With demand increasing for energy storage, more solar customers have questions about how batteries can work with their solar systems. Let’s talk about how to couple PV and batteries as well as the various types of inverters on the market.
There are essentially 2 different ways an inverter can output power:
- Interactive, aka grid-tied, aka current mode, aka anti-islanding
- Stand-alone, aka off-grid, aka voltage mode, aka islanding
A multimode is an inverter that can do both, which can also be called a bimodal inverter.
When you connect a battery to an inverter, it is also an inverter/charger.
If you have a system that works with grid-tied and battery backup, it typically is going to use a multimode inverter with a battery.
If you have a stand-alone PV system without grid-tied, then you only need an inverter that connects to the battery when you have a dc-coupled system.
With ac-coupling and dc-coupling, you are defining how PV will charge batteries. If you have to convert from dc PV to ac and then back to dc to charge the batteries, then it is ac-coupling. If you can go directly from dc PV to dc batteries, then it is dc-coupling.
A typical system that does ac-coupling will have interactive solar inverters that can be connected to the backed-up loads and the output of the battery inverter on a panelboard (subpanel or main service panel). When the grid goes down, these circuits will be isolated from the grid and you will essentially be operating an ac microgrid from the output of your battery inverter and you can keep the loads and the interactive inverters on.
A dc-coupled system in its simplest form will have 4 main components:
- Charge controller
The PV is connected to the charge controller, so you do not overcharge the batteries.
The batteries are connected to the output of the charge controller.
The inverter is connected to the batteries (sometimes through the charge controller).
With a dc-coupled system in this explanation, if you are operating in stand-alone mode only, then you do not need a multimode inverter. If you are connected to the grid, you can have a multimode inverter.
Often the multimode inverter will have an internal automatic transfer switch (aka microgrid interconnect device or MID), where you will have 2 outputs, one which is for operating backed-up (stand-alone) loads and the other which is for loads that will be connected to the interactive output and do anti-islanding.
This MID is sometimes external to the inverter and can be used for ac-coupled and dc-coupled systems.
A battery inverter is a term that is often used to describe an inverter that is connected to a battery that can also do charging. The NEC defines an inverter as something that can convert dc to ac. With a battery, we can also convert ac to dc when we are charging, so a perhaps better word to use would be a battery inverter/charger or a battery electronic power converter. Because a battery inverter does convert dc to ac, I think it is still ok to call it an inverter, it’s just important to note that it can do more.
A stand-alone inverter output circuit is a constant voltage and frequency source that will increase current as loads demand it.
An interactive (grid-tied) inverter output circuit will match the grid’s frequency and voltage, and then send out current as the sunlight makes power available or as batteries are instructed to export. An interactive inverter is unable to make voltage and frequency on its own.
A “hybrid inverter” is essentially a charge controller and a multimode inverter in a single box that is connected to PV and batteries. (Hybrid inverter is not defined in the NEC and neither is microinverter).
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