In addition to the solar panel you will require some kind of solar controller to keep the panel from overcharging your battery if left connected for long periods of time.
A solar controller, or charge controller is basically a voltage and/or current regulator to keep batteries from overcharging. It regulates the voltage and current coming from the solar panels going to the battery. Most "12 volt" panels put out about 16 to 20 volts, so if there is no regulation the batteries will be damaged from overcharging. Most batteries need around 14 to 14.5 volts to get fully charged.
Even small solar panels can erode the battery plates and boil away electrolyte on lead-acid batteries requiring expensive battery replacement. Some solar panels are called "self-regulating" - which is not entirely true. A self regulating solar panel has fewer cells, giving a lower open circuit output voltage closer to 15 VDC, hence less likely to overcharge your battery. Charge controllers are high efficiency (low loss) solar controllers that are wired between the solar panel and the battery bank being charged. Their function is to disconnect the solar panel when the battery has fully charged and automatically re-connect the panel when the battery voltage drops. If not disconnected, a solar panel will try to drive the battery voltage to as high as 18-19 VDC - a level most batteries take exception to.
Many charge controllers have set points that can be varied so both wet and gell batteries can be protected. Some charge controllers have indicator lights that tell you whether the solar panel is charging the battery or "floating" so you don't have to guess whether it's working. The simplest charge controller have a fixed voltage set point and no indicators of any kind to tell you what's happening. The most sophisticated charge controllers have digital LCD display of battery voltage, charging current, and load current and may offer "Load Disconnect" - the ability to remove any drain from the battery if the battery voltage drops too low.
A basic charge controller simply performs the necessary function of ensuring that your batteries cannot be damaged by over-charging, effectively cutting off the current from the pv panels (or reducing it to a pulse) when the battery voltage reaches a certain level.
A new type of regulator called MPPT (Maximum Power Point Tracking) is beginning to be offered all over the world, the voltage across the terminals of a 12 volt battery varies from about 11 volts when discharged, to about 14.5 volts when fully changed. To charge a battery you need to apply more voltage than is presently across the terminals. To be safe, most solar panels output around 17 volts (when the panel cell temperature is at 25 degrees). For most panels this output voltage is lower when they are hot (about 15v on a very hot day) and higher when they are cold (about 18v on a very cold day).
An MPPT controller can perform some magic on the power coming from the panels. It first looks at the voltage across the battery terminals. It then looks at the voltage coming from the solar panels and “adjusts” this voltage so that it is just enough to charge the battery. In the process of this adjustment it is able to take the additional (unused) voltage and turn this into useful current (amps) to charge this battery. This is called the maximum power point. The MPPT controller constantly monitors or tracks these voltages and that is why it is called a Maximum Power Point Tracking controller.
A Maximum Power Point Tracker controller performs an extra function to improve your system efficiency.
The efficiency loss in a basic system is due to a miss-match between voltage produced by the pv panels and that required to charge the batteries under certain conditions.
A 24 volt battery will require upto about 28 volts to fully charge it. When the battery being charged is in a fairly low state, it's voltage (under charge) may only 24 volts.
Our PV panels, which we refer to as 24 volt panels, need to be able to charge the batteries on a bright day (not only in full sunshine) so are designed to produce at least 24 volts in those conditions. In bright sunshine hover, these panels may be cable of producing 40 volts. In-fact, they are likely to produce their rated output power (volts x amps) at 32 - 34 volts.
When the battery is at 24 volts, it will be pulling the panel voltage down to 24 (assuming no voltage drop in your cables). This results in the panels producing significantly less than their rated output and therefore there is a loss in efficiency.
An MPPT controller, in addition to performing the function of a basic controller, also includes a DC voltage converter, converting the voltage of the panels to that required by the batteries, with practically no loss of power. In other words, they attempt to keep the panel voltage at their Maximum Power Point, while supplying the varying voltage requirements of the battery.
Furthermore, a 24 volt system with an MPPT charge controller may have the panels wired in series to produce 48 volts, maintaining the ability to provide some charging current in dull conditions, when a standard system would not provide any charge.
Manufacturers claim upto 40% power increase from your panels using MPPT, which is most likely to be achieved when battery levels are low and/or light levels are low.
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