Magnum Energy ACLD-40, 4 kW AC load controller for AC-coupled systems to provide 3-stage battery charging
The Most Sophisticated Way to Add Three-Stage Charging to your AC-Coupled System
The ACLD monitors the battery voltage of a backup battery bank, and if the voltage rises to a predetermined level, the ACLD connects a diversion load of sufficient size, to the 240 Volt AC side of the Magnum inverter, to prevent the battery voltage from increasing any further. The controller will continue to engage and disengage the load as often as necessary, using Pulse Width Modulation (PWM), to prevent battery overcharge. The ACLD will do this in a way that makes the batteries follow a proper 3-stage charging profile, with bulk, absorb, and float charging.
An AC Load Diversion controller is used to divert excess energy to an AC load in an effort to keep the battery bank that is connected to a back-up inverter from being overcharged, when used in an AC Coupled application. A possible diversion load could be a heating element in a hot water tank. Standard 240 Volt AC resistive loads can be used with the ACLD, which are easy to find and cheap!
What is AC-Coupling?
The term “AC-coupling” is used for systems that use grid-tie inverters in an off-grid setting. For example, a grid-tied solar system with a string inverter or micro-inverters is normally used connected to the grid, feeding power back to that grid. Grid-tie inverters need to see a stable grid to feed power to it, and they will switch off when the grid is down. To make grid-tie inverters work in an off-grid scenario the “grid” is provided by the off-grid battery-based inverter, such as a Magnum MS4024PAE etc., and the grid-tie inverter is connected to the 240 Volt AC side of the battery-based inverter. This fools the grid-tie inverter into believing the grid is present and it will feed power to that as usual. That is AC-coupling, or an AC-coupled system.
The power provided by the grid-tie inverters(s) will first be used by the local loads hooked up to the off-grid system. If more power is produced than can be used locally, the excess will flow back over the battery-based inverter, as charge current into the battery bank. That is how an AC-coupled system can charge the batteries from a grid-tie inverter.
What Are the Issues With AC-Coupling?
AC-coupling works great with many of the higher-end off-grid inverters, such as Magnum, Xantrex, Conext/Schneider, and Outback! However, there are two potential pitfalls:
The first potential problem is that the battery-based inverter will not provide charge control for the power funnelling back into the batteries. This can cause overcharging of the batteries if nothing is done to prevent it. The generic solutions used to prevent over-charging with AC-coupling is to have a contactor disconnect the grid-tie inverter(s) when the battery Voltage indicates that they are nearing full charge. Usually there is an auxiliary relay somewhere (the Magnum inverter has one, so does the Classic charge controller), and it can drive a contactor to do the job. Working off of Voltage is not a great indicator of state-of-charge of the batteries though, as you can essentially only go to the point of absorb charging before having to disconnect the AC inverter(s), or risk getting into ‘equalize’ territory with the batteries where the Voltage goes up further and they start gassing greatly (which is hard on the batteries).
This is where the Magnum ACLD-40 comes in. It will provide proper 3-stage charging from AC-coupled sources by diverting power away to a diversion load of your choice, such as a water heater. This is great in two ways; it prevents overcharging of the batteries, and it provides a way to use excess energy from solar (for example) that would otherwise be lost, to do something useful, such as water heating.
The second potential problem with AC-coupled systems is that there is no control over the current flowing back into the battery-based inverter. This means that a 10kW PV grid-tie inverter can produce much more current than a 4kW off-grid inverter can handle! AC-coupled systems should be sized so the battery-based inverter has a current rating that is at least as large as the maximum (charge) current that the grid-tie inverter can supply. Failure to do so, and not providing a way to divert excess power, will damage or destroy the off-grid inverter.
- Controls up to 4000 Watt of excess power to prevent battery overcharge
- Works with 12, 24, or 48 Volt systems
- Allows the use of common, resistive AC household loads instead of hard-to-find DC loads to divert excess power
- Easy access inverter and network ports
- Standard RS485 MagNet protocol to communicate with Magnum inverters and remotes
- Can utilize power generated from wind, solar, or hydro systems
- Provides PWM (Pulse Width Modulation) Voltage when powering diversion load to run load without flicker