Generating 120-VAC from 12-VDC

Electrical and electronic topics for small boats
jimh
Posts: 11674
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Generating 120-VAC from 12-VDC

Postby jimh » Thu Apr 19, 2018 9:47 am

A mid-April storm with freezing rain and ice caused our home's 120-VAC utility power to be interrupted for about 30-hours. We were able to survive the environmental effects of no heat and outdoor air temperature about 29-degrees by wearing more layers of clothing, compensate somewhat for the loss of illuminating light from electrical lamps by lighting candles, but the lack of power also shut off the internet access modem and WiFi base and router, depriving us of a network connection, and that was intolerable.

On my test bench I have a 70-Ampere-hour 12-Volt storage battery, which is usually kept close to full-charge by a small 1-Ampere float charger. In the closet was a 300-Watt Morningstar SureSine ultra-pure sine wave inverter, along with all the stuff needed to wire it up. (I bought all this with the idea of perhaps installing it on my boat someday.) It only took me a few minutes to lash up the bench battery to the inverter, and attach a standard 120-VAC duplex outlet. I had a long extension cord, too, and I ran the 120-VAC from the inverter through the house to the cable modem and router. Once re-powered, we had WiFi access to the internet restored.

12V_InverterLashUp.jpg
The bench lash up of a Morningstar SureSine inverter to generate 120-VAC from a 12-Volt storage battery (not shown). The outlet ground connection has not been made to earth due to the exigent circumstances.
12V_InverterLashUp.jpg (49.19 KiB) Viewed 7065 times


The load on the battery was just the inverter, which is rated for very high efficiency, 90-percent. The load on the inverter was just a Motorola Surfboard modem, perhaps 10 to 20-Watts, and an Apple router and WiFi hub, perhaps another 20-Watts. If the inverter were 90-percent efficient, then the load on the battery would be about 40/0.9= 45-Watts. At a battery voltage of nominal 12.6-Volts, that is a current drain of about 3.5-Amperes. If the battery was at full-charge and had 70-Ampere-hours of energy, the system should run for about 70/3.5= 20-hours, or perhaps less as the battery voltage might decline below the inverter's low-voltage limit.

We enjoyed access to the internet via the battery-inverter 120-VAC power for about 45-minutes. The 120-VAC shut off, and the inverter began to beep. I measured the battery terminal voltage at 11.8-Volts with no load, which correlates to a state of discharge of approximately 75-percent. I suspect under load it was even lower. Our internet access via battery and inverter power had ended.

I mention this to demonstrate that even with efficient voltage conversion and very modest electrical loads on the 120-VAC circuit, creating power from a storage battery is a challenge.

As for why my particular 70-Ampere-hour battery gave up in less than one hour at 3.5-Amperes, I can surmise several causes:

--the initial state of charge may have not been at full-charge; this could very well be the real cause, as I can't say for certain what the battery terminal voltage was at the start. The float charger may have been disconnected when we were out of town for about two weeks sometime earlier; the battery had powered my chartplotter for about 24-hours in a recent experiment and the float charger may not have been keeping up with its power drain; the chart plotter load would be about 4-Amperes, the float charger can supply 1-Ampere; the net discharge would have been 3-Amperes for about 24-hours, or 72-Ampere hours.

--the battery is eight years old, and it has not had many discharge cycles; it's been held at float voltage for years; perhaps its storage capacity has significantly diminished over the years, and the minimal use it gets as my bench 12-Volt power supply never put a sustained load on it without a charger attached at the same time.

What I learned from this experience: if you want to generate 120-VAC, even at a modest 45-Watt maximum load, you better have plenty of 12-Volt storage battery capacity available.