I need to determine the 12VDC Ah capacity of a battery pack, to provide current to a 30A 120VAC circuit thru an inverter with 88% efficiency, where the voltage must not drop below 10vdc, over a period of 3hrs. This is the worst possible scenario. I would expect that the current of 30A would begin to ramp down to 20A after the first 15 min., then hold steady at 20 amps for the remainder of the three hours. As a bit of a buffer, perhaps 80% efficiency should be used for the performance of the inverter. The battery pack will be in a well ventilated position to control fumes and temperature, as will the inverter. I would expect to use deep cycle batteries, but am unsure of size this battery pack will need to be.
Battery support capacity for inverter supply of 30A over 3hrs
energyinverterlead-acid
Related Solutions
"Modified Sine" outputs are very bad approximations of AC
This is a capture of the output of an APC 650 recorded by Jesse Kovach, while under load.
Notice the severe over-amplitude events at the extremes (the spikes at top and bottom). In reality they are actually much greater in amplitude, but the oscilloscope in the image was not fast enough to capture it.
Sharp edges in the time domain equate to broad-spectrum noise in the frequency domain. All of this high-frequency content represents additional energy that must be absorbed by protection circuits. If not, it can exceed isolation withstanding limits in the various input stage components and "burn through". If this doesn't burn out the input it will result in a cascading failure where it will cause something to fail on the secondary side from the resulting overvoltage.
...and that's just one failure mode. There are others. Psuedo-sine waves are poor matches to sine-wave inputs. :(
Go DC-DC instead of DC-AC-DC
A much better (and much more efficient!) approach is to go DC-to-DC directly (note: you can't actually go DC-to-DC directly if your input voltage is lower than your output voltage, but the details of this are well contained inside a "DC-DC converter").
Self-contained switch-mode power supplies for Dell laptops that take DC inputs are available in the marketplace. Here's an example:
which I sourced from:
http://www.amazon.com/Adapter-Charger-Dell-Latitude-D630/dp/B002BK7JEC#
Please note that I have no personal experience with this particular product and many cheap DC converters are poorly designed internally. Be careful.
There are really two questions here.
- To calculate the run time you first need to estimate the amount of
current being drawn by the inverter. If you assume an efficiency of
85% (your millage may vary) then you can calculate the current as
((120V / 24V) * 7A) / 0.85= 41A. This number is a bit on the high side since the voltage will initially start around 26V and drop down as you discharge the battery. The run time for 50% discharge is then calculate as a) (200 AH/41A) / 50% = 9.7 hrs. In case b) you have double the capacity so the runtime is increased by two. - You don't get any more charge discharge cycles by combing batteries in series. However, if you combine them in parallel then you would only be discharging the banks by half as much for a given a given amount of time. So even though your charge/discharge cycles wouldn't change, you would get twice as much work out of the batteries.
Best Answer
$$P = V \cdot I = 120 V \cdot 30 A = 3,600 W = 3.6 kW$$
You're looking for a 4 kW UPS with 3-hour uptime. (I've ignored the reduction to 20 A to give some margin for deterioration of batteries over time.)
Have a look at readymade UPS solutions before you try to roll your own. You'll find all sorts of benefits from reliability, self monitoring, alarms, automatic mains switchover (if mains is available), stable frequency and voltage, etc. As the UPS manufacturers purchase batteries in bulk you may find a readymade solution at lower cost than purchasing components and building your own.