a open circuit. Voc = voltage open circuit, and yours appears to be 4.3 volts. Estimated guess is Vmp (voltage maximum power) is 3.45 volts. NiMH and NiCD are the most challenging batteries to charge. There are three ways to go about it, but all involve Constant Current methods.I have made two small solar panels each measures 4.3V 1A with no load.
1. Apply a C/10 current for 15 hours. If a timer cannot be used then apply C/20 for at least 30 hours or indefinitely. This is this simplest method but is not practical for solar as that could take two weeks or more.
2. Apply a C/3 current for 5 hours and terminate.
3. Apply 1C rate but you must use a delta V and Delta T monitoring circuit to measure the sudden rise in voltage and temp that signals full charge and terminates the charge immediately. Otherswise if you do not terminate the charge, the battery will terminate itself with a BOOM.
But to answer your question the easiest way to regulate the voltage is just a simple series diode. Diodes have a voltage drop of .2 volts for geranium, and .7 volts for silicon.
This is why you do not see NiMH or NiCd used in solar applications. However there is one exception and that is in the Remote Controlled Toy biz like RC Planes. You can buy NiMh solar charge controllers at most RC hobby web sites.MSEE, PEComment
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I have measured short circuit current, connected panel to multimeter directly. I am bad in electronics, sorry. Thank you for help and your time, I appreciate your help. inetdog, thank you too, I have got some valuable information from your posts.Comment
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Th eLM317 can be used as either a current source or voltage regulator. Problem is his panel voltage is too low. The output needs to reach at least 6 volts minimum to charge 4 MiMh cells in series. He could make it work for 3 or less cells, but 4 is just out of reach. His panel Vmp is around 7 volts and the LM-317 needs at least 1.5 volts minimum between input and output.
If you are going to build your own charger it is assumed you have the electronic background to do so. Otherwise you should not attempt.MSEE, PEComment
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