6A, 6V Relay Solar Charge Control

This 6V solar charge regulator meets most small-scale 6V application requirements. With a current rating of 6A, it can handle up to a 50W solar panel. Its principle of operation is very simple and there is only one adjustment: cut-out voltage. Many inexpensive commercial solar charges available use, I believe, this technique. However, this particular design is superior in that it is powered via the solar panel rather than the battery –the implications of this are obvious: low battery discharge current. The switching is accomplished via an inexpensive 10A relay.

One of the interesting features is the secondary load output that can be used for other purposes within specific loading limits –in other words, there are two outputs: one for charging the battery and the other for use when the battery is not charging so that the solar power may be more fully utilized. This is a low voltage extension of the 6A, 12V relay based solar charge control. While the new schematic may appear very similar, there are subtle differences that are geared to make it function for 6V applications. Also, I have specified an alternative 10A Schottky rectifier in order to stretch the current rating.

The application for this type of charge control is one in which the battery capacity is large in respect to the charging current; e.g. 6A solar panel charging a 60AH battery in which it takes perhaps a full day’s sunshine to fully charge the battery. For much smaller batteries, the high charging rate and relatively high battery internal resistance results in excessive terminal voltage so that the control immediately interrupts charging –the end result is that the battery cannot charge fully –a linear charge regulator is more appropriate in such cases. Most of the chargers that I have posted on electroschematics to date have been of the linear type. I have plans for at least two more solar chargers, so keep watching.

Schematic of the Solar Charge Control Circuit

6V Relay Based Solar Charge Control Schematic

Bill of materials


Circuit function

U2 provides a 2.5V reference voltage. U1 is a LM339 quad comparator connected as a S-R flip flop with wired OR outputs so that it may be conveniently driven via open collector output structure. U3 is a CMOS 555 clock generator that generates 30mS pulses every 30S. Its function is to set the flip flop so that charging may commence. (Another descriptive name for this topology is “minimum on-time regulator.”) D1 effectively changes the totem pole output of the 555 to open collector. As soon as the battery terminal voltage exceeds the set point (7.2V or so), comparator U1D resets the flip flop and drops the relay. At this point, it remains dormant until the next clock pulse. When fully charged, the relay flips on and off briefly upon each clock pulse. U1C and Q1 constitute a relay driver –the NPN open collector output of the LM339 and Q1 (PNP) make up the classic composite transistor connection. This is necessary to boost the low power output of the LM339 sufficiently for driving the relay coil.

Clock timing issue

Unlike the 12V version, the 555 is powered via a variable voltage –when the relay drops or closes, the solar panel input voltage makes step voltage changes. The timing capacitor (C2) cannot follow this change in voltage and causes the clock pulse timing to be somewhat variable. Fortunately, this is of no consequence as the circuit continues to function. This info is for the observant technician.

Secondary output limitation

While this neat feature is provided, its application is limited. Note that the load resistance is not allowed to drag the solar panel output below about 5V under marginal lighting conditions or else the relay may not have sufficient voltage to pick up. This load could conceivably operate a low-power heater etc. The big challenge is to obtain the proper load resistance –a number of aluminum resistors such as the Vishay /Dale TMC50 series may be a good solution. Also, before selecting the resistors make actual measurements to see what works –even then, experimentation may be required for best results.

Alternative rectifier for 10A current rating

(Note that this upgrade is applicable for just about all previously published solar charge controls.)
The current rating is limited by D5 (6A) and the relay contacts (10A). By using a 10A rectifier such as the Micro Commercial Schottky SR1045, a 10A current rating is feasible. To go beyond this, both a high current rectifier and relay must be selected. Note that these rectifiers run hot at the rated current, so additional PCB foil area or other heat sink is be required. On one previous project, I fabricated heatsinks by hammering pieces of AWG#14 wire into flat strips and soldering them to both ends of the rectifier. Note that the Schottky rectifier is very desirable and well suited for this low voltage application due to its low voltage drop (0.48V @ 6A or 0.55V @ 10A). This means that it will function well if the solar panel generates only 8V instead of the specified 9 to 10V.


To set the Max Voltage Adjust (R12) potentiometer, start with it turned CW, monitor the voltage and wait for the battery terminal voltage to reach the desired voltage (e.g. 7.2V). At this point, turn R12 CCW until the relay drops. When the relay picks up again, recheck the voltage at which it drops.

Charging a dead battery

If the battery voltage is too low (e.g. 2 or 3V), the relay may drop out immediately due to undervoltage. To get around this problem, connect the solar panel directly to the battery until the voltage charges to at least 5V or so, or flash it by connecting it directly across a charged battery for a few seconds.


For the future

  • 6A, 6V SSS based solar charge control
  • Solar charge control with synchronous rectifier
  • MPPT buck converter solar charge control
  • Selection guide for the various solar charge controls

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