Understanding AC & DC Coupled Battery system

Over the past couple of years, battery technology has improved significantly with many new lithium battery types emerging as manufacturers explore different ways to add or couple batteries to new or existing solar systems. The original Tesla Powerwall was the first ‘high voltage’ DC battery system. Ever since its introduction, higher voltage (200-500V) batteries have become increasingly popular and are used with specialised hybrid inverters.

DC-coupled systems are the typical battery-based PV systems where PV is connected to a charge controller that charges a battery. The most common DC-coupled systems use solar charge controllers (also known as solar regulators) to charge a battery directly from solar, plus a battery inverter to supply AC power to the household appliances.

Advantages:

• High efficiency – up to 99% battery charging efficiency (using MPPT)

•  Low-cost setup for smaller-scale off-grid systems up to 5kW

• Ideal for small auto or marine 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.

Disadvantages

• More complex and expensive to set up systems above 5kW as often multiple strings are required in parallel and multiple charge controllers are required.

• Slightly lower efficiency if powering large AC loads during the day due to the conversion from DC(PV) to DC(batt) to AC.

AC-coupled systems use a string solar inverter coupled with an advanced multi-mode inverter. How this works is that the battery-based inverter tricks the utility-interactive inverter into thinking that there is a utility present in order to turn on the utility-interactive inverter.

Although relatively simple to set up and very powerful, they are slightly less efficient (90-94%) at charging a battery compared to DC-coupled systems (98%). However, these systems are very efficient at powering high AC loads during the day and some can be expanded with multiple solar inverters to form micro-grids.

Majority of the modern off-grid homes use AC coupled systems due to the advanced multi-mode inverter/chargers, generator controls and energy management features. Also since string solar inverters operate with high DC voltages (600V or higher), larger solar arrays can be easily installed.

Advantages

• Higher efficiency when used to power AC appliances during the day such as air-conditioning, pool pumps, and hot water systems, (up to 96%).

• Generally lower installation cost for larger systems above 5kW.

• Can use multiple string solar inverters in multiple locations (AC coupled micro-grids)

• Most string solar inverters above 3kW have dual MPPT inputs, so strings of panels can be installed at different orientations and tilt angles

Disadvantages

• Lower efficiency when charging a battery system – approx 92%

• Quality Solar inverters can be expensive for small systems.

• Lower efficiency when powering direct DC loads during the day.

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