Ve6fi Amateur Radio




Home Station Solar Charge Controller



A home solar system will either use batteries for storing energy or will use a grid-tie system where the power grid will store the excess energy coming from the solar panels. This article will discuss a method for storing energy in batteries. Typically home systems will vary in size from 100 watts to 5000 watts. For this range of wattage and at 12 volts there will be currents of 8 amperes to 400 amperes respectively and one half of that current on a 24 volt battery system. A typical solar panel will be rated at 200 watts.

Analog Charge Controllers

One could build an analog three terminal controller from the 7800 product line but they are only rated at 1 ampere. One could use this three terminal regulator to drive five power transistors (2n3055) to up the current rating to 10 amperes as described in the ARRL Handbook.

There is an analog/digital Solar Switch described in October 1993 QST by WB8VGE. It is rated at 6 amperes and uses Hexfets to switch the charging current to the batteries. The advantage of this one is that it is all solid state.

Digital Charge Controllers

Most commercial controllers for controlling the charging of solar batteries use the Pulse Width Modulation (PWM) technology. With PWM the charging is controlled by the width of the charging pulse. When charging a discharged battery the charging pulse is wide. When the battery is fully charged the charging pulse is narrow. These types of charge controllers, which can handle 10 to 15 amperes, are available at Canadian Tire and Home Depot. Larger capacity controllers are available from firms such as Morning Star and Outback.


The Problem

This digital pulse width method of charging involves the use of square wave pulses. Square pulses produce many harmonics known to us in Amateur Radio as NOISE. Fourier analysis shows that the squarer the pulses the more (harmonics) noise will be introduced. In an Amateur Radio station the man-made noise should be kept as low as possible.

A Charge Controller with Negligible Noise

  • General

The following article will introduce a method of controlling the charge on batteries from solar panels without introducing noise. This design monitors the charging voltage/charging current on batteries. With lead acid batteries the charging voltage limit is 2.5 volts/cell. On a 24 volt battery system this would be a 31 volts; on a 12 volt system this would be 15.5 volts. In this design once the charging voltage rises to 31 volts, the solar panels are disconnected from charging the batteries. This controller will disconnect the panels for a set amount of time and then reconnect them automatically.

Solar panels are a constant current source. They will generate current as long as the sun is shining. Disconnecting the output of a solar panel is not harmful the panel will just stop producing current.

  • Technical Description

The schematic is shown on the bottom of the page. The controller utilizes a five volt reference chip LT027DCN8, a comparator LM339 chip and a timer chip CD4060BE. The reference chip will produce a constant five volt output that the comparator uses as a reference. The comparator has four internal comparators of which one will be used. The timer chip has settings which allow the solar panels to be disconnected for either 2, 4, or 8 minutes.

Smaller 12 volt driver relays are used for low level switching and one main contactor is used for the higher current switching between the solar panels and the batteries.

A 12 volt wall wart supplies 12 volts DC power. It provides all the + Vcc voltage to the reference chip and the relays.

  • Operations

Voltage Comparator A sample of the battery voltage on pin 7 of the Lm339 is adjusted to trip the comparator output to a Low when the battery voltage reaches 15.5 volts. Adjustment is by the 1.0k ohm pot on the input to the Lm339. (It compares the input sample on pin 7 with the reference voltage of +5 volts on pin 6 of the Lm339). The sampling of the battery voltage is continuous.

Relay Driver Rel 1 When the comparator trips, the Low from the comparator will energize the relay Rel 1. When the relay Rel 1 energizes the following occurs:

Transistor Q2 is turned ON and sends a Low to relay Rel 3, the main relay contactor, which had been allowing current to flow from the panels to charge the batteries.

Provides a holding circuit for Rel 1 relay.

Provides 12 volts to start the CD4060 timer.

Timer Operation The solar panels have been disconnected from the batteries by the above sequence. The time that the panels are disconnected is a function of the timer chip CD4060. The Timer has different timing outputs available on pins 1, 2, and 15. The timer is now in operation and once the output on pin 15 goes High, Q1 the 2N3906 transistor will have a Low output and will energizes the Rel 2 relay. When relay Rel 2 energizes the following occurs:

The 12 volts from the operating strap (o/s) of relay Rel 2 is removed resulting in the output of Q2, the 2N3906 transistor, to go High. The input to the Rel 3 relay coil is now High resulting in the relay Rel 3 dropping and allowing charging current from the panels to once again flow to the batteries through the normal closed contacts.

Holding current for relay Rel 1 is disconnected and relay Rel 1 drops out.

Timer is reset to zero as the voltage on pin 16 goes to zero.

Power to the timer is disconnected.


  • Indicators

Some LEDs are shown on the circuit to indicate that different portions of the circuit are operational. On the front panel one LED indicates when the panels are disconnected. Also on the front panel there is a DC voltmeter that monitors the battery voltage. These voltmeters are available as a two terminal device on ebay. (Search digital voltmeters on )  A picture is shown at the bottom of this article.

  • Construction

One can use a four inch square vector board to mount the components on and directly wire the components. Mount the chips and driver relays in sockets. Mount the octal relays, if they are being utilized, in sockets. Parts are available at

Relay Re 3 on the upper right of the schematic shows the relay or contactor that will switch the current between the solar panels and the batteries. You will decide what relays or contactors you will use, as it is not known how many panels and wattages you may have, however here are a few suggestions:

If one has more than one set of panels one can switch them individually with individual relays for each set. Only switch 5 amps on each set of contacts on the Octal relay Z3036-Nd from Digikey

Use an additional timer circuit CD4060, driver relay, and contactors and disconnect some panels at a slightly different voltage.

Use Rel 3 to energize a larger contactor that can handle larger current interruption.

Caution with Relay contacts Connecting and disconnecting solar panels can create problems on relay contacts since one is disconnecting a direct current source. The current will want to continue to flow and could arc across the contacts. Once an arc is established it may not extinguish itself and the contacted will be welded closed. It is not like alternating current which passes through zero and is easy to turn off. The ordinary octal type relays will not handle current much above 5 amps per set of contacts when switching on and off (Octal relay Z3036-Nd Digikey) even though the contacts are rated at 10 amps. It is strongly suggested that you use DPST contactors for switching 10 amps and above. A contactor can interrupt more current since it acts fast and has a greater distance between its contacts. This is one portion of the circuit that you want to over design.

  • Conclusion

The author has a 1200 watt solar array setup at his home utilizing three set of panels each producing up to 16 amps at 24 volts to charge a 24 volt battery string. To switch this 16 ampere circuit a triple pole single throw open face relay is used. There are three sets of these relays switching the three sets of panels. The controller has been is service for three years without any failures.

In the above schematic replace the 2N3906 with 2N3904 (PNP)



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