Charge Controllers


The function of the charge controller is to limit the maximum voltage at the battery in order to avoid damage to the battery from overcharging.  We call it a charge controller to differentiate it from the device used with an engine driven alternator, called a voltage regulator.  Charge controllers are rated firstly by the maximum amperage (and sometime wattage) they can handle.  If the maximum amperage from the solar panel is less than one percent of the battery capacity (in amp-hours at the 20 hours rate), you probably can do without a controller.  If the battery is AGM or Gel, if the solar panel can put out more than 2% battery capacity, you probably need a controller.  Some controllers have a display that gives some indication of the charge mode and/or the energy going to the battery, in some cases as simple as an LED light, and in others an LED display that tells how much energy is going to the battery.  Some controllers are called “smart” because they change the voltage setting depending on the battery state of charge, making it possible to recharge the battery more rapidly.  The voltage at which a battery should be charged depends to some extent on the battery temperature.  The higher the temperature, the lower the voltage.  Batteries that are stored in the engine compartment or experience large swings in temperature should have controller with temperature compensation.  Some controllers come with a battery temperature sensor (BTS) and others have it available as an option.


load 001
Hotwire’s resistive load bank
12/120 water heater element

Wind generator need a different kind of controller.  With solar panels, the controller can open the circuit, preventing current from going to the battery until the voltage falls.  Solar panels are perfectly happy with this.  Operating them open circuit or short circuit does not harm them.  Wind generators, however will go to very high voltages when open circuited, and then when the circuit is completed again, cause a sudden rush of energy that can damage the switching devices in a solar controller.  With wind generators, we divert the current into a diversion load in order to prevent overcharging the battery.  We have two different diversion loads available: a set of wire-wound ceramic power resistors or a dual voltage water heater that has two elements in one; a 12 volt side and a 120 volt side, so you can still make hot water with shore power or a genset, and actually get something (at least sometimes) from the excess energy from the wind generator.  The 12/120 element fits most US-made water heaters, and requires an adaptor for some square flange water heaters.  You’ll want a 1 1/2 inch socket to replace the original element, and if your water heater is more than 10 years old, you probably do not want to attempt element removal.


Some charge controllers have an additional feature called Maximum Power Point Tracking (MPPT), that actually increases the output of the solar panel.  Most solar panels are made with some additional voltage available to make up for the loss of voltage at higher temperatures.  It is possible to take this additional voltage and “transform” it into more amps going into the battery.  For more info on how it works, see: Although the manufacturers say you can get up to 30% more energy, in my testing, MPPT added more like 10% on a regular basis, still enough to make it worthwhile for most installations.  Many MPPT controllers can take a high voltage panel (like most of the ones over 150 watts) o charge a 12 volt battery, making it possible to use less expensive (per watt) large panels.

Controllers from smallest to largest:

Solar Converters 3 amp MPPT, LED light,  $113.85  Small MPPT controller info    The smallest MPPT controller available.  (call 727-943-0424)

Morningstar SG-4 3.5 amp   MORE INFO     $32  Morningstar SG-4 info  The smallest reliable PWM controller.    (call 727-943-0424)

Solar Converters 5 amp,  MPPT, LED light, BTS, MORE INFO   $136.80   (call)

Morningstar SS6 6 amp, no frills, Optional LVD,    MORE INFO   $48     (call)

Morningstar SS10 10 amp, no frills, Optional LVD,   MORE INFO,  $58   (call)

prostar-splash[1]Morningstar PS15  15 amps, meter (built in),  LEDs, optional LVD,  MORE INFO  $150  BUY NOW (call)


Morningstar SS20 20 amp,  no frills,  MORE INFO  $97 BUY NOW (call)

Blue Sky Energy 2000e  25 amps  MPPT, built-in meter, optional BTS  MORE INFO   $289   BUY NOW



Blue Sky Energy 2512i HV 25 amps, MPPT  MORE INFO $213

Blue Sky Energy 3024iL 30 – 40 amps  MPPT optional remote display, optional BTS, MORE INFO  $389   (call)  optional wind and solar $448

Xantrex (now Schneider Electric) C-35 35 amps, solar or wind (diversion), optional meter (solar only), optional BTS,   MORE INFO   $115   (call)

Xantrex  (now Schneider Electric) C-40  40 amps, optional BTS, wind or solar, optional display (solar only)  $165 (call)

Morningstar Tristar 45,  45 amps,  wind or solar,  Optional MPPT,  Optional BTS,  Optional Meter,  $175 or $451 (MPPT)

Morningstar Tristar 60, 60 amps, wind or solar, Optional MPPT, Optional remote meter,  Optional BTS   MORE INFO   MORE INFO (MPPT)    $224,  $567 (MPPT)

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