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I've got a small solar cell panel rated at 20V on a good sunny day. I need to regulate this down to 13.8V to charge a lead battery.
Can I use a "Buck Step Down Voltage Regulator" for this? I'm uncertain about whether the regulator will provide 13.8V regardless of input voltage, or if it just provides an output voltage relative to the input voltage.
Specifically, I'm looking at an LM2596 regulator.
A quick look at your part on Ebay and the chip datasheet (thanks, @whatsisname) suggests that it has a regulated adjustable output. So yes, if you set it for 13.5V it will hold to that voltage.
Provided that you don't violate the input requirements. A buck converter can only ever put out a lower voltage than the input; so you have to make sure the input voltage never falls below ~14V in order to guarantee the 13.5V output. You say your solar panel is rated at 20V on a "good day". What panel output do you get on a "bad day"? If it falls below ~14V you'll probably have problems charging your battery.
A buck regulator that produces 13.8 volts may work down to an input voltage of 14 volts or 15 volts (circuit dependant). Once the input voltage gets too low, either the buck will shut-down or just turn on hard dropping a few hundred millivolts; if your voltage dropped to 10V then this type of circuit might be able to output 9.5V under load.
If you want a regulator that provides a constant output of 13.8 volts when the input range is from below 13.8 volts to above 13.8 volts then look for a boost-buck type circuit.
A point that seems to be being missed so far is that the POWER output of the regulator is <= POWER in. If the PV panel is not able to provide the Power (= Vout x Iout) that the battery requires at a given voltage then the regulator will not be able to maintain the voltage.
Result depends on circuit used BUT the ebay sourced module that you show probably attempts to maintain set voltage regardless of Vin. It will only do so if the available power is >= the required output power. The circuit is probably somethink like fig 20 in the LM2596 data sheet that whatsisname referenced.
If the battery is provided with 13.8V it will attempt to draw a current based on its charge state at the time. The current may be relatively low if the battery is fully charged and relatively high if the battery is relatively uncharged. In all cases your available input POWER needs to be >= the power your battery is trying to take. If P_solar is < P_desired_battery then Vout_buck will fall to a level where it is able to meet the resultant load.
You need to advise PV panel max rated power in full sun. Ideally also rated voltage and current at max power point in full sun.
You also need to advise the battery capacity and battery type (flooded, absorbed glass mat, ...). Knowing actual brand and model of battery may help.
Just applying 13.8V to a battery to charge it would be VERY bad for its health if your charger had enough capability to always do this. Lead acid batteries are usually charged at constant current up to a certain voltage and then constant voltage to an end point and then some twiddly magic may be done at the end. 13.8V is the usual target endpoint for certain modes of charging. Fortunately (for most) in cases like yours the max available current from the PV panel is probably less than or not vastly more than the max you should use in CC mode. This of course depends on panel Watts and battery capacity. If this is a eg 5W panel and a 12V 7Ah battery all should be well. If it's a eg 1.2 Ah battery and a 300W panel the battery will neither live long nor prosper.
What is the 'right' voltage depends on whether you want float or deeper discharge modes of operation or more. Lead acid charging is a far from straight forward process.
Battery University - chargin lead acid batteries is worth reading.
A buck switching mode regulator will provide a constant voltage output (or mostly constant, under heavy load, it will droop a few millivolts) regardless of your input voltage so long as it is within the allowed input voltage range, in this case at least above 14V. Obviously the input has to be higher than the output
Generally a switching mode regulator will regulate using a feedback loop, usually set by a simple voltage divider. These resistors will connect to the FB pin and the output voltage. What I'm saying is the regulator will output a voltage based on its already output voltage.
