# Battery / Off-Grid system

2 Posts
Mar 13, 2009 11:38 am
Battery / Off-Grid system

When would you use 12v, 24v, 48v, or 96v batteries for an off-grid system? Is it strictly cost related or is there an advantage of one over the next?

97 Posts
Mar 13, 2009 12:36 pm
Re: Battery / Off-Grid system

Hi Andrew R.  I am sure there are many factors to determine choice of operating voltage for alternative energy systems.  I chose a 48v system, because of the reduced current requirements, and for less I2R losses.  With reduced current on system components, there are less losses due to resistive heating of the interconnection wiring, the inverter and controller semiconductors, and one can get by with a smaller size of wiring.  Copper wire is expensive, particularly when you get into heavy gauge wire, and when a long run of wire is required for a wind turbine, as an example.  Hopefully others here will give you more input on this.  Jon C.

Mar 14, 2009 09:12 am
Re: Battery / Off-Grid system

Jon C. pretty much summed it up for you.
There is no "Code" within the NEC (National Electrical Code) that I am aware of that dictates where the change should occur.
There is no mathematical formula that decides where, for example, a 12 volt nominal system should stop and a 24 volt nominal system should start and so on.
But think about it this way...

If you have a 5000 watt PV array wired for 12 volts nominal you will have to provide ample size conductors and supporting electrical devices like fuses, breakers, charge controllers, and all in order to safely utilize a total of 400 amps, give or take.

That same 5000 watts at 48 volts nominal would equal only 100 amps give or take. This translates into big \$\$\$ savings.

IMPORTANT NOTE!(These are not real world examples of an actual PV system. These are examples to help illustrate the gist of Ohm's law.)

Its not just for PV arrays ether. This applies to all electrical installations from the biggest nuclear power plant down to the battery in kids small toy. If you would like to learn more, just search - Ohm's law and then price the difference between 500 mcm and #2 - copper wire per foot.

Another important fact that Jon C. pointed out is power loss. When speaking in terms of conductors, such as but not limited to wire, Lower voltages suffer a much higher rate of power loss over a compareable distance to a higher voltage. This is the sole reason that Tesla won out over Edison in the AC vs. DC war.
Alternating currents can be transformed by way of inducing the current flow from one set of windings (pronounced: wine-dings) onto a different set of windings because it is constantly going from, pos. to off to neg. to off and so on. Think about those high tension power lines you see all over the country side and in the cities. Chances are the are carrying some where around 200,000 vac more or less but by the time it gets to a typical home its been transformed several times down to 120/240 vac. Each time the amperage increases. (Not taking into account the number of "taps" for a multitude of homes and business's or industries.)
An example of this in a Direct current can be found in the ignition system of earlier automobile engines. One set of windings inside the coil receives 12 vdc through the closed points but when the points open (turn off), the electric field inside the coil collapses and is induce on another set of windings that provide nearly 10,000 vdc potential to the grounded spark plug gap. But it is at a much lower amperage than the 12 volt line side.

If your really interested in an RE system for your home or whatever, I can tell you, one of the best investments that I made was a subscription to http://www.homepower.com/home/ it had the greatest payback value of all in my opinion.
« Last Edit: Mar 14, 2009 09:18 am by Thomas Allen Schmidt »

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