There have been a couple (first second) of threads in the past year or so, asking about the need for tiny solar charger with no idle/dark consumption current. In both threads the poster asked/desired to charge with a tiny solar panel and no charge controller. Both cases had very low load or very intermittent loads. On both threads I have suggested a simple Zener regulator circuit. Some have suggested that it wouldn’t work for one reason or another. I think many have failed to consider it because they don’t understand the concept. So this thread is to shed some light and give anyone interested in learning a chance to look into this possible solution.
Note, this is not a battery “charger”. It is a circuit that will allow the battery to be charged from a solar panel and not be charged above a specific voltage. The circuit also can limit the charge current or the panel’s output can be the limit, but the current must stay well within the range of the zener’s ability to deal with the heat. I am using a 5watt Zener but without a heatsink and cooling it might handle 1 watt…maybe.
The basic circuit is explained on this web page. R(load) is the battery. Vin is the solar panel. I purchase some of these 12v zeners to do my demo with. I am using a 4watt panel. I have 50 ohms for Rs(current limit) to keep the current limited to about 150ma.
Principle of charging: In the case of using a 12v Zener, when the battery is not connected, all current from the panel would be shunted to negative/ground by the Zener. That is why it needs to be properly heat managed. When the battery is not below 12v and the battery is connected, the battery will not share the current and again the Zener will shunt all current to negative. When the battery is less than 12v, the battery charging load will cause the voltage of the I(load) point (see diagram on web page) to be pulled down to the battery voltage. When this happens, the Zener will not be above its 12v threshold and will not conduct current. The battery will slowly charge based on the battery’s equivalent circuit and the power source current limit (panel or resister Rs). As the battery gets charged to 12v, when the battery voltage might want to go above 12v, then the Zener starts hogging the current, shunting the power to negative, and stopping further battery charging. This state continues as long as the power source is active (until dark) or until something causes the battery voltage to drop below 12v.
Things to note, the battery will never be “fully charged” but will rest at the Zener/circuit set voltage. If you set the Zener to a desired “float” voltage, that is where the battery will be charged to. This may not be ideal for some battery type, but the circuit will prevent voltage from going beyond the Zener voltage point. I have looked and it is hard to find the exact Zener value and watts rating that we might desire. In the 12v example, if you would put a Schottky diode in series with the 12v Zener, you could raise the charge cutoff by about 0.2-0.3 volts, to about 12.2-12.3v. If you put a silicone diode in series with the Zener, you can add 0.6v to the cutoff giving you 12.6v.
My testing/demo is with a 3 cell Li-ion pack so 12v will give me 4v per cell, which is a pretty good state of charge. I discharged the pack down to 11.3v and put it in the sun yesterday morning. This morning the battery was at 11.75v, so it is not “there” yet. The sun is now on the panel and the meter says 11.81v with 0.08 amps.
More later.
Note, this is not a battery “charger”. It is a circuit that will allow the battery to be charged from a solar panel and not be charged above a specific voltage. The circuit also can limit the charge current or the panel’s output can be the limit, but the current must stay well within the range of the zener’s ability to deal with the heat. I am using a 5watt Zener but without a heatsink and cooling it might handle 1 watt…maybe.
The basic circuit is explained on this web page. R(load) is the battery. Vin is the solar panel. I purchase some of these 12v zeners to do my demo with. I am using a 4watt panel. I have 50 ohms for Rs(current limit) to keep the current limited to about 150ma.
Principle of charging: In the case of using a 12v Zener, when the battery is not connected, all current from the panel would be shunted to negative/ground by the Zener. That is why it needs to be properly heat managed. When the battery is not below 12v and the battery is connected, the battery will not share the current and again the Zener will shunt all current to negative. When the battery is less than 12v, the battery charging load will cause the voltage of the I(load) point (see diagram on web page) to be pulled down to the battery voltage. When this happens, the Zener will not be above its 12v threshold and will not conduct current. The battery will slowly charge based on the battery’s equivalent circuit and the power source current limit (panel or resister Rs). As the battery gets charged to 12v, when the battery voltage might want to go above 12v, then the Zener starts hogging the current, shunting the power to negative, and stopping further battery charging. This state continues as long as the power source is active (until dark) or until something causes the battery voltage to drop below 12v.
Things to note, the battery will never be “fully charged” but will rest at the Zener/circuit set voltage. If you set the Zener to a desired “float” voltage, that is where the battery will be charged to. This may not be ideal for some battery type, but the circuit will prevent voltage from going beyond the Zener voltage point. I have looked and it is hard to find the exact Zener value and watts rating that we might desire. In the 12v example, if you would put a Schottky diode in series with the 12v Zener, you could raise the charge cutoff by about 0.2-0.3 volts, to about 12.2-12.3v. If you put a silicone diode in series with the Zener, you can add 0.6v to the cutoff giving you 12.6v.
My testing/demo is with a 3 cell Li-ion pack so 12v will give me 4v per cell, which is a pretty good state of charge. I discharged the pack down to 11.3v and put it in the sun yesterday morning. This morning the battery was at 11.75v, so it is not “there” yet. The sun is now on the panel and the meter says 11.81v with 0.08 amps.
More later.