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16 Charge Controller Musings
The following are my opinions.
They are based on speaking with both manufacturers and end
users of photovoltaic charge controllers as well as from personal
experience using different types of controllers since the late 1970's.
Please take these musings for what they are worth to you.
There are basically four kinds of photovoltaic charge controllers: the on/off, the shunt,
the Pulse Width Modulation and the Maximum Power Point Tracking controllers.
On/Off controllers disconnect the pv modules from the battery(s) when the battery
voltage comes up to a pre-set point and reconnects them when the battery voltage
drops to the reconnect voltage.
In some cases a blocking diode is required to prevent power from flowing back into
the pv modules at night.
This type of controller is very gentle on the batteries which reduces outgassing and
They also do not produce radio interference emmissions.
Shunt controllers usually shunt (short circuit) the pv module outputs together when the
batteries are full.
Some shunt controllers will divert incoming pv power to an auxillary load to prevent
overcharging of the batteries.
This type of controller usually has a blocking diode to prevent back-feeding to the
modules at night and to protect the batteries.
Pulse Width Modulation controllers pass power from the pv modules to the
batteries in the form of high voltage (within the operating voltage of the modules)
/ short duration spikes.
Many of these can also perform battery equalization charges (a controlled over-charge)
to help prevent stratification of the acid in the electrolyte.
This may also help bring up a weak cell in a battery.
A down side is that these chargers can produce radio frequency interference.
At least one manufacturer has directions in the owners manual as how to
change their PWM controllers into an on/off controller if this is an interference
problem in a specific application.
Some PWM controllers can be set for different types of batteries to reduce the
high end voltage as well as to shut down the equalization feature with sealed
A PWM controller will, as a rule, cause a battery to require more watering than
an on/off controller.
Maximum Power Point Tracking controllers are an outgrowth of the linear current booster
technology used in battery-less solar powered water pumping.
In my simple terms, am MPPT controller "hides" the batteries from the pv modules.
This allows the modules to output a voltage close to the open circuit voltage.
There is a "sweet spot" in the voltage/amperage curve in a pv module.
It is called the Maximum Power Point Voltage and is where the module puts out the
An MPPT controller "chases" the voltage to find this sweet spot.
There are many claims of a 20 to 35% increase in module output.
My simple little mind does not accept that unless you are using a tracker with
reflectors/concentrators and have a very low cell temperature.
Early in the morning and late in the day the module voltage drops to the point where the
controller is chasing a voltage that it can not find.
Under low light conditions an MPPT controller can output less wattage than other kinds
of charge controllers.
An MPPT charge controller is very effective when the pv modules must be placed some
distance from the batteries.
The pv array can be wired for a much higher voltage than the nominal battery voltage.
By using a higher voltage pv array, smaller wire can be used between the array and
The controller will drop the incoming voltage down to the charging voltage of the
In my opinion, this is the best reason for using an MPPT controller.
As of this writing we are using eight charge controllers.
One is an on/off controller by Flexcharge.
One is an MPPT controller by Solar Converters.
One is a PWM controller by Steca.
The rest are PWM controllers by Morningstar.
Please note that I am not an electrical engineer nor a middle-aged whiz kid,
My pedigree consists of sliced thumbs, busted knuckles, thermal and acid burns.
I know what has worked for me and what has not.
My recommendation is to let your application determine the type of charge controller
you will need.
And don't go on the cheap.
If you would like to get into a highly technical discussion with someone,
it is certainly not me.
If you have questions, just call and ask.
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