Can grid-tie inverters charge UPS batteries if connected to the UPS output?

Question

If you have a UPS powering some load in an off-grid situation, and you have a solar panel with a grid-tie microinverter, is it possible to connect the grid-tie inverter to the UPS output, such that the microinverter assists with powering the load plugged into the UPS, and if there's an excess of power available, actually charge the UPS batteries?

I've read that it's possible with certain kinds of inverters, and that Tesla's "Powerwall 2" is apparently one of them. It accepts power from grid-tie inverters on its output and uses it to charge its batteries, or if they are full to feed back into the grid.

I thought this kind of function would require additional circuitry to extract the extra incoming power, but from what I've read, it sounds like there are some inverter designs that inherently have this capability.

Is this true? What sort of designs would work this way? Is there any quick way you could tell whether a given UPS is capable of this, without getting hold of a schematic?

Answer 1

I thought this kind of function would require additional circuitry to extract the extra incoming power, but from what I've read, it sounds like there are some inverter designs that inherently have this capability.

Yes, As long as the voltage criteria are satisfied a mosfet will happily allow current flow in either direction. So it's not difficult to design an inverter that can feed power in both directions. Indeed APC made some UPSes which they called "delta conversion" that explicitly exploited this, the battery bank was connected to the output through a single converter, then a separate "delta converter" was used to push power from the input side of the UPS to the output side.

Having said that though, the devil is in the details. I would not connect a grid-tie inverter to the output of a UPS that was not explicitly designed to support it. I would be concerned about the potential for bad interactions between the control systems of the two converters, and about the possibility for excessive voltages on the DC bus of the UPS.

Answer 2

UPS devices are not designed to be used this way. They are designed to be powered from the input, not from the output. I can see a number of safety hazards in your connection:

• If somebody turns UPS off, the grid tie inverter starts to free-run. If somebody then turns the UPS back on, the UPS may not be synchronized to the grid-tie inverter. Most UPSs have a power on/off button.
  • Edit: Perhaps this may not be an issue after all, as the inverter may be designed to avoid danger when the power turns on.
• The UPS may turn off, noting the lead-acid battery is below a given voltage. When the lead-acid battery has rested, its voltage may rise, and if the UPS is poorly designed, the previous scenario (UPS turning off and then back on) can happen automatically due to the increased rest voltage!
• The electrical cables needed to connect the inverter to the UPS output, not to the input, may be easily misused. A permanent installation instead of detachable cables could solve this issue, but most UPSs are not designed for permanent installations.

After all these safety hazards, you probably find that you cannot charge the UPS battery from the output. UPSs are designed to charge batteries when there is voltage on the input. It would be foolish to design UPS to power its charging circuitry from the output, as it most use cases it would be just a circular energy transfer, reducing UPS efficiency.

I wouldn't do what you are planning.

Do the proper thing and convert the inverter to the UPS input, not to the UPS output.

Answer 3

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