UPS doesn't work on off-grid solar power system

Question

My APC 350 UPS worked fine when plugged into standard utility company power in the States. Now it's being powered by an off-grid solar power system in Central America, using 120 volt power from a 1500 watt inverter that gets its juice from a 12 volt battery bank. It just clicks every 3 or 4 seconds, and the red Building Wiring Fault indicator is lit. Also, there is no output.

The manual says:

If the red Building Wiring Fault indicator on the rear panel of the Back-UPS is lit, one of the following conditions exists:

• Open or high resistance ground
• Hot or neutral polarity reversed
• Overloaded neutral circuit

The manual also says:

Note: Improper building wiring will not prevent the Back-UPS from operating, but it will limit its protection capability

The inverter juice seems to work just fine with everything else I've plugged into it for the last 10 years, but thinking I might have a ground problem, I installed a good deep dedicated earth ground for that outlet. No difference.

I reversed the polarity at the outlet. (Changed black for white wires) No change.

I'm not sure what "Overloaded Neutral Circuit" means. Any help there would be appreciated.

Another consideration is the inverter. Its output is a modified sine wave, not a pure sine wave. Any thoughts on that as a cause?

I called APC, but their tech was clueless. Sent me another unit, which works great here in the States, but so did the other one. I'm afraid that if I lug the heavy thing to Costa Rica, I'll just wind up with two of them there and neither of them will work.

Answer 1

The UPS relies on the AC waveform to determine the status of the line voltage. The modified sine wave from the inverter is probably too crude for what the UPS needs. The wiring fault indicator is probably an additional problem, and that may be due to your jury-rigged power setup not emulating the utility power grid well (e.g., the way the neutral is referenced to ground).

Just an observation: You have limited power available. From your battery bank, you're going through an inverter. The UPS will convert that back to 12V to charge its own battery, then put it through another inverter to power your PC. Each time you convert the power from one thing to another, you have losses. The UPS will also waste power continuously on itself. You can reduce your power requirement by eliminating some of these steps.

Your best solution, at least from a power standpoint, will be a laptop (which will use less power than a PC, anyway). The laptop's battery provides a built-in UPS. You can plug it's power adapter into your inverter. However, a more efficient solution would be to run power cord from your battery bank to a cigarette lighter socket at your working location. Charge or power the laptop using a car adapter.

Answer 2

Don't plug the UPS's line side into an inverter. Connect the UPS batteries into the MPPT charge controller and let solar charge the batteries, while the UPS provides line voltage. :-)