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Solar Charge Controller FAQS
Q.What is a solar charge controller?
A solar charge controller, or solar regulator 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 are around 14 to 14.5 volts when they are fully charged.
Q.Do I always need a charge controller?
Not always, but usually. Generally, there is no need for a charge controller with the small maintenance, or trickle charge panels, such as the 1 to 5 watt panels. For example, if you use a 2W panel for each 50 battery amp-hours, or if you use a 5W panel to charge a 210 amp-hours golf car battery, then you don't need a charge controller.
Q.Why is my voltage meter show 12 Volt Solar Panel is 17 Volts in reading?
The reason is that if solar panel is designed to charge a 12 volt battery under different weather conditions. The panels need to provide some extra voltage so that when the sun is low in the sky, or you have heavy, cloud cover, or high temperatures, you still get some output from the panel. A fully charged "12 volt" battery is around 12.7 volts at rest (around 13.6 to 14.4 under charge), so the panel has to put out at least that much under worst case conditions.
Q.Can I use a standard controller for high voltage panels?
Standard controllers (that is, all but the MPPT types), will often work with high voltage panels if the maximum input voltage of the charge controller is not exceeded. However, you will lose a lot of power,  from 20 - 60% of what your panel is rated at. Charge controls take the output of the panels and feed current to the battery until the battery is fully charged, usually around 13.6 - 14.4 volts. A panel can only put out so many amps, so while the voltage is reduced from say, 33 volts to 13.6 volts, the amps from the panel cannot go higher than the rated amps - so with a 175 watt panel rated at 32 volts/7.6 amps, you will only get 7.6 amps at 12 volts or so into the battery.  Amount of power in term of watts is volts x amps, so your 175 watt panel will only put about 90 watts into the battery.
Q.What type of controller should I use with high voltage panels?
The only way to get full power out of high voltage grid tie solar panels is to use an MPPT controller. Since most MPPT controls can take up to 150 volts DC (some can go higher, up to 600 VDC) on the solar panel input side, you can often series two or more of the high voltage panels to reduce wire losses, or to use smaller wire. For example, with the 175 watt panel mentioned above, 2 of them in series would give you 66 volts at 7.6 amps into the MPPT controller, but the controller would convert that down to about 29 amps at 12 volts.
Q.How many types of Charger Controller?
There are three major types of charge controls.
Simple 1 or 2 stage controls which rely on relays or shunt transistors to control the voltage in one or two steps. These essentially just short or disconnect the solar panel when a certain voltage is reached. They are hard to find in the market now.
3-stage and/or PWM such as Morningstar, Xantrex, Blue Sky, Steca, and many others. These are pretty much the industry standard now, but you will occasionally still see some of the older shunt/relay types around, such as in the very cheap systems offered by discounters and mass marketers.
Maximum power point tracking (MPPT), such as those made by Midnite Solar, Xantrex, Outback Power, Morningstar and others. These are the ultimate in controllers, with prices to match - but with efficiencies in the 94% to 98% range, they can save considerable money on larger systems since they provide 10 to 30% more power to the battery. The 4-stage MPPT controllers have advantage function call equalization (see bellow).
Most controllers come with some kind of indicator, either a simple LED, a series of LED's, or digital meters. Many newer ones, such as the Outback Power, Midnite Classic, Morningstar MPPT, and others now have built in computer interfaces for monitoring and control. The simplest usually have only a couple of small LED lamps, which show that you have power and that you are getting some kind of charge. Most of those with meters will show both voltage and the current coming from the panels and the battery voltage. Some also show how much current is being pulled from the LOAD terminals.
Q.What is Equalization?
Equalization does somewhat what the name implies - it attempts to equalize - or make all cells in the battery or battery bank of exactly equal charge. Essentially it is a period of overcharge, usually in the 15 - 15.5 volt range. If you have some cells in the string lower than others, it will bring them all up to full capacity. In flooded batteries, it also serves the important function of stirring up the liquid in the batteries by causing gas bubbles. Of course, in an RV or boat, this does not usually do much for you unless you have been parked for months, as normal movement will accomplish the same thing. Also, in systems with small panels or oversized battery systems you may not get enough current to really do much bubbling. In many off-grid systems, batteries can also be equalized with a generator+charger.
Q.What is PWM?
PWM stands for Pulse Width Modulation. PWM is often used as one method of float charging. Instead of a steady output from the controller, it sends out a series of short charging pulses to the battery - a very rapid "on-off" switch. The controller constantly checks the state of the battery to determine how fast to send pulses, and how long (wide) the pulses will be. In a fully charged battery with no load, it may just "tick" every few seconds and send a short pulse to the battery. In a discharged battery, the pulses would be very long and almost continuous, or the controller may go into "full on" mode. The controller checks the state of charge on the battery between pulses and adjusts itself each time.
Q.What is a Load and "LVD" output?
Some controllers also have a "LOAD", or LVD (Low Voltage Disconnect) output. The Load output can be used for smaller loads, such as small appliances and lights. The advantage is that the load terminals have a low voltage disconnect, so it will turn off whatever is connected to the load terminals and keep from running the battery down too far. The LOAD output is often used for small non-critical loads, such as lights. Do not use the LOAD output to run any but very small inverters. Inverters can have very high surge currents and may blow the controller.
Most systems do not need the LVD function - it can drive only smaller loads. Depending on the rating of the controller, this may be from 6 to 60 amps. You cannot run any but the smallest inverter from the LOAD output. On some controllers, such as the Morningstar SS series, the load output can be used to drive a heavy duty relay for load control, gen start etc. The LOAD or LVD output is most often used in RV & remote systems, such as camera, monitor, and cell phone sites where the load is small and the site is unattended.
Q.What are the "Sense" terminals on my controller?
Some charge controllers have a pair of "sense" terminals. Sense terminals carry very low current, around 1/10th of a milliamp at most, so there is no voltage drop. What it does is "look" at the battery voltage and compares it to what the controller is putting out. If there is a voltage drop between the charge controller and the battery, it will raise the controller output slightly to compensate.
These are only used when you have a long wire run between the controller and the battery. These wires carry no current, and can be pretty small - #20 to #16 AWG. We prefer to use #16 because it is not easily cut or squished accidently. They attach to the SENSE terminals on the controller, and onto the same terminals as the two charging wires at the battery end.
Q.What is a "Battery System Monitor"?
Battery system monitors are not controllers. Instead, they monitor your battery system and give you a pretty good idea of your battery condition, and what you are using and generating. They keep track of the total amp-hours into and out of the batteries, and the battery state of charge, and other information. They can be very useful for medium to large systems for tracking exactly what your system is doing with various charging sources.
Eason Zhong

Eason Zhong

Eason Zhong