Post edited 4:42 – March 2, 2010 by Pianotuna
Post edited 4:44 – March 2, 2010 by Pianotuna

Hi all,

How to choose a charge controller.

Why do I need one?

You don't—if there is a huge battery bank and a very low wattage solar panel.

Once the solar panel bank reaches 5 amps (60 watts) per 100 amp hour battery a charge controller is pretty much a "must do". (or you will have fried batteries with your ham and eggs).

In order for a battery to charge there has to be a higher voltage presented to it from the solar panels. Solar cells are often 0.5 volts per cell and a twelve volt battery when fully charged "at rest" is 12.7 volts. Full charged when charging is a different number—usually listed as 13.5 volts.

The cheapest charge controllers check the voltage on the battery and if it is "charged" allow very little current to go to the battery. That works ok up to about 80% of fully charged—but after that these very simple controllers will take 72 hours to finish charging the battery bank.

The next level up of charge controller will use a "pulse" mode to maintain the battery and to help prevent (but not eliminate) sulfate build up on the plates of the battery. 

Many solar panels produce 16 volts or more when "under load". If the load is a 12 volt battery, the the voltage will drop to 12. If the panel has an output of 100 watts @ 16 volts, then the output if directly connected to a 12 volt battery drops by about 25%–so the 100 watts becomes 75 watts. Panel output is in an inverse relationship with heat—the hotter it is the lower the output.

There is a special type of charge controller using what is called Maximum Power Point Technology. What they do is change the DC voltage from the solar panels to AC power and then transform it back to DC power for charging the batteries controlled by a microprocessor. This allows them to recover that 25% wattage loss. If you have four panels that's the equivalent of  having one extra panel for free.

Lead Acid batteries operate using a chemical reaction. The speed of the reaction is affected by the temperature of the battery. Higher temperatures allow speedier reactions. As temperatures drop lead acid cells require a higher charging voltages to get to the same "state of charge".

There are some MPPT charge controllers that have a temperature sensor that is often clamped to a battery post. They are kinder to the batteries and the batteries will last longer.

There are some MPPT charge controllers that have an "equalize" function. If you use flooded cell batteries this is an excellent feature to look for.

Look for an MPPT charge controller that will accept a high input voltage. This will help keep system losses low and will take maximum advantage of the MPPT feature.

There are also some MPPT units which allow for doing some charging on another battery bank once the primary bank is full. For example the Blue Sky 3024di has that feature. I'm sure some others units do this as well.

Conclusions:

Find a charge controller that features MPPT, high input voltage, and a temperature compensation sensor. A display is a nice perk, too, so you can actually know if the batteries are getting a charge.