Battery Storage Knowledge Bank

Inverters/Chargers

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Solax Battery-1.png

Function of the inverter / chargerinverter_charger.png

The battery is connected to the electrics in the property using an inverter /charger.

The charger controls the electricity flowing to the battery during the re-charging phase. It provides a charge profile appropriate to the battery size and type, in particular controlling voltage and current, and adjusting the charge profile to suit the battery cell temperature.

A DC charge controller is used as a direct interface between a DC source (such as a solar array) and the battery. It converts the variable DC input voltage to the precise required DC charge voltage.

An AC charger provides a DC charge from an AC source (e.g. the mains grid, or AC supplied by the solar PV inverter.

An inverter-charger can work as an AC charger when charging the battery (from an AC source), and as an inverter to invert the battery charge from DC back to AC when taking DC power out of the battery.

DC vs AC coupling

Batteries are charged with DC electricity. PV systems generate DC electricity, which has to be converted to AC electricity by the PV Inverter.

There are two ways of coupling the battery system to the PV system / electricity network:

  • AC Coupling – connect the battery charger to the AC side of the network, as in the schematic above. In this case the charger is in fact an “Inverter / Charger”, converting AC back to DC before storage in the battery. The battery Inverter / Charger is separate from the PV Inverter.

    A typical “AC Coupled” schematic is as follows:

ac_coupled.png

  • DC Coupling – connect the battery charger to the DC side of the PV system, thus electricity generated by the PV panels is stored before it is converted to DC.

    DC Coupled systems have a single unit acting as PV Inverter and Battery Charger, making them ideal for “new PV installations”, as opposed to “retro-fit” applications where there is already a PV system in place with a pre-existing inverter.

    The main products currently falling into this category are the Solax X Hybrid, the Samsung SDI 3.6kWh All-in-One System and the SolarEdge “StorEdge” system.

    The schematic for the Solax system is as follows:

solax_system.png

Implications of DC coupling for the Feed-in Tariff

Note that with DC Coupling, the generation meter which measures “generation” by the PV system for tariff purposes is installed after the DC / AC Inverter, and thus after the battery. The total generation will be reduced by the round-trip storage losses (typically 10- 15%) incurred in respect of any energy which is stored in the battery. This will reduce the overall tariff payable on the system.

Thus where there is a high Feed-in Tariff applying to a system (certainly applies for any system installed in the UK prior to any 2016 tariff cut), it can make more financial sense to retro-fit an AC coupled system rather than a DC coupled system.

For new systems which do not attract a high tariff, DC coupling is likely to be superior because it reduces the number of AC/DC inversions, thus reducing the AC/DC inverter losses.

For more information download our free eGuide to Storage or call us on 0118 951 4490