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6-2. Controllers
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Marshall Islands March 31-April 11, 2008
Marshall Islands March 31-April 11, 2008
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Why have a Charge Controller
• Open cell batteries can lose water quickly if
overcharged. If electrolyte falls below the top of the
plates damage occurs.
• Sealed batteries may be ruined if frequently
overcharged
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Why have a Discharge Control?
• To prevent damage to batteries from excessively deep
discharge
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Most Important Controller
Characteristic
•
•
Assuming the controller turns the charging and discharging on and
off at the right voltages, by far the most important characteristic of
a controller is reliability. When a controller fails completely it may
be days or weeks before the problem is reported and then weeks
or months before a replacement is installed. The battery is usually
damaged as a result. More subtle controller failures (changes in
set points, high internal losses, etc.) are difficult to troubleshoot
and may not be found at all. Deterioration of the battery is usually
rapid and battery failure is what shows that the controller is not
working correctly.
A highly reliable controller that does the basic on-off switching of
charging and discharging currents with reasonable accuracy is
much better than a controller that is less reliable but has highly
accurate voltage settings and uses more effective charging
methods such as tapered charging or PWM charging. For open
cell batteries that can be a relay type controller. For sealed
batteries, a well designed and accurate voltage semiconductor
controller with excellent lightning protection is best.
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Shunt (Parallel) Controller
•Placed between the panels and
the battery
•To prevent overcharge, the
output from the panel is shorted
by the controller using a
semiconductor switch
•Because the panel wires go to
the battery, a blocking diode has
to be installed or shorting the
panel output would also short the
battery
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Advantages of the Shunt Controller
• Simple and cheap. Lends itself to local production
• Less likely to be damaged by excess current flow than
series switching control.
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Disadvantages of Shunt Control
• All the panel power is shorted through the
controller and converted to heat. So large
panels generate a great deal of heat and it is a
problem to get rid of it. Shunt controls are
therefore best suited to small (under 50Wp) PV
systems.
• Sensitive to lightning
• Requires a blocking diode with its attendant
power loss
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Series (switching) Charge Control
• Placed between the
panels and the
battery
• A transistor switch or
a relay is used to
disconnect the panel
from the battery so
overcharging cannot
occur
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Advantages of Relay Type Switching
Controller
•
Relay switching is very resistant to lightning strike damage
•
Very low voltage drop and internal losses
•
There is no heat dissipation problem so it can be used with any
size of PV system without problem.
•
Typically has lower power losses than the shunt controller since
there is no blocking diode
•
Simple circuitry that can usually be repaired locally
•
No microchips required so less vulnerable to moisture and
corrosion
•
No high frequency circuitry so less vulnerable to dirt and salt
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Disadvantages of relay type
controllers
• Usually more expensive than semiconductor controllers
• Difficult to incorporate other than on/off type of
switching. Semiconductor switches can do high
frequency pulse charging, tapered charging and other
more sophisticated charging methods.
• May draw more current for its internal operation than
semiconductor circuits though good designs minimize
this potential problem.
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Relay type controller
High reliability relay type controller circuitry
designed by S.P.I.R.E. and constructed in
Kiribati by the Kiribati Solar Energy Co.
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Disadvantages of Semiconductor type
Switching Controllers
• Semiconductor switching type of controller is sensitive
to lightning damage (not the case for relay types)
• Easy to damage by excess current flow through the
semiconductor controller (not true for relay types)
• Higher internal voltage drop than relay controllers
• In the Pacific, the experience has been that the
operating life of relay controllers is around twice that of
all but the best semiconductor controllers
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Pulse Width Modulation (PWM)
• In charging a battery, the charging current and rate of
charge has a lot to do with how quickly and efficiently
charging can be accomplished. By charging using
pulses of energy, high current can be used without
overheating the battery. The rate of the charging can be
adjusted by changing the ratio between the amount of
time the pulse occurs and the amount of time between
pulses. Pulse Width Modulation or PWM is a charging
approach that can be used by semiconductor
controllers to “punch” in the charging at a fairly high
current but to avoid the high heat that is generated from
continuous charging. The charge rate is adjusted by
changing the width of the pulses. Wide pulses with little
time between them provides high charging rate. Narrow
pulses with much time between them a low rate.
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Changing PWM charging rate
Red area represents energy going to battery for charging
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SHS Semiconductor Controller
•
•
•
•
ProStar controller
12/20/40 Amperes
Up to 48V battery
Lightning
protection
included
• “On-Off” type
controller
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Large semiconductor Controller
• Morningstar TS
style
semiconductor
controller
• Remote data port
• Manually
controlled
equalization
• 12, 24 or 48V
operation
• Up to 60A
• Both PWM and
“on-off” charge
control available
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Discharge controller
• Placed between the battery and the loads
• All are of the series switching type. Often use a relay
even if the charge controller uses semiconductors but
many use a semiconductor switch
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Maximum Power Point Tracking
Controller (MPPT controller)
• A complex but effective electronic controller that always
takes energy from the panels at its maximum power
point increasing energy delivery for charging a battery
by 10-15% or more over standard controllers plus
allowing higher voltage strings of panels for lowered
wiring losses.
• Cost effective for battery charging only on large
installations (typically 1kWp and larger).
• Often used for direct connection of pumps to solar
panels thereby avoiding batteries while still maintaining
efficient use of the solar energy
• Difficult to troubleshoot and usually impossible to repair
locally.
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Sample MPPT controller
• Outback MX-60 MPPT controller
• Includes data logger and remote
monitoring
• Programmable equalization
• Input voltage up to 140 DC to
charge a battery ranging from
12V-60V
• Claims up to 30% more energy
than non MPPT controllers but
more likely 10%-15% if panel
voltage is matched to that of
batteries.
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Charging methods
• On/Off charging. Panel power turned off when full
charge is reached.
• Controlled “taper” charge. Amperes are gradually cut
back by the controller when full charge is approached
and a trickle charge current always flows while the sun
is shining.
• Pulse charging. Requires semiconductor switch.
• Maximum power point tracking controller electronically
channels the maximum energy to the battery under
changing conditions. Only semiconductor type can do
this. Usually only used for high Wp installations.
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Marshall Islands March 31-April 11, 2008
Programmable controllers
• Using a small micro-processor, the controller can be
programmed to match the needs of different types of
batteries, panels and loads in order to optimize
charging conditions.
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• MPPT controllers are usually programmable
• Can be complicated. Tends to be sensitive to heat and
salt air.
• Sensitive to nearby lightning strikes unless special
lightning protection is included in the controller
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Charge Set Point
• For SHS, controllers depend on battery voltage to
determine whether or not full charge (or maximum
discharge) has been reached.
• Usually a specific voltage is chosen as the cut off point
for charging. For open cell batteries it is typically 14.4
volts. For sealed batteries typically 13.8 volts. This is
the charge cutoff “set point”
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Discharge Control Set Point
• A discharge control set-point voltage is established.
When the battery voltage falls to the set-point, usually
called the LVD (Low Voltage Disconnect) the discharge
controller disconnects the load. According to the type
of battery used and the system design, that set point
may vary from essentially full discharge to 50%
discharge or higher for the disconnect.
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Reset conditions
• After cutting off the charging, charge controllers usually
do not re-establish full charging flow until the battery
voltage falls one volt or more. Necessary because the
battery voltage falls as soon as the charging current is
stopped. Without the “dead band” between charge
cutoff voltage and reset voltage, when the charge cut
off the voltage would drop and turn charging on. Then
the voltage would rise above the charge cutoff voltage
causing the controller to oscillate between on and off.
• After cutting off the load, discharge controllers usually
do not re-connect the load until the battery has
recharged somewhat as determined by a rise in battery
voltage of one to two volts.
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Equalizing Charge
• When cells in a battery have significant differences in
specific gravity, the implication is that sulfation is
progressing. To reverse the early effects of sulfation, a
controlled overcharge can be applied that will bring all
the cells back into full service causing their SG to
become about equal. This is an equalizing charge.
• Programmable controllers usually can be set to
automatically do an equalizing charge at given times or
under specific sets of conditions (e.g. an LVD has
occurred)
• Manual equalization can be done by connecting panels
directly to the battery and not connecting a load until
the battery voltage equals or exceeds 2.6V per cell.
Water level in the cells should be checked each day
and lost water replaced but never overfilling a cell.
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Causes of Controller Failure
• Nearby lightning strikes. The controller switch is directly
in the path of induced voltages in the long wire
connecting the panel and the battery.
• High temperatures caused by blocking the ventilation
around the controller or placing it in a hot location
• Damage by technicians and users
• Insect damage
• Corrosion of circuit boards due to poor design
associated with salt exposure, high humidity and high
temperatures
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Choosing a controller
• Reliability is the result of a simple design
• Avoid complex extra “features” such as many indicator lights, LCD
screens, micro-processor controls
• Choose a control that fits the type of battery being used especially
if it is for a sealed battery since they require specific controller
features for maximum life.
• Be sure there is adequate current capacity for both charging and
for operating the loads
• Low power loss with small internal voltage drop and low internal
energy use.
• Housing prevents insect entry and water entry
• Good lightning protection
• Proper voltage of operation
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Important Installation Note
• There should NEVER be more than 2 meters (6 feet) of
wire connecting the controller to the battery and if
possible it should be shorter than that. The wire should
be the same size as the panel wire or larger.
• Longer wires between the battery and controller result
in inaccurate sensing of battery voltage by the
controller and improper operation.
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Integrated controllers
• Some solar refrigerators and some inverters designed
for solar use have integrated controllers and a separate
controller need not be installed if the refrigerator or
inverter having an integrated controller is the only thing
connected to the panels and battery.
• It usually is not a technical problem to include a
separate controller even if the refrigerator or inverter
has its own controller too though of course that
inreases the cost and may add some more internal loss
to the solar system.
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