I have a device that needs to run 24/7 away from any power source. It can run off a 12V supply and I want to use a 12V battery that will be charged during the day via a number of solar panels. As far as I am aware both the voltage and current of a solar panel may change depending on the sunlight intensity. What is the best way to charge the 12V battery at 13.8V from the minimum number of solar panels ? Some of the circuits I have seen are simply a number of solar panels in series connected to the battery via a diode.
What you are looking for is a power point (or maximum power point) tracker. I thought they were fairly common for larger solar installations. Try a search on those terms, you should find a fair amount of material.
Not recommended. When the ambient temperature drops, the open loop voltage of the solar panel rises. You can overload a battery this way, reducing the battery lifetime considerably. Opt for a regulator.
As I understand them, solar panels look just like a current source. They can have a high open circuit voltage up to around 19V if they are not loaded down, so you may want a way to stop charging the battery to avoid overcharging it. Mostly, it is just the current that changes with light intensity, not the voltage. You can maximize solar panel efficiency, by deliberately keeping the voltage high, for a given load, by using a step down switcher, that is set up to regulate both the input voltage, and output voltage. I did this for a design many years ago, and it worked quite well.
Some areas of the world do not receive much light at certain times of the year, so you will need to size your panel up to 40 times larger than the average current draw that you expect to have. Also plan on having at least 2 weeks of standby battery supply available, to account for times when there may be very little light available.
Do you perhaps have an example circuit diagram ? I am not sure what you mean when you say you regulate both the input and output voltage. If one connects a resistive load directly to a solar panel, wouldn't both current and voltage change depending on the amount of sunlight received by the panel ?
I am in South Africa, so the difference between summer and winter is fairly small, and the amount of sunlight received fairly high compared to places like europe. I do not want to have to use more solar panels than I have too because my charging circuit is less efficient than it can be. If I can get away with a cheaper less effiicient panel then having a more complex charging circuit would be worthwhile.
In principal would a buck, boost or buck-boost regulator be the most appropriate for charging a 12V battery from a (cheap) solar panel ?
Yes both current and voltage changes into a resistive load. The idea is that you want to maximize the power transfer. At a given light level, the panel will output basically the same current (since it looks like a current source with a limited output voltage), regardless of the output voltage, so you will get more power out if you keep the panel voltage high. But, remember that your battery does not look like a resistive load. The battery voltage will not start to climb, until it is reaching the end of charge point.
The basic idea is that you assign a priority to the the two control loops. You always limit the output voltage to your battery, but you try to regulate the panel voltage at the same time. The main feedback loop tries to keep the input voltage regulated to say 19V or so, but if the output voltage gets too high, then this will back off the output, regardless of what happens at the input.
The design that I played with, used a 78S40 regulator that has an extra uncommited opamp. The main feedback to control the regulator came off of the input side which loaded the panel down appropriately to maintain the set input voltage. Then the additional opamp was used to buffer a feedback voltage from the output side and fed through a diode to pull down on the input side control point, so trick the control into thinking that the panel voltage was now too low, which backed the regulator off. This control method is the reverse of what most regulators do. The 78S40 was nice for this, as you can pick the direction of the feedback control.
You just need a simple step-down regulator. Or, if there is a microcontroller involved, and you are not worried about maximizing efficiency, then you could just short the panel out with a FET to turn it off (or use a P-FET to switch the high side) when the micro reads that the battery is at full charge.
These are the onse I have seen that only uses a diode to connect to the 12V battery. This I believe is far from the most efficient way to use a solar panel, and will also reduce the life of the battery quite dramatically depending on solar conditions. The ones I have seen with anything more sofisticated are much more expensive. If you know of actual manufacturers of oem panels with charging circuits, I would appreciate it.
The ones I've seen have proper charge controllers. They have to - typically they're used by "boondockers" with large battery banks who definitely expect their batteries to last for years. Obviously, proper controllers cost more than a simple diode, but they're necessary, since the panels typically produce 16-20 volts.
There are lots of questions. What is your power consumption? Is this a one-off item, or are you planning to mass produce them? If it's one-off, where will it be installed (what latitude)? Actually, latitude is only part of the environmental considerations - you have to plan sufficient capacity to tide you over stormy days, for example. What's the budget? Again, if this is a one-off item, the best bet is probably an RV setup. If you're manufacturing, do a Google search on "solar photovoltaics manufacturers" (without the quotes) and pick someone accessible to you. I'm sure they'll be happy to discuss design considerations.
If you mean by efficient, getting the most power into a battery it will be more efficient without a diode, as long as you don't mind the batteries getting discharged by the panel when it gets dark. But assuming that you do want the diode then it should be a low forward voltage type like a Schottky.
To protect the batteries, a decent system will use a shunt regulator to dump excess current. If you don't want to waste the power and make it more efficient, put more batteries in to cope with the peak charging current.
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This whole solar panel business is sort of a side line. My product is a GSM device that can communicate with a central station when certain alarm conditions occur. When no GSM activity is present the device consumes less than 1mA at 3.3V this goes up to a peak of 1.8A when the GSM module is connected to the network and data is being transmitted. It is possible to reduce the 1mA by using the powerdown modes of the MCU (ATMega128). I have had inquiries for an application where the device will be used where no external power is available, hence the solar panel. I have been unable to find a solarpanel with the appropriate charging circuits at a reasonable price. The prices here in South Africa of a solarpanel without charging circuitry is in the order of 10x cheaper than one with the circuitry. The latitude is +- 23deg South, with an average AFAICR of over 90% sunny days per year. I am currently looking for one or two units for a trial, and if succesful a potential for quite a few more. The units for the trial must be the same as the ones that will be used in production if this ever happens.
There are much better ways of increasing efficiency, withou worring about the diode drop.
Alas, it is not the peak charging current that is the issue. The issue is that eventually, the batteries will reach full charge, and it is hard on them to continue charging at this point. So usually there is a voltage limit put in place. The current is already limited, based on the sola panels spec., which looks like a constant current source that changes with light level.
Quite an interesting article. Of coarse I will need quite a lot of batteries in series if I want to charge a 3.2V Lithium cell.
I found an interesting NASA article where they put more solar cells in parallel in stead of in series. They then tie a DC-DC converter to the solar cell array, and the output of the DC-DC (Which is isolated) in series with the solar array. The output of the DC-DC converter is controlled to keep a fixed output voltage.
"Be vewy vewy vewy careful" if you are talking about Lithium batteries. They have a way of "letting the magic smoke out" (and all the rest of their insides for that matter) if they are overcharged. Lithium batteries can be very dangerous if overcharged.
-- Mike "mikey" Fields
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