16 Charge Controller Musings

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

water consumption.

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 a 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

batteries.

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

most wattage.

An MPPT controller "chases" the voltage to find this sweet spot.

There are many claims of 20 to 35% increase in module.

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.

The controller will drop the incoming voltage down to the charging voltage of the

batteries.

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, just middle-aged.

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.

If you would like to get into a technical discussion with someone, it is certainly not me.

John

12/5/07

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