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An Introduction to Charge Controllers
A charge controller is an essential part of nearly all power
systems that charge batteries, whether the power source is
PV, wind, hydro, fuel, or utility grid. Its purpose is to
keep your batteries properly fed and safe for the long term.
The basic functions of a controller are quite simple. Charge
controllers block reverse current and prevent battery overcharge.
Some controllers also prevent battery overdischarge, protect
from electrical overload, and/or display battery status and
the flow of power. Let's examine each function individually.
Blocking Reverse Current
Photovoltaic panels work by pumping current through your
battery in one direction. At night, the panels may pass a
bit of current in the reverse direction, causing a slight
discharge from the battery. (Our term "battery" represents
either a single battery or bank of batteries.) The potential
loss is minor, but it is easy to prevent. Some types of wind
and hydro generators also draw reverse current when they stop
(most do not except under fault conditions).
In most controllers, charge current passes through a semiconductor
(a transistor) which acts like a valve to control the current.
It is called a "semiconductor" because it passes current only
in one direction. It prevents reverse current without any
extra effort or cost.
In some controllers, an electromagnetic coil opens and closes
a mechanical switch. This is called a relay. (You can hear
it click on and off.) The relay switches off at night, to
block reverse current.
If you are using a PV array only to trickle-charge a battery
(a very small array relative to the size of the battery),
then you may not need a charge controller. This is a rare
application. An example is a tiny maintenance module that
prevents battery discharge in a parked vehicle but will not
support significant loads. You can install a simple diode
in that case, to block reverse current. A diode used for this
purpose is called a "blocking diode."
Preventing Overcharge
When a battery reaches full charge, it can no longer store
incoming energy. If energy continues to be applied at the
full rate, the battery voltage gets too high. Water separates
into hydrogen and oxygen and bubbles out rapidly. (It looks
like it's boiling so we sometimes call it that, although it's
not actually hot.) There is excessive loss of water, and a
chance that the gasses can ignite and cause a small explosion.
The battery will also degrade rapidly and may possibly overheat.
Excessive voltage can also stress your loads (lights, appliances,
etc.) or cause your inverter to shut off.
Preventing overcharge is simply a matter of reducing the
flow of energy to the battery when the battery reaches a specific
voltage. When the voltage drops due to lower sun intensity
or an increase in electrical usage, the controller again allows
the maximum possible charge. This is called "voltage regulating."
It is the most essential function of all charge controllers.
The controller "looks at" the voltage, and regulates the battery
charging in response.
Some controllers regulate the flow of energy to the battery
by switching the current fully on or fully off. This is called
"on/off control." Others reduce the current gradually. This
is called "pulse width modulation" (PWM). Both methods work
well when set properly for your type of battery.
A PWM controller holds the voltage more constant. If it has
two-stage regulation, it will first hold the voltage to a
safe maximum for the battery to reach full charge. Then, it
will drop the voltage lower, to sustain a "finish" or "trickle"
charge. Two-stage regulating is important for a system that
may experience many days or weeks of excess energy (or little
use of energy). It maintains a full charge but minimizes water
loss and stress.
The voltages at which the controller changes the charge rate
are called set points. When determining the ideal set points,
there is some compromise between charging quickly before the
sun goes down, and mildly overcharging the battery. The determination
of set points depends on the anticipated patterns of usage,
the type of battery, and to some extent, the experience and
philosophy of the system designer or operator. Some controllers
have adjustable set points, while others do not.
Control Set Points vs. Temperature
The ideal set points for charge control vary with a battery's
temperature. Some controllers have a feature called "temperature
compensation." When the controller senses a low battery temperature,
it will raise the set points. Otherwise when the battery is
cold, it will reduce the charge too soon. If your batteries
are exposed to temperature swings greater than about 17 C,
compensation is essential.
Some controllers have a temperature sensor built in. Such
a controller must be mounted in a place where the temperature
is close to that of the batteries. Better controllers have
a remote temperature probe, on a small cable. The probe should
be attached directly to a battery in order to report its temperature
to the controller.
An alternative to automatic temperature compensation is to
manually adjust the set points (if possible) according to
the seasons. It may be sufficient to do this only twice a
year, in spring and fall.
Control Set Points vs. Battery Type
The ideal set points for charge controlling depend on the
design of the battery. The vast majority of RE systems use
deep-cycle lead-acid batteries of either the flooded type
or the sealed type. Flooded batteries are filled with liquid.
These are the standard, economical deep cycle batteries.
Sealed batteries use saturated pads between the plates. They
are also called "valve-regulated" or "absorbed glass mat,"
or simply "maintenance-free." They need to be regulated to
a slightly lower voltage than flooded batteries or they will
dry out and be ruined. Some controllers have a means to select
the type of battery. Never use a controller that is not intended
for your type of battery. Typical set points for 12 V lead-acid
batteries at 25 C
(These are typical, presented here only for example.)
High limit (flooded battery): 14.4 V
High limit (sealed battery): 14.0 V
Resume full charge: 13.0 V
Low voltage disconnect: 10.8 V
Reconnect: 12.5 V
Temperature compensation for 12V battery:
-.03 V per C deviation from standard 25 C
Low Voltage Disconnect (LVD)
The deep-cycle batteries used in renewable energy systems
are designed to be discharged by about 80 percent. If they
are discharged 100 percent, they are immediately damaged.
Imagine a pot of water boiling on your kitchen stove. The
moment it runs dry, the pot overheats. If you wait until the
steaming stops, it is already too late!
Similarly, if you wait until your lights look dim, some battery
damage will have already occurred. Every time this happens,
both the capacity and the life of the battery will be reduced
by a small amount. If the battery sits in this overdischarged
state for days or weeks at a time, it can be ruined quickly.
The only way to prevent overdischarge when all else fails,
is to disconnect loads (appliances, lights, etc.), and then
to reconnect them only when the voltage has recovered due
to some substantial charging. When overdischarge is approaching,
a 12 volt battery drops below 11 volts (a 24 V battery drops
below 22 V).
A low voltage disconnect circuit will disconnect loads at
that set point. It will reconnect the loads only when the
battery voltage has substantially recovered due to the accumulation
of some charge. A typical LVD reset point is 13 volts (26
V on a 24 V system).
All modern inverters have LVD built in, even cheap pocket-sized
ones. The inverter will turn off to protect itself and your
loads as well as your battery. Normally, an inverter is connected
directly to the batteries, not through the charge controller,
because its current draw can be very high, and because it
does not require external LVD.
If you have any DC loads, you should have an LVD. Some charge
controllers have one built in. You can also obtain a separate
LVD device. Some LVD systems have a "mercy switch" to let
you draw a minimal amount of energy, at least long enough
to find the candles and matches! DC refrigerators have LVD
built in.
If you purchase a charge controller with built-in LVD, make
sure that it has enough capacity to handle your DC loads.
For example, let's say you need a charge controller to handle
less than 10 amps of charge current, but you have a DC water
pressurizing pump that draws 20 amps (for short periods) plus
a 6 amp DC lighting load. A charge controller with a 30 amp
LVD would be appropriate. Don't buy a 10 amp charge controller
that has only a 10 or 15 amp load capacity!
Overload Protection
A circuit is overloaded when the current flowing in it is
higher than it can safely handle. This can cause overheating
and can even be a fire hazard. Overload can be caused by a
fault (short circuit) in the wiring, or by a faulty appliance
(like a frozen water pump). Some charge controllers have overload
protection built in, usually with a push-button reset.
Built-in overload protection can be useful, but most systems
require additional protection in the form of fuses or circuit
breakers. If you have a circuit with a wire size for which
the safe carrying capacity (ampacity) is less than the overload
limit of the controller, then you must protect that circuit
with a fuse or breaker of a suitably lower amp rating. In
any case, follow the manufacturer's requirements and the National
Electrical Code for any external fuse or circuit breaker requirements.
Displays and Metering
Charge controllers include a variety of possible displays,
ranging from a single red light to digital displays of voltage
and current. These indicators are important and useful. Imagine
driving across the country with no instrument panel in your
car! A display system can indicate the flow of power into
and out of the system, the approximate state of charge of
your battery, and when various limits are reached.
If you want complete and accurate monitoring however, spend
about R1500 for a separate digital device that includes an
amp-hour meter. It acts like an electronic accountant to keep
track of the energy available in your battery. If you have
a separate system monitor, then it is not important to have
digital displays in the charge controller itself. Even the
cheapest system should include a voltmeter as a bare minimum
indicator of system function and status.
Have It All with a Power Center
If you are installing a system to power a modern home, then
you will need safety shutoffs and interconnections to handle
high current. The electrical hardware can be bulky, expensive
and laborious to install. To make things economical and compact,
obtain a ready-built "power center." It can include a charge
controller with LVD and digital monitoring as options. This
makes it easy for an electrician to tie in the major system
components, and to meet the safety requirements of the National
Electrical Code or your local authorities.
Charge Controllers for Wind and Hydro
A charge controller for a wind-electric or hydro-electric
charging system must protect batteries from overcharge, just
like a PV controller. However, a load must be kept on the
generator at all times to prevent overspeed of the turbine.
Instead of disconnecting the generator from the battery (like
most PV controllers) it diverts excess energy to a special
load that absorbs most of the power from the generator. That
load is usually a heating element, which "burns off" excess
energy as heat. If you can put the heat to good use, fine!
Is It Working?
How do you know if a controller is malfunctioning? Watch
your voltmeter as the batteries reach full charge. Is the
voltage reaching (but not exceeding) the appropriate set points
for your type of battery? Use your ears and eyes-are the batteries
bubbling severely? Is there a lot of moisture accumulation
on the battery tops? These are signs of possible overcharge.
Are you getting the capacity that you expect from your battery
bank? If not, there may be a problem with your controller,
and it may be damaging your batteries.
Conclusion
The control of battery charging is so important that most
manufacturers of high quality batteries (with warranties of
five years or longer) specify the requirements for voltage
regulation, low voltage disconnect and temperature compensation.
When these limits are not respected, it is common for batteries
to fail after less than one quarter of their normal life expectancy,
regardless of their quality or their cost.
A good charge controller is not expensive in relation to
the total cost of a power system. Nor is it very mysterious.
I hope this article has given you the background that you
need to make a good choice of controls for your power system.
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