A solar charge controller is an integral part of a solar system and is described as the ‘heart’ of the solar setup. They ensure the battery is protected and charged and power is harvested from the solar array.
Lithium batteries are a new addition to the solar PV market with Lead-acid batteries have been the go-to batteries for solar systems. Lithium batteries are built different. They are light yet dense, fast charging, provide a constant voltage, and last longer making them your preferred choice for storing power
Because Lithium batteries are charged differently from Lead Acid batteries they require a different setup in charge controllers. Charge controllers usually come with predefined settings for Sealed, AGM, and Gel batteries but none have really come with settings for lithium phosphate batteries.
The thing you might not need ‘special’ controllers just because you are using lithium batteries. Any controller which allows you to enter custom settings (which you get from the battery manufacturer) will work. We have listed some of them at the end of this article.
Isn’t the Battery Protection Circuit enough?
Lithium batteries are made up of cells that can be safely charged to 4.1 V or 4.2 V per cell and not higher. They should not also be discharged too quickly and are completely empty when discharged to 2.5 V per cell.
Overcharging the battery will damage the battery and can create a fire hazard, discharging a lithium battery too low reduces the cell lifetime.
A Lithium battery has an inbuilt battery protection circuit that prevents overcharging, over-discharging, and discharging too quickly by the load. And that’s as far as it goes.
Why you still need a Charge Controller for Lithium Batteries
So this question is asked very often. If I have this battery protection in lithium batteries why do I need a charge controller in my solar system?
A charge controller and the battery protection circuit have an overlap function of protecting the battery from overcharging. But that’s where it ends. A charge controller’s main function is to regulate the voltage from the solar panels and make sure the battery is being charged.
The Open Circuit Voltage from the Solar panels is irregular and changes according to sunlight. The work of the controller is to bring down this voltage to your battery’s voltage and ensure the battery is charged.
Charge controllers typically have bulk, absorption, and float battery charging stages. Bulk is where the full power of the panels is used to charge the battery, absorption is where a little power trickles in and float is just to keep the battery topped off.
Setting your Charge Controller for Lithium Batteries
Lithium Batteries may just use bulk mode until a target voltage is achieved. You may set for absorption mode for a couple of minutes before going to float. But it is possible to go straight from Bulk to float.
The basic is to ensure the bulk voltage is set at around 14.4V, 14.6 for the absorption stage, and 13.5 or 13.6 for float.
But each battery should come with specifications from the manufacturer:
An Example for the Renogy LFP Batteries:
Standard Charge shall consist of charging at 0.2C constant current rate until the battery reaches 14.6V.
The battery shall then be charged at a constant voltage of 14.6V while tapering the charge current.
Charging will terminate when the charging current has tapered to a 0.02CA. Charge Time is approximately 7 hours. Safe Charging consists of temperatures between 32 ºF and 113 ºF. Battery Standard Discharge is a constant current of 0.2C to 10V.
Also, make sure to turn off Temperature compensation and Equalization features as these settings are not required for lithium batteries.
|High Voltage Disconnect||16 V|
|Boost Voltage||14.4 V|
|Boost Return Voltage||13.2 V|
|Low Voltage Reconnect||12.6 V|
|Under Voltage Warning||12 V|
|Low Voltage Disconnect||11.0 V|
|Discharging Limit Voltage||10.6 V|
Battery Charging Parameters for lithium batteries
Ensure you check with manufacturers’ recommended parameters for your battery.
Awakening a Dead Lithium Battery with a Charge Controller
When you store a lithium battery pack in a discharged state for a long period of time self-discharge slowly depletes any remaining charge in the battery. The protection circuit will turn off in this case making the battery unusable.
Some MPPT charge controllers like the Renogy rover have a reactivation feature to awaken this ‘sleeping’ lithium battery. It does this by applying a small charge current to activate the protection circuit and when a correct cell voltage is reached it then starts a normal charge ‘waking up the battery’.
Victron SmartSolar Charge Controller
The Victron Charge Controller is a Premium Choice for PV Offgrid systems. With options of up to 100 amps and 250V, this is the safest bet when you are planning to consistently add panels to your system.
This is by far the best MPPT charge controllers for lithium batteries. It can accurately track charging voltage to two decimal points.
It has a friendly user interface that you connect with your phone through the Bluetooth module. It has pre-programmed settings for SLA and Gel batteries and for lithium batteries you’ll need to set custom parameters. You’ll just need to go to the settings and set the battery’s parameters as recommended by the manufacturer.