Battery-Based Inverters

Using AC Power Sources and Other Tips
Intermediate

Inside this Article

Modern battery-based inverters offer more than just a DC-to-AC power conversion—they can charge batteries, select from multiple power sources, and control external functions. Here are a handful of common features and what they can do for your RE system.

Integrated Battery Charging

A battery charger in a battery-based inverter greatly enhances the inverter’s versatility and utility. First, it provides an alternate means for charging the battery bank from a power source other than the sun, which is useful in times of short days, poor weather, and/or high loads. It also increases complexity, weight, and cost, but the additional application flexibility usually outweighs these negatives. Several inverter manufacturers offer inverter/chargers, including Schneider Electric (formerly Xantrex), Magnum Energy, and OutBack Power.

Connecting to an AC Power Source

Connecting an inverter/charger to an AC power source opens up a whole new realm of application flexibility. Generators have been the most common backup source for off-gridders, and they can enhance system performance and flexibility. A generator can be used to power large, infrequent loads. During a stretch of cloudy weather, it can simultaneously power loads and help recharge the system’s batteries. A generator can decrease the cost of a PV-based system by allowing it to be sized for average annual insolation rather than for low winter daylight hours. And, if the batteries or inverter fail, a generator can serve as a stand-alone backup to the inverter system.

An inverter/charger can similarly use energy from the grid for backup, or be connected to the grid and a backup generator via a transfer switch, which may be internal or external. This capability can meet a wide range of useful applications and benefits. A PV-based RE system is still used to power designated loads, but the grid can provide a seamless backup. And in the event of cloudy weather and a grid failure, a generator provides yet another layer of backup.

One method of doing this is with “low battery transfer”—if the battery voltage falls below a predetermined set point for a prescribed period of time, the inverter/charger automatically transfers loads to an AC source and turns on the battery charger. When the battery voltage exceeds the “high” set point for a prescribed time period, the inverter transfers the loads from the grid back to the battery bank.

Other methods border on “smart grid” status. For example, a “grid use” feature can be set to instruct the inverter when to connect to the grid. Typically, this setting coordinates with time-of-use (TOU) billing, connecting loads to the grid when rates are low to operate AC loads and perhaps help recharge the batteries. When TOU rates are high, (usually during daylight hours), the inverter/charger disconnects from the grid, and the RE power source and the batteries power AC loads. The result is reduced peak demand and lower utility bills, with a reliable and quiet backup system.

“Grid support” capability uses the RE system to reduce power consumed from the grid. When the inverter recognizes that the solar charge controller has completed its bulk-charge cycle and is operating in absorb or float mode, the inverter then diverts any excess production to power AC loads, thereby reducing AC power supplied by the grid—while the RE system continues to recharge the batteries.

“Peak shaving” initially uses grid energy to supply loads up to a set AC current limit, but then draws on the RE source to provide additional energy if required. This can help reduce use of peak rate utility energy (i.e., surge demand or top-tier TOU), while maintaining the battery bank at a high state of charge as a backup source.

Another sophisticated inverter feature is the AC source input current-limit setting, which limits the total current that will be drawn from an external power source such as a generator, thereby keeping it from being overloaded. If the sum of the regular AC load current and the charger’s AC current exceeds the setting, the inverter “backs off” the AC current drawn. In effect, the charger becomes a variable “opportunity” load.

For example, if the AC loads draw 25 A, and the charger is set to draw 10 A, but the input current limit setting is 30 A, the inverter will reduce the charger load from 10 A to 5 A to keep the total load current at the 30 A limit. However, if a 5 A load is turned off and the downstream AC load total is reduced to 20 A, the inverter will increase the charger load to its 10 A setting, and the total current will still be at or below the 30 A input limit.

The issue here is that the combination of loads might exceed a generator’s output current rating. It’s usually no problem to pull 40 A from the grid, but a generator may only be able to supply 30 A. Setting the AC input limit helps prevent the generator from being overloaded. Some inverter/chargers have two AC input settings: one for “AC 1,” i.e., the grid, and another for “AC 2,” the generator.

These features allow system owners with access to the grid to install and gain from RE systems without having to sever all ties with the grid. The system size can be influenced more by space and budget than by absolute power requirements and location. The owner uses some amount of renewable energy, has backup for assigned loads, and utility electricity use and cost are reduced.

It’s also possible to use these features to suit other situations. For example, if a PV system is impractical and the local grid is unreliable, then a scaled-back battery/generator system without RE may work—basically, it’ll be a big uninterruptible power supply (UPS). This approach offers flexibility in sizing, operating, and maintaining the inverter and battery bank—and adding a PV system at a later date can be fairly easy.

Controlling External Functions

Other battery-based inverter features often include an auxiliary output connection (AUX). Common AUX applications include a “Fan” or “High Battery V” setting to ventilate a flooded-cell battery bank enclosure, exhausting hazardous hydrogen gas released as the batteries are recharged. When the grid is down or for a system without a backup power source, a “Load Shed” setting can be used to disconnect a noncritical load, minimizing battery damage if the battery voltage falls below a set point. For example, if specific circuits for non-essential lights, fans, pumps, hair dryers, etc., are disconnected when the battery state of charge (SOC) drops to 70%, then remaining battery energy is conserved for powering essential loads such as necessary lights, a fridge, and communications equipment. Alternately, this feature could be used to signal the system’s owner that the batteries are getting low and corrective action based on loads and weather patterns may be required.

An automatic generator start (AGS; usually an optional module) feature is used to automatically start and stop a backup generator according to a wide range of parameters, including day of the week, time of day, battery voltage, scheduled exercise, etc. Some inverter/charger manufacturers offer their own optional AGS module; others may require a third-party solution such as an Atkinson Electronics Generator Start Control Module.

Powering 240 VAC Loads with 120 VAC

While some battery-based inverters are available with 120/240 VAC output, with a little outside help, a single-phase 120 VAC inverter/charger can power a 240 VAC load by connecting an autotransformer to the inverter’s output. This is a popular and cost-effective application for systems that have mostly 120 VAC loads but need to occasionally power a 240 VAC load, such as a well pump. In this example, placing the pressure switch, which will disconnect the 240 VAC pump when the water storage tank is full, between the inverter and autotransformer eliminates the transformer’s standby loss when the pump is not operating.

Stacking

Despite all of these features and flexibility, a single inverter/charger sometimes just isn’t up to the task of large loads. Fortunately, many manufacturers’ models can be “stacked” to deliver more power, either through higher voltage (changing 120 VAC to 120/240 VAC), more current, or both. By networking a pair of 120 VAC inverters, their outputs can be wired in series to supply 120/240 VAC split-phase power. Similarly, a pair of 2,500 VA, 120 VAC inverters could be stacked in parallel to provide up to 5,000 VA at 120 VAC.

A potential drawback to operating multiple inverter/chargers is that their collective idling losses can really add up. For example, a single inverter idling at 20 W will consume 480 Wh per day (20 W × 24 hrs./day). Two inverters idling would consume almost 1 kWh each day. Fortunately, these inverters are typically smart enough so that standby units can be user-programmed to “sleep” in a very low-power mode (about 3 W instead of 20 W) when loads are low. They wake up and assist the main inverter when loads exceed what a single inverter can handle, but then return to their low-power sleep mode when loads are once again reduced.

Independent Inverters

Another multi-inverter strategy can reduce whole-system inverter losses and increase low-power efficiency. A smaller inverter, without a built-in battery charger, is selected to efficiently run a range of regular and/or always-on loads, like CFLs, answering machines, cordless phones, and Internet routers. A large inverter/charger is also connected to the battery, but is used to power large (usually, short-term) loads, such as a microwave oven or shop tools, connected to circuits isolated from the small inverter. It’s only turned on when it senses a load via its low-power “search” mode. Using two inverters also provides an additional measure of equipment backup should one fail, and the larger inverter/charger can help charge the batteries if required. Exeltech, Samlex, and Schneider Electric (Xantrex) make smaller sine wave inverters useful for this strategy.

Networking & Monitoring

Inverter/chargers typically offer their own networking features, and there are also third-party solutions for monitoring and some amount of remote control. Networking is required to connect control and display devices; to facilitate most AGS functions; to share common data (i.e., battery temperature); to coordinate operation of multiple devices (i.e., inverters and charge controllers); to troubleshoot system maladies; and, often, to set or adjust system parameters such as input current limits, battery charger set points, and AUX operation.

Monitoring allows a user to track system operation and performance, diagnose potential problems, and sometimes control functions. Monitoring is available locally via a PC, and some products offer remote capability. Examples include greenHouse Computers, Right Hand Engineering, WattPlot, and APRS World.

Choosing Your Features

Not all of these features are available from every battery-based inverter/charger, and multiple features can’t necessarily be used simultaneously. When you’re ready to meet the needs of your particular application, download the user manuals for several inverters, give them a good read, and then contact your dealer and/or the manufacturers for additional information. Additionally, online assistance may be available from manufacturers, and independent forums and list servers. You’ll then be on your way to a system configuration that offers a variety of practical features to help provide improved system performance and convenience tailored to your particular needs.

Access

Jim Goodnight has more than 35 years of design and project management experience, covering a broad range of technical fields. He has been designing and optimizing PV systems since 2002. He began providing technical consulting and field support to OutBack Power Systems in 2004, and formally joined the company in 2008. Jim joined Schneider Electric in 2010 as a senior sales application engineer.

Inverter Manufacturers:

Atkinson Electronics • www.atkinsonelectronics.com

Exeltech • www.exeltech.com

greenHouse Computers • www.greenhousepc.com

Magnum Energy • www.magnumenergy.com

OutBack Power Systems • www.outbackpower.com

RightHand Engineering • www.righthandeng.com 

Samlex • www.samlex.com 

Schneider Electric • www.xantrex.com

WattPlot • www.wattplot.com

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