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  • solar panel battery charge method

    Been thinking of getting a solar panel for general use in the field, camping mostly. It would be nice to have a way of keeping the battery up if I'm parked a few days and use the lights at night, that sort of thing.

    Anyway, I was thinking that since most panels put out 17 volts or more under light load, I could use one to charge a capacitor, then have a timer circuit that would dump the charge into the battery when the voltage reaches a certain level. Between pulses, a circuit would read the battery voltage and decide when to terminate the pulses. I know that nicads in particular like to receive the charge in pulses, so this could be a good way to charge them. I don't think the solar panel will care if it's used this way- it might even be more efficient since it's basically a current generating device.

    The timer circuit might work through an inductor, which could have the effect of allowing current to flow into the battery even during parts of the cycle where the input voltage is lower than the battery voltage.

    It's just a thought- I don't know if the panel would produce more VA product just being hooked up direct, or if it was fed through such a circuit. Anybody have any thoughts on this?
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  • #2
    You need to run a controller between the solar panel & the battery or it will drain the battery at night also you can get a device that lets your vehicle battery only go so low so it will still start.
    "Let me recommend the best medicine in the
    world: a long journey, at a mild season, through a pleasant
    country, in easy stages."
    ~ James Madison


    • #3
      Many panels will produce much more than 17V. You really need to use a solar charge controller. A type of voltage regulator for the charge element in the system. You can build one or buy one. They really are very low cost and you really can't beat the prices. Look here:

      They have an input from the panel(s) and an output to the battery as well as an output for the load. For devices that can use battery power directly these work great. The output wires can be shut off if the battery charge is pulled very low. If you use an inverter to create 120V AC then you can pull directly from the battery and not from the charge controller. The inverters typically have a built in shutdown if the battery voltage drops too low.


      • #4
        Like other have said, just get a charge controller.

        I got ahold of some of the Solyndra solar cells. They are pretty neat, About an inch in diameter and about a meter long. Each one puts out about 114v open circuit at 4w. I took 4 of them and put them in series and charged up a pair of electrolytics to 415v in cloudy weather. Then blew a hole in a pop can with it!

        HSC has some if you are local to them:

        Solyndra solar cells by macona, on Flickr


        • #5
          I don't know the answer to all of your questions but, here is what I can tell you.

          First, the VA output, also called power and measured in Watts, that your panel can produce is a function of the panel size and the light's intensity. The law of conservation of energy says that it will not be any better no matter what circuit is attached to it. The circuit can only waste this output by using it efficiently.

          Next, a rechargeable battery, nicad or otherwise, will only charge when the charging source presents a higher Voltage to that battery than the present Voltage state of the battery itself. So if the battery presently measures 1.1 Volts, then the charger must supply a Voltage that is higher than that (1.2V) for any charging to take place. Since the battery has some internal resistance, the charge current will be determined by the difference in these Voltages divided by that internal resistance (Ohms law). The higher the charging current, the faster the battery will be charged. And the lower the charge current, the SLOWER it will be charged. So it is generally desirable to have a charging Voltage that is high enough to provide at least a moderate charging current. If it is too high, it will produce excessive heat in the battery and cause damage to it so there are limits on both ends.

          One thing that the above means is if you hook up a solar battery producing 16 V to a battery pack that is already charged to 16.1 Volts, absolutely no charging will take place and you are just spinning your wheels. Adding a capacitor and perhaps some switches to the circuit will not change that as long as the solar cell is producing less than the present Voltage level of the battery. This is because the capacitor will only charge up to that 16V. Capacitors do not amplify. Nor will adding inductors.

          There are ICs that are described as switching regulators that, with some external components like resistors, capacitors, and inductors, can boost the Voltage level of the source Voltage. I would think that you really want one of these devices to insure that you have a sufficient Voltage to charge the battery to it's full rated Voltage. Thus, you can take a solar cell that is producing say 5 or 10 Volts and boost that Voltage to perhaps 20 Volts to charge your nicads to that level. Thus, charging can continue even when the light level is too low to provide a charting Voltage directly from the solar battery. Additional circuitry should be used to turn it off when the battery reaches full charge so you do not pump more energy into it which will produce heat and damage the nicad cells.

          I am presently learning about switching PS ICs on another project. I have a future project in mind where I want to scavenge the power in room light to charge some nicads or other cells for occasional use driving some small motors. But I haven't gotten to that yet. I do know enough to feel confident that simple capacitors or inductors just added to the circuit will not cut it. You do need an active device to boost the Voltage. Oh, on the efficiency thing, some of these IC circuits can be 95%+ in the efficiency department. Power Out = 95% of Power In.

          I can not guarantee all of the above, but that is my present understanding.
          Paul A.

          Make it fit.
          You can't win and there is a penalty for trying!


          • #6
            Thanks for the responses. I am aware of most all that's been said. My question is mostly regarding an alternate way that power can be taken from the solar panel and delivered to the battery. I realize that charge controllers exist, and they are not expensive- I suppose I could ask whether they are typically linear circuits, or switch mode circuits using an inductor- the latter possibly being able to 'condition' the voltage and current available from the panel at any particular moment to keep it working at its most efficient operating point. I would guess that the number of cells in series is designed to permit the charging of a 12v battery in a fairly optimum way, so linear operation could be hard to beat. So the question in this regard is- if there is any switching circuitry in a charge controller, does it do a better job of extracting power from the panel?

            Much of what I'm asking and pondering about here is academic anyway- I doubt that there's a lot to be gained by having an elaborate control circuit, and what would it really matter in the real world- if the sun is shining, you will be able to put a charge into your battery in an amount somewhat related to the specs of the panel, and the 'optimum' condition under which it's working.

            Back to my original suggestion, that of charging a capacitor, then dumping the charge into the battery in a pulse- supposing that the battery is good with that, does the repetitive charging and discharging of the cap operate the panel outside of it's preferred range of operation? Academic again, but that's pretty much what I was wondering about. I have no problem rigging up a charge controller circuit, or just buying one, and I do know that in normal circumstances one is required. I think pulse charging is interesting, and I was merely wondering if a solar panel would be a happy part of such a scheme.
            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


            • #7
              The charge controllers are almost certainly a switched mode DC/DC converter. Linear is way to lossy to be of much use here.

              I dont think a battery is going to be happy with a high current pulse. I think you will see early death. I am just not sure what the point of doing it that way is. Either way the energy is going into the battery. Charging a cap and then discharging into a battery is going to waste power. Assuming 100% efficiency of a dc/dc converter and a pulse arrangement it will still have the same average power input to the battery. The battery would charge at exactly the same rate in a perfect world.


              • #8
                If you don't have enough solar power to run your load without batteries you won't have enough solar power to run your load and charge your batteries.

                You can charge your capacitor and your batteries at the same time by wiring them serially. It only introduces another lossy passive component (the capacitor) and results in wasted energy. Same as if you charge only the capacitor with the solar charger then charge the battery with the capacitors. You lose energy charging and discharging that capacitor. If all you wish to do is charge unused batteries during the day and discharge them after dark then you need as much daytime capacity as your nighttime load plus any inefficiencies.

                Other factors to consider is peak load vs average load. Solving that is actually very messy.


                • #9

                  He said charge in the day and use at night. There is no point in having the lights on in the daytime. This is the proverbial solar powered, night light. So perhaps 12 or more hours of charging and 2-4 hours of load. Sounds feasible to me. He just has to get rid of the idea that a capacitor or inductor, without the switching IC, will somehow improve performance. It will not and in fact they would just add more losses.

                  As for the notion that charging a nicad in pulses is better than a steady, DC charging current, I do not know. Perhaps there is an advantage there but I have never heard about it. Most chargers operate from the 60 Hz AC line and pulses are just there and would cost $ to filter out with a capacitor. Probably just cheaper to omit the capacitor. Besides, the battery itself will act much like a capacitor anyway so the pulses will not be seen at the load.
                  Last edited by Paul Alciatore; 06-07-2013, 03:46 AM.
                  Paul A.

                  Make it fit.
                  You can't win and there is a penalty for trying!


                  • #10
                    Regarding charge controllers. They come in regular and MPPT (multiple power point tracking). You should be ok with only 17 volts from your panel for a 12 volt system but if you get into the higher voltage panels you need the mppt controller to get the max power from your panels to the battery.

                    I have some panels that are rated at about 150 watts at 40+ volts and with a regular charge controller I only get about the amperage that I would get if they were 14 volt output panels. A mppt controller would maximize the output to the battery bank and is on my list before I use it again.

                    OPEN EYES, OPEN EARS, OPEN MIND

                    THINK HARDER




                    • #11
                      I have a friend who lives off the grid. Doesn't have to, but that's a philosophical position. He was showing me his setup with solar panels, controller and battery bank and was particularly proud that the meters were showing amps in and amps out - greater out than in.

                      "Look there, more power out than going in."

                      I said, "No, just more amps out since it's doing voltage matching."

                      "No," he said, "More power out."

                      I just shrugged. No use bickering with friends over terminology and he can think what he wants to think but he'll never get more power out of a device than he puts in. Kind of a funny guy. He'd also be a believer in the 130 mile per gallon carburetor and would bend my ear about a monetary theory that banks are creating money because they charge interest and we need to get rid of it because it's a drag on the system. Sigh!
                      "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill


                      • #12
                        Don't reinvent the wheel, you can't make a more efficent solar charger then you can buy and likey can't make one for less then the parts/pcb will cost you.

                        As for linear verus SMPS, the cheapo ones are linear and connect the solar panel DIRECTLY to the battery, rather efficent, then they use a dump load to prevent overcharge once the battery is fully charged. Can only use say 15~17v cells to charge a 12v battery, can't really use 24v+ solar cells since you wouldn't get anymore current.

                        Expensive ones use SMPS, I think its called MPPT? Multi point power tracking? or something along those lines. basicly they are designed to load the solar cell to its 'ideal' voltage for a given amount of sun and can get a few more % power out of your solar cell then the linear ones, but not all that much.

                        at 17W your still likey better off buying a linear charger and buying a bigger solar cell if you need more.
                        Last edited by Black_Moons; 06-07-2013, 12:34 PM.
                        Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.


                        • #13

                          I have a battery bank consisting of 10 L16 batteries at my off the grid cabin in northern Minnesota.
                          There are 5 120 watt solar panels that get some sun about 4 hours a day.
                          I have them connected directly to the battery bank without a controller.
                          The battery bank has two 2500 watt 110 volt inverters connected to supply three wire 220 volt AC power to the cabin.
                          The main reason for the solar panels is to maintain a full charge over the winter when I am not at the cabin. The batteries would self discharge and sulfate over the winter without the solar panels
                          When we are there, a 30KW propane generator recharges the battery bank automatically when the voltage drops to 11.8 volts.
                          The generator also supplies my shop machinery, welders and the well pump directly.

                          I was told not to connect the solar array directly to the batteries but the first set of L16s lasted for 12 years and the current set are 5 years old and going strong.
                          I cut it off twice and it's still too short!


                          • #14
                            Originally posted by Black_Moons View Post
                            Don't reinvent the wheel, you can't make a more efficent solar charger then you can buy and likey can't make one for less then the parts/pcb will cost you.

                            Sure you can make a better one. There is always room for improvement. But it will take a good amount of careful engineering. As for cost, that is harder to beat because they buy parts in bulk and they are free to compromise quality for price.

                            I can do it fast, cheap, and well. Pick any TWO.
                            Paul A.

                            Make it fit.
                            You can't win and there is a penalty for trying!


                            • #15
                              Originally posted by Paul Alciatore View Post

                              He said charge in the day and use at night. There is no point in having the lights on in the daytime.
                              Doesn't matter - the math still holds up. There are easy calculations that can be made for the design of a trickle charger - no external load, just the trickle charger. That becomes the baseline. Add to that the nighttime load that has to be recovered during the day. Additional need-to-knows are: geographical location and insolation data for that region, times of year this will be used, temperature range the batteries will be exposed to, the expected nighttime load (add a fudge factor for daytime load, too). The numbers you get to work with will tell you how many square meters of solar panel is required.

                              Then there are practical considerations. Installing "house" batteries, for example, to prevent discharging the engine's starter battery. Imagine you are on a boat, your only battery is dead, and your radio-telephone is useless. Same with a camper on a long lonely desert road. A proper isolation circuit that removes the panels from the circuit to prevent them draining the batteries. If multiple batteries are used then isolators are needed to keep the batteries from talking to each other. Definitely use the most efficient lighting available. Strings of small lights distribute the limited light very effectively.

                              Here is a helpful site:
                              Last edited by dp; 06-07-2013, 06:03 PM.