Solar Charge Controller


This little charge controller could save your battery if you are using a PV panel for a drainback  OR  trickle down solar  heating system... Closed loop systems do not require as much power because they do not require a minimal flow rate. For this reason closed loop pump systems do not require a battery backup although their efficiency is greatly improved with the addition of PV controller.  Unfortunately PV powered drainback systems do require a battery backup and a charge controller is used to protect the battery.

These  parts are all you need to build a simple charge controller capable of handling as much as 20 Amps worth of current. It's designed to disengage the load baring relay when battery voltage drops below 11 volts, but it should be used in conjunction with a solar application that's active when the sunlight is intense. This prevents battery voltage from rising above 15 volts. After the charge controller relay is deactivated the voltage on the battery starts to rise in proportion to the intensity of sunlight available to the PV panel. Once the battery is fully charged  near 15 volts the charge controller relay is activated and the differential controller is free to activate the pump based on a differential temperature.

A  battery protected with a charge controller acts as a buffer between the sun and the pump. Without a battery the supply voltages swings available to the pump would be extreme and not necessarily proportional to the temperature in the collector. Remember DC pumps are designed to work best within a limited voltage range. The charge controller prevents the  low voltage damage and the pump load prevents over voltage damage.



From this simple schematic you should be able to build your own charge controller. Watch this video to better understand the nature of all charge controllers as well as this simple basic charge controller.


Closed loop systems can actually harvest some heat without the aid of a differential controller, but heat collection efficiency can be greatly improved with the addition of a PV controller.  Without a PV controller the intermittent nature of sunlight may cause a pump to turn on pre-maturely before a collector has had time to warm up. Also without a differential controller a pump may also stay on after storage temp is higher than collector temperature. 

Drainback systems must at least pump water to the top of a collector in order to collect solar heated water. If a collector is hot but the sunlight energy is insufficient to pump water high enough no heat will be collected. This is why PV panels are used with a battery backup when they�re connected to a drainback system. Another reason for using a battery back up in conjunction with PV panel has to do with supply voltage regulation. 12 VDC pumps are designed to operate best between 10 volts and 14 volts. Below 10 volts the flow rate of the pump may be insufficient, Above 14 volts the bearings inside the pump may be damaged. A battery helps stabilize the supply voltage fluctuations within a practical range and a charge controller provides assurance that this range is maintained. If the power requirements of a drainback pump are less than 30 Watts a PV controller may be used with a battery backup and a charge controller.

Charge Controller Calculations


The analog charge controller resistors must be carefully chosen to keep the battery voltage between 10.5 volts and 14.5 volts. I use the simple voltage divider of 4.7K and 3.3K to sample the battery voltage and bring it close to the 5 volt reference. This voltage is then buffered with a voltage follower and placed on the left side of the 2.2K resistor. The voltage that appears on opamp  pin 10 is the voltage that's compared to the 5 volt reference. If this voltage is higher than 5 volts the charge controller relay is activated. If this voltage is lower than 5 volts the relay is deactivated. To determine the actual voltage on pin 10 we first need to simplify this circuit to two resistors and 2 voltage supplies...

The actual voltage across these two resistors would be their voltage difference or 9.8V-4.3V = 5.5 volts... Now the total current across both resistors would be  5.3V/12.2K = .45ma... SO... The voltage drop across the 2.2K resistor is 2.2K x .45ma = 1 volt.

AND the voltage on pin 10 is...  4.3 V + 1 V = 5.3 volts.
As long as the voltage on pin 10 remains higher than 5 volts the relay remains activated, but if the battery voltage continues to drop the battery load will be disconnected..
By changing R3 to 1K... and R4 to 22K the voltage on pin 10 only has to reach 11.5 volts to disengage the charge controller relay... 




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