Solar Battery System Types – AC vs DC Coupled Batteries

What is AC or DC coupling? AC or DC coupling refers to the way solar panels are coupled or linked to an energy storage or battery system.

The type of electrical connection between a solar array and a battery can be either Alternating Current (AC) or Direct Current (DC). AC is when the current flows rapidly forward and backwards (this is what the electricity grid uses to operate) and DC is where the current flows in one direction. Most electronic circuits use DC, while solar panels produce DC, and batteries store DC energy. However, most electrical appliances operate on AC. This is why all homes and businesses have AC circuits. DC can be converted to AC using an inverter but, as explained below some energy is always lost in the conversion.

The Solar battery evolution

Simple DC coupled solar battery systems were once used only for remote power systems and off-grid homes, but over the last decade inverter technology advanced rapidly and led to the development of new AC coupled energy storage configurations. However, DC coupled systems are far from dead, in fact charging a battery system using a solar charge controller or hybrid solar inverter is still the most efficient method available.

Over recent 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. Since then higher voltage (200-500V) batteries have become increasingly popular and are used with specialised hybrid inverters. More recently, AC batteries have been developed by many leading solar manufacturers including Tesla, LG Chem, Enphase, BYD and LG Electronics.

With the many complex varieties of battery storage systems now available, here we explain the advantages and disadvantages of each type.

The 4 Main Solar Battery System Types

1. DC coupled systems

2. AC coupled systems

3. AC Battery Systems

4. Hybrid Inverter Systems

1. DC Coupled Systems

DC coupled systems have been used for decades in off-grid solar installations and small capacity automotive/boating power systems. 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.

For micro systems, such as those used in caravans/boats or huts, the simple PWM type solar controllers are very low-cost way to connect 1 or 2 solar panels to charge a 12 volt battery. PWM (pulse width modulation) controllers come in many different sizes and cost as little as $25 for a small 10A version.

For larger systems, MPPT solar charge controllers are up to 30% more efficient and available in a range of sizes up to 100A. Unlike the simple PWM controllers, MPPT systems can operate at much higher string voltages, typically up to 150 Volts DC. However this is still relatively low compared to grid-tie solar string inverters which operate 300-900 V.

View our MPPT solar charge controllers in our DIY Store Online here.

Due to the common 150V upper voltage limitation of many MPPT solar controllers, only 3 panels can be linked in series which means for larger solar systems above 3kW the installation can become more complex and involves combining strings of panels in parallel with fuses.

Higher voltage MPPT solar charge controllers

More powerful, higher voltage solar controllers are available; up to 250V from Victron Energy and 300V from AERL in Australia. There are also even higher 600V units available from Schneider Electric and Morningstar. These are much more expensive and don’t have multiple MPPT’s inputs like many solar string inverters used in AC coupled systems. However, MPPT charge controller are still a relatively cheap and very secure way of ensuring batteries are charged even in the event of a AC inverter shutdown – this is especially important in remote locations.

Modern all-in-one hybrid inverters work much like DC coupled systems. These inverters incorporate high voltage MPPT solar controllers and a battery inverter/charger inside a compact plug and play unit. However, they have several limitations as explained in section 4 below.

Advantages

• Very high efficiency – up to 99% battery charging efficiency (using MPPT)
• Great 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.
• If an electricity service provider restricts or limits the capacity of grid-tie solar allowed (ie. 5kW max), additional solar may be added by DC coupling a battery system.

Disadvantages

• More complex to setup systems above 5kW as often multiple strings are required in parallel, plus string fusing.
• Can become expensive for systems above 5kW 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(PV) to DC(batt) to AC.
• Many solar controllers are not compatible with ‘managed’ lithium battery systems such as the LG Chem RESU or BYD B-Box.

Recommended DC Coupled Systems

• Victron Energy Multiplus inverter/chargers and solar charge controllers,
• Selectrconic SP PRO multi-mode inverter/chargers DC coupled with the AERL Coolmax MPPT solar charge controllers.
• Outback Power Radian, FXR Inverters & FLEXmax MPPT sol ar controllers
• Magnum Energy Inverters and Charge Controllers
• Phocos AnyGrid

2. AC Coupled Systems

AC coupled systems use a string solar inverter coupled with an advanced multi-mode inverter or inverter/charger to manage the battery and grid/generator. Although relatively simple to setup 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.

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.
• Advanced AC coupled systems can use a combination of AC and DC coupling (Note: this is not possible with some lithium batteries)
• 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.

Recommended AC Coupled systems

• Selectronic SP PRO inverter/chargers and Fronius solar inverters
• SMA Sunny Island and SMA sunny boy inverters
• Victron Energy Multiplus or Quattro inverters and Fronius solar inverters.

3. AC Batteries

AC batteries are a new evolution in battery storage for grid connected homes which allow batteries to be easily AC coupled to your new or existing solar installation. AC batteries consist of lithium battery cells, a battery management system (BMS) and inverter/charger all in one compact unit.

These systems combine a battery with an battery Inverter/charger and are generally only designed for grid-connected systems, as the (transformerless) inverters are typically not powerful enough to run most homes completely off-grid. The most well known AC battery is the Tesla Powerwall 2, along with the SonnenBatterie which is more common in Europe and Australia. Leading micro inverter company Enphase Energy also manufacturer a very compact AC battery system for home use. These systems are generally simple to install, modular and one of the most economical choices for storing solar energy for later use.

AC coupled battery inverters *

A more recent trend is to use a ‘retrofit’ AC coupling inverter to create a AC battery system. These systems use a specialised AC coupled battery inverter such as the SMA sunny boy storage together with a common DC battery such as the popular LG Chem RESU.

Advantages

• Easy retrofit – can be added to homes with an existing solar installation

• Economical way to add energy storage.

• Generally simple to install.

• Modular system to allow expansion.

Disadvantages

• Lower efficiency due to conversion (DC – AC – DC) – approx 90%

• Some AC batteries cannot function as a back-up supply (Enphase)

• Not designed for off-grid installations.

Recommended AC Battery Systems

• SonnenBatterie ECO

• Tesla Powerwall 2

• SMA sunny boy storage and LG chem RESU.

• Goodwe SBP series – AC Retrofit inverters

• Huawei SUN2000

4. Hybrid Inverter Systems

Hybrid systems can be described as a grid-connected DC coupled solar battery systems. They come in many different configurations and typically use a hybrid or multi-mode inverter. Modern hybrid inverters incorporate high voltage MPPT controller/s and battery inverter/chargers inside a common unit. The first generation hybrid inverters were compatible with 48V lead-acid or lithium battery systems, however over recent years higher voltage (400V+) hybrid systems have become increasingly popular.

High Voltage or Low Voltage? The new generation ‘high voltage’ batteries operate in the range of 300-500V DC (400V nominal) as opposed to the traditional 48V battery systems. This offers several advantages, including increased efficiency as the solar array typically operates at 300-600V which is very similar to the battery voltage.

The new generation higher voltage (400V) batteries and compatible hybrid inverters use lithium battery systems operating between 120-500V DC, rather than 48V. Higher voltage batteries can be configured in two different ways:

1. DC coupled between the solar array and inverter.

2. DC coupled directly to a compatible hybrid inverter (as shown below).

Since most solar arrays operate at high voltages around 300-600V, high voltage batteries use efficient DC-DC converters with very low losses. The first generation Tesla Powerwall was the first 400V battery available and was mated to the popular SolarEdge Storedge hybrid inverter.

The new LG Chem RESUH battery range is now one of the most popular LV 400V battery systems available being compatible with many hybrid inverters including SolarEdge Storedge, SMA sunny boy storage and Solax X-hybrid Gen 3.

Advantages

• Economical and simple to install

• Compact, modular battery options

• Smaller cable size and low losses using high voltage (400V battery systems)

• Can be retrofitted to ‘some’ existing solar installations.

• High efficiency battery charging – approx 95%

• Growing number of hybrid inverters becoming available

Disadvantages

• Some systems cannot function as a back-up power supply

• Many systems with back-up have a 3-5 second delay during a blackout

• Generally not suitable for off-grid installations due to transformerless hybrid inverters with low surge rating and no generator controls.

Recommended Hybrid systems

• SolarEdge StorEdge inverter and LG Chem RESUH

• Sungrow SH5K-20 and LG chem RESU

• Huawei SUN2000L and LG Chem RESUH

• SMA Sunny Boy Storage and BYD HV

AC Vs DC Coupled For Off-Grid Systems

Why would you use an AC coupled off-grid system rather than DC coupled?

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.

Note: If an AC coupled system is used for off-grid installation the solar inverter/s must be compatible with the main (multi-mode) battery inverter/charger to enable charge control. This is required so the solar generation can be ‘managed’ or ramped up and down to ensure safe and accurate battery charging. There are several management systems used by different manufacturers such as frequency ramping (used by SMA and Victron) or direct communication – refer to manufacturers specifications.

As explained previously DC coupled systems are extremely cost effective for small to medium size systems. Another advantage of DC coupled systems is solar controllers are very flexible and scalable meaning additional panels can be easily added if required using relatively low cost DC solar controllers.

AC And DC Coupling Combined

Most modern interactive inverter/chargers and such as Selectronic’s SP PRO, SMA sunny island and Victron multiplus can function in both AC and DC coupled configurations. This offers the best of both worlds and provides back-up DC battery charging in the event of an AC shutdown.

If there is a shutdown in a remote location due to low battery voltage or low SOC the DC coupled solar charge controllers will continue to function (without AC operation) and charge the battery system which preserves the battery life and can restart the system if configured correctly. This creates a more fail-safe setup unlike a pure AC coupled system which are unable to be automatically or remotely restarted unless a back-up generator/source is functioning. A combined AC and DC coupled off-grid solar system – Selectronic SP PRO AC coupled to a Kaco solar inverter. The lead-acid battery bank is also charged with two DC coupled Victron MPPT solar controllers.

Please note: this is a guide only. All solar and energy storage systems must be installed by a licensed electrical/solar installer. Solar & battery systems generate huge amounts of energy which can result in damage or serious injury if the installation does not meet all relevant regulations, standards & industry guidelines.

Read more similar articles on Clean Energy Reviews