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 display battery status and the flow of
power. Let's examine each function individually.
Blocking Reverse Current
Photovoltaic (PV) 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, but 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
in only 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. It
switches off at night, to block reverse current. As it turns on and
off, there is an audible clicking sound.
If you are using 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 PV module
that trickle-charges a battery and compensates for battery discharge
in a parked vehicle but will not support significant loads. In this
situation, you can install a simple diode 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 an 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.
This can be illustrated by an analogy:
The Energy Chef is watching a pot of
water on a gas burner, which is fed by a tube coming from the sun. He
has one hand on the gas valve. He's thinking, "I need to get this
water as close to a boil as possible before the sun goes down, but I
must never boil the water.
In this analogy, the temperature of the
water represents battery voltage; the flow of gas represents charging
current; boiling represents overcharge; and the energy chef manipulating
the valve is like the charge controller.
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,
called pulse width modulation (PWM). Both methods work well when the
voltage set points are properly selected 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 pattern of use, 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 battery 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 reduces the charge too
soon. If your batteries are exposed to temperature swings greater than
about 30ºF (17ºC), compensation is essential.
Some controllers have a temperature sensor built
in. This type of controller must be mounted in a place where the temperature
is close to that of the batteries. Better controllers have a remote
temperature sensor 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 battery design.
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," "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.
Low Voltage Disconnect
The deep cycle batteries used in renewable energy systems are designed
to be discharged a maximum of 80 percent (20% state of charge). 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 reconnect
them only when the voltage has recovered due to some substantial charging.
When overdischarge is approaching, a 12 volt battery will drop below
11 volts (a 24 V battery will drop below 22 V).
A low voltage disconnect (LVD) 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, your
loads, and 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.
Maximum
Power Point Tracking
A new feature is showing up in charge
controllers. It's called maximum power point tracking (MPPT). It extracts
additional power from your PV array under certain conditions.
The function of MPPT is analogous to the
function of a transmission in a car. When the transmission is in the
wrong gear, the wheels do not receive maximum power. That's because
the engine is running either slower or faster than its ideal speed range.
The purpose of the transmission is to couple the engine to the wheels
in a way that lets the engine run in a favorable speed range in spite
of varying acceleration and terrain.
Let's compare a PV module to a car engine,
with voltage analogous to engine speed. At the ideal voltage, the PV
can deliver maximum power. This is the maximum power point, also called
peak power voltage (Vpp). Vpp varies with sunlight intensity and with
solar cell temperature. The voltage of the battery is analogous to the
speed of the car's wheels. It varies with battery state of charge, and
with the loads on the system (any appliances and lights that may be
on). For a 12 V system, it ranges from about 11 to 14.5 volts.
In order to charge a battery (increase
its voltage), the PV module must apply a voltage that is higher than
that of the battery. If the PV module's Vpp is just slightly below the
battery voltage, then the current drops nearly to zero (like an engine
turning slower than the wheels). To play it safe, typical PV modules
are designed with a Vpp of around 17 volts when measured at a cell temperature
of 77ºF (25ºC) on a cool day. They do that because it will
drop to around 15 volts on a very hot day. However, on a very cold day,
it can rise to 18 volts!
What happens when the Vpp is much higher
than the voltage of the battery? The module voltage is dragged down
to a lower-than-ideal voltage. Traditional charge controllers transfer
the PV current directly to the battery without giving you the benefit
of this added potential.
Now let's make one more analogy. The car's
transmission varies the ratio between speed and torque. At low gear,
the speed of the wheels is reduced and the torque is increased. Likewise,
MPPT varies the ratio between the voltage and current delivered to the
battery in order to deliver maximum power. If there is excess voltage
available from the PV array, it is converted to additional charging
current for the battery. It's like an automatic transmission. As the
Vpp of the PV array varies with temperature and other conditions, it
"tracks" this variance and adjusts the ratio accordingly.
That's why it's called a maximum power point tracker.
What advantage does MPPT give in the real
world? That depends on your array, climate, and seasonal load pattern.
It gives you an effective current boost only when the Vpp is more than
about 1 volt higher than the battery voltage. In hot weather, this may
not be the case unless the batteries are at a low state of charge (SOC).
In cold weather however, the Vpp can rise as high as 18 volts. If your
energy use is greatest in the winter (typical in most homes) and you
have cold winter·weather, then you can gain a substantial boost
in energy when you need it the most!
Here is an example of MPPT action on a
cold winter day:
The outside temperature is 20ºF (-6.6ºC).
The wind is blowing a bit, so the PV cell temperature rises to only
around 30ºF (-1.1ºC). Vpp = 18.0 V. The batteries are a bit
low, and loads are high, so battery voltage = 12.0 V.
Ratio of Vpp to battery voltage is 18:12
= 1.5:1
Under these conditions, a perfect MPPT
(with no voltage drop in the array circuit) would deliver a 50 percent
increase in charge current! In reality, there are losses in the conversion
just as there is friction in a car's transmission. Reports from the
field indicate that increases of 20 to 30 percent are typically observed.
MPPT controllers are a new technology
that is just starting to become available. This sidebar is a simplified
introduction to a complex topic. Watch for more information in future
issues of Home Power.
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 US$200 for a separate digital device that includes
an amp-hour meter. It acts like an electronic accountant that keeps
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 the 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.
Control Your Charge!
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 quality or 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.
Author: Windy Dankoff, Dankoff Solar Products, Inc., 2810 Industrial Rd., Santa
Fe, NM 87505-3120. 888-396-6611 or 505-473-3800. Fax: 505-473-3830 pumps@danksolar.com · www.dankoffsolar.com
Published in September
1999 Home Power Magazine,
the article "What is a Charge Controller?"
is shown with permission of Home Power Magazine and author Windy Dankoff.
