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AC coupling vs DC coupling - Solar battery storage

Jason Svarc

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

  • AC coupled systems

  • DC coupled systems

  • AC batteries

  • DC In-string batteries

AC coupling or DC coupling simply refers to how solar panels are coupled or linked to a battery system. DC coupled systems were the first type of system available and charge a battery directly from the solar array using solar controllers to regulate the battery charging. For a long time this was the only option for off-grid systems. Since solar panels produce DC and batteries store DC energy, this still is the most efficient method of charging a battery using solar.

Some of the many different battery types available, From L to R: Aquion AHI, LG chem Lithium DC, Narada lead-carbon, BAE sealed lead-acid, Tesla AC battery, Pylon Tech Lithium LFP.

Some of the many different battery types available, From L to R: Aquion AHI, LG chem Lithium DC, Narada lead-carbon, BAE sealed lead-acid, Tesla AC battery, Pylon Tech Lithium LFP.

So why would you use an AC coupled system? Most modern solar grid-tie systems use solar inverters to convert the solar DC power to AC power. The cost of solar inverters is much lower for larger systems as they can handle very high DC voltages which mean the solar panels can be installed in long strings, which is much easier and simpler to install. AC coupled systems are more efficient during day when there is high AC demand such as air-conditioning systems and pool pumps.

Below are the advantages and disadvantages of each type...

DC Coupled systems

DC coupled systems use solar charge controllers (also known as solar regulators) to charge the battery system directly from a solar array. The very simple PWM type Solar controllers are a very low cost, effective way to regulate a small battery and come in many different sizes costing as little as $50 for a small 10A version. 

The larger more efficient MPPT type solar charge controllers are much more powerful (up to 80A) but are still limited and most cannot operate above 150 Volts DC so usually only 3 panels can be linked in series, this means for larger solar systems above 1kW it is more complex and involves combining strings of panels in parallel. There are higher voltage solar controllers (600V) available but they are generally very expensive and dont have multiple (MPPT) inputs. Many of the modern hybrid solar inverters are basically DC coupled systems with integrated solar controllers and housed inside a common enclosure.

Basic layout diagram of a DC coupled solar battery system - off-grid setup

Basic layout diagram of a DC coupled solar battery system - off-grid setup

Advantages

  • Very high efficiency - up to 99% battery charging efficiency (using MPPT)
  • Great low cost setup for smaller off-grid systems up to 3kW
  • Ideal for small auto or marine systems requiring only 1 - 2 solar panels.
  • Scalable - Additional panels and controllers can be easily added if you require more solar panels.
  • Very efficient for running DC appliances and loads
  • If a electricity service provider restricts the size of solar allowed to be installed (ie. 5kW max), additional solar can then be added using DC coupling.

Disadvantages

  • More complex to setup for larger systems above 3kW as multiple strings are required to be installed in parallel.
  • Can be more expensive to setup for systems above 5kW as multiple higher voltage solar charge controllers are required.
  • Lower efficiency when powering large AC loads during the day.
  • Many solar controllers are not compatible with modern lithium batteries - Battery management systems - BMS.

Recommended system: Victron Energy Multiplus inverters and Victron Bluesolar charge controllers

The best solar panels for 2017 from leading manufacturer's Sunpower, LG, REC, Winaico and more...

The best solar panels for 2017 from leading manufacturer's Sunpower, LG, REC, Winaico and more...

AC Coupled systems

AC coupled systems use a solar inverter which is coupled to a specialised off-grid (multi-mode) inverter/charger to charge the battery. Although simple to setup this is slightly less efficient when charging as the solar inverter converts the solar DC power to AC and the battery inverter/charger converts the AC back to DC. However solar inverters can operate with very high DC voltages (up to 1000V DC), so very large systems can be installed. AC coupling is also well suited for larger 3 phase commercial systems.

Basic layout diagram of an AC coupled solar battery system - Grid connected (hybrid) setup

Basic layout diagram of an AC coupled solar battery system - Grid connected (hybrid) setup

Advantages

  • Higher efficiency when used to power AC appliances during the day such as air-conditioning.
  • Lower installation cost for larger systems above 3kW.
  • Can use multiple solar inverters in multiple locations (micro-grids)
  • Most solar inverters above 3kW have dual inputs (2 x MPPT) so solar panels can be setup in different orientations and tilt angles.
  • AC coupled systems can use a combination of AC and DC coupling (Note: this can be problematic 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 system: Selectronic SP PRO multi-mode inverters and Fronius Primo solar inverters

 

AC Batteries

AC batteries are a new concept and simply combine a lithium battery with a battery inverter/charger in one simple unit. At this stage they are only available for grid-connected systems and are not designed to function completely off-grid. The most well known AC battery is the Tesla Powerwall 2, Enphase Energy also manufacturer a smaller AC battery system for home use. These systems are compact, modular and can be a very economical choice for self use of solar energy.

Basic layout diagram of a AC battery coupled with a AC solar system - Grid connected (no backup shown)

Basic layout diagram of a AC battery coupled with a AC solar system - Grid connected (no backup shown)

Advantages

  • Economical and simple to install
  • Can be added to homes with an existing solar installation
  • Modular systems which can be easily expanded

Disadvantages

  • Lower efficiency due to conversion (DC - AC - DC) - approx 90%
  • Some AC batteries cannot function as a back-up supply (Enphase)
  • Cannot function in completely off-grid installations.

Recommended system: Tesla Powerwall 2

 

DC In-string Batteries

DC batteries which are connected between the solar panels and solar inverter are another type of battery configuration for residential use. Since typical solar arrays operate at higher voltages (300-500V) these batteries are often high voltage and use inbuilt DC-DC converters to charge the battery. The first generation Tesla Powerwall was an in-string DC coupled battery which was mated to a specialsed hybrid inverter. The inverters used with in-string batteries must be compatible (and communicate) with the battery in order to discharge energy when required.

One way to explain this type of system is the battery is charged during the day and once night falls it discharges energy at the same voltage as a solar array so the solar (hybrid) inverter still generates power as if the sun is still shining.

Basic layout diagram of a DC in-line battery coupled solar system

Basic layout diagram of a DC in-line battery coupled solar system

Advantages

  • Economical and simple to install
  • Can be added to 'some' existing solar installations.

Disadvantages

  • Moderate efficiency - approx 94%
  • Some DC in-line batteries cannot function as a back-up supply
  • Not suitable for off-grid installations.

Recommended system: SolarWatt MyReserve battery with a Fronius or SMA solar inverter