AC and DC Coupled Batteries: know the difference

When connecting a solar array with a battery, it is not an easy matter to just connect a battery of suitable capacity with a solar array. 

Advanced AC coupled off-grid systems use modern solar inverters to convert solar DC power directly to AC which can then be used immediately by most appliances during the day. This is very efficient, especially when powering high loads such as air-conditioning systems, modern kitchen appliances and water/pool pumps. Additionally, the installation cost of AC coupled string solar inverters is lower for larger systems above 5kW, due to the multiple MPP trackers, higher string voltage up to 1000V, and higher capacities up to 10kWp single phase.

AC Coupled vs DC Coupled

You can have two types of electrical connection between a solar array and a battery; which are alternating current (AC) and direct current (DC). AC is when the current flows rapidly forward and backwards while DC is where the current flows in one direction.

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    In most electrical circuits, DC is used. Solar panels produce DC and batteries store it. However, most appliances at home or in the workplace use AC current. This is because not all appliances can handle DC power, so AC currents are converted to run them instead. DC can be converted to AC using an inverter but, as explained below some energy is always lost in the conversion.but depending on your setup, there might be energy losses.

    Main solar battery configurations

    1. DC coupled Battery Systems – Grid connected
    2. AC coupled Hybrid Systems – Grid connected
    3. DC coupled systems – Off-grid
    4. AC coupled systems – Off-grid

    Grid-connected DC coupled battery systems

    Off-grid DC coupled

    DC Coupled systems are commonly used for off-grid installations and automotive/boating power systems. Typically they use a solar charge controller that charges the battery directly from the solar panels, with a battery inverter that changes the DC load from the battery and panels into AC which can be then used with your appliances. 

    A solar charge controller, also known as a solar regulator, is essentially a solar battery charger connected between the solar panels and battery. Its job is to regulate the battery charging process and ensure the battery is charged correctly, or more importantly, not over-charged.

    Two main types of solar charge controllers, PWM and MPPT.

    Most solar charge controllers are PWM, which are more simple than MPPT type controllers, and use a basic mechanism where a switch turns off and on to modulate charge voltage, and maintain a consistent battery voltage. Usually solar panels have a higher voltage than batteries, thus the PWM controllers reduce the load and makes charging less efficient.

    MPPT controllers are more advanced than PWM controllers and have the ability to enable a solar panel to operate at its maximum power output and optimum voltage. These solar charge controllers can be up to 30% more efficient, depending on the battery voltage and operating voltage of the solar panel. 

    Pros and cons of Off-grid DC coupled systems

    Advantages:

    • Very high efficiency – up to 99% battery charging efficiency, using MPPT
    • Great low cost setup for smaller scale off-grid systems, up to 6kW
    • Ideal for mobile homes and small boat systems, requiring only 1 – 2 solar panels.
    • Modular – Additional panels and controllers can be easily added if required.
    • Very efficient for powering DC appliances and loads.
    • If an electricity service provider restricts or limits the capacity of grid-tie solar allowed, additional solar may be added by DC coupling a battery system and MPPT charge controllers.

    Disadvantages:

    • More complex to setup systems above 6kW as often multiple strings are required in parallel, and may need string fusing
    • Can become expensive and complex for systems above 6kW as multiple higher voltage solar charge controllers are required
    • Slightly lower efficiency if powering large AC loads during the day due to the conversion from DC(from panels) to DC(batteries) to AC

    Off-grid AC coupled

    AC coupled systems are often used for larger-scale systems, and the difference between AC coupled and DC coupled systems are; they don’t use solar charge controllers and instead use a string inverter along with a multi-mode inverter to manage the battery and grid/generator. While still simple to set up, they are less efficient at charging batteries. However, these systems are more efficient at powering high AC loads during the day and some also can be expanded with multiple solar inverters to form micro-grids.

    Many modern off-grid homes are being powered by AC coupled systems, thanks to the use of string solar inverters which can operate with higher DC voltages (up to 600V or higher) than DC coupled systems. This allows much larger solar arrays to be easily installed at lower cost and complexity compared to DC coupled systems that would require multiple MPPT charge controllers for large voltages (around 10 to 20kW+ solar inverters).

    Advantages:

    • Higher efficiency when used to power AC loads during the day such as air-conditioning, pool pumps, and hot water systems (up to 97% efficient)
    • Generally lower installation cost for larger systems above 6kW
    • Can use multiple string solar inverters in multiple locations (i.e AC coupled micro-grids)
    • Most string solar inverters above 3kW have dual MPPT inputs, so long strings of panels can be installed at different orientations and tilt angles

    Disadvantages:

    • Lower efficiency when charging a battery system – approx 85% to 90%
    • Solar Inverter capacity may be limited to the Inverter-charger size 
    • Quality Solar inverters can be expensive for small systems
    • Lower efficiency when powering DC loads

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