What is a Solar Charge Controller?
Why do I need one?
A
charge controller, or charge 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 need around 14 to 14.5 volts to get fully charged.
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. A rough rule is that if the panel puts out
about 2 watts or less for each 50 battery amp-hours, then you don't need
one.
For
example, a standard flooded golf car battery is around 210 amp-hours.
So to keep up a series pair of them (12 volts) just for maintenance or
storage, you would want a panel that is around 4.2 watts. The popular 5
watt panels are close enough, and will not need a controller. If you are
maintaining AGM deep cycle batteries, such as the Concorde Sun Xtender
then you can use a smaller 2 to 2 watt panel.
Why 12 Volt Panels are 17 Volts
The
obvious question then comes up - "why aren't panels just made to put
out 12 volts". The reason is that if you do that, the panels will
provide power only when cool, under perfect conditions, and full sun.
This is not something you can count on in most places. The panels need
to provide some extra voltage so that when the sun is low in the sky, or
you have heavy haze, 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.
*Contrary to intuition, solar panels work best at cooler temperatures. Roughly, a panel rated at 100 watts at room temperature will be an 83 watt panel at 110 degrees.
Detailed information on MPPT charge controllers.
The
charge controller regulates this 16 to 20 volts output of the panel
down to what the battery needs at the time. This voltage will vary from
about 10.5 to 14.6, depending on the state of charge of the battery, the
type of battery, in what mode the controller is in, and temperature.
(see complete info on battery voltages in our battery section).
Using High Voltage (grid tie) Panels With Batteries
Nearly
all PV panels rated over 140 watts are NOT standard 12 volt panels, and
cannot (or at least should not) be used with standard charge
controllers. Voltages on grid tie panels varies quite a bit, usually
from 21 to 60 volts or so. Some are standard 24 volt panels, but most
are not.
What happens when you use a standard controller
Standard
(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 to 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 to 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 23 volts/7.6 amps, you
will only get 7.6 amps @ 12 volts or so into the battery. Ohms Law tells us that watts is volts x amps, so your 175 watt panel will only put about 90 watts into the battery.
Using an MPPT controller 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. See the link above for detailed into on MPPT
charge controls. 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.
Charger Controller Types
Charge controls come in all shapes, sizes, features, and price ranges. They range from the small 4.5 amp (Sunguard)
control, up to the 60 to 80 amp MPPT programmable controllers with
computer interface. Often, if currents over 60 amps are required, two or
more 40 to 80 amp units are wired in parallel. The most common controls
used for all battery based systems are in the 4 to 60 amp range, but
some of the new MPPT controls such as the Outback Power FlexMax go up to 80 amps.
Charge controls come in 3 general types (with some overlap):
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. For all practical purposes
these are dinosaurs, but you still see a few on old systems - and some
of the super cheap ones for sale on the internet. Their only real claim
to fame is their reliability - they have so few components, there is not
much to break.
3-stage and/or PWM
such 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. For more information, see our article on
MPPT.
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.
All
of the charge controllers that we stock are 3 stage PWM types, and the
MPPT units. (in reality, "4-stage" is somewhat advertising hype - it
used to be called equalize, but someone decided that 4 stage was better
than 3). And now we even see one that is advertised as "5-stage"....
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 to 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.
What is PWM?
Quite
a few charge controls have a "PWM" mode. 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.
The
downside to PWM is that it can also create interference in radios and
TV's due to the sharp pulses that it generates.
What is a Load, or "Low Voltage Disconnect" output?
Some
controllers also have a "LOAD", or LVD output, which 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. A few, such as the Xantrex C12, can also be used as a lighting controller, to turn lights on at dark, but the Morningstar SLC lighting controller is usually a better choice for that. 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.
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 accidentally. They attach to the SENSE terminals on the
controller, and onto the same terminals as the two charging wires at the
battery end.
What is a "Battery System Monitor"?
Battery system monitors, such as the Bogart EngineeringTriMetric 2025
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. They are somewhat overkill for small systems, but are kind of a
fun toy if you want to see what every amp is doing :-). TriMetric's new PentaMetric model also has a computer interface and many other features.
via Solar Electric
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