I Have a 12volt Battery That I Need to Reduce Feed to 3volts

Question: Should I choose a 12 volt, a 24 volt or a 48 volt stand-alone power system?

Reply: In short, your energy consumption should determine the voltage of your power system. You should not have continuous currents greater than 100 Amp.

Check out our off-grid system examples and find out how consumption relates to voltage. The examples list typical appliances found in average households; get an obligation free quote while you are there.

Basics

• Power (Energy) (P) = Watts
• Current (Flow) (I) = Amps
• Voltage (Pressure) (V) = Volts
• Cell = Single component of a Battery
• Battery (Battery Bank) = Collection of Cells wired in series or parallel

Power - Current - Voltage

• 1,000 watts = 83 amps @ 12 volts
• 2,000 watts = 83 amps @ 24 volts
• 4,000 watts = 83 amps @ 48 volts
• 20,000 watts = 83 amps @ 230 volts

The higher the current (measured in Ampere or Amps) the larger the wiring and circuit protection components need to be. High currents require larger diameter cables and fuses/breakers, both of which are expensive. By doubling the voltage (I = P/V) you get double the power (Watt) at the same current.

Dealing with currents over 100A is costly (and therefore inefficient) and potentially dangerous. A perspective: a standard household extension cord rated at 10 amps max current (a common value). 100A would melt it and could start a fire!

Industry Standard

12 volts used to be a standard for extra low voltage power systems. Today, most systems are 24V or 48V and include a 230V AC inverter. This means the wiring of the house does not have to be different from any other grid-connected household and cabling cost is greatly reduced.

For 230V (low voltage) wiring you must get a qualified electrician to wire your house for 230V AC. This way you can use standard AC appliances and lighting, most of which are a lot more affordable to buy and many are increasingly more efficient.

System Size

In the past we tried to reduce the cost of an off-grid system by limiting its size. This was achieved by using 12V or 24V appliances & lighting that do not require an inverter. In recent years, inverters and solar panels have become more efficient and a lot more affordable. In addition, most customers seem to want more power over the years. A 12V DC system with a tiny inverter is difficult if not impossible to upgrade/upsize. Not to mention the fact that only very few companies sell extra low voltage appliances or lighting and cater largely to the RV market. Further, the movement towards a greater use of Lithium based battery chemistry limits economics to 24 & 48V based on economies of scale of manufacture.

To summarise: Most systems we design are 24V or 48V with a 230V inverter. The criteria we use is power consumption and scalability. We would only suggest a 12V DC power system (such as Rainbow Power Pouch) if you need a few lights in a shed or caravan and wish to wire it yourself.

Battery Bank Size

Limitations

With solar panels as the primary energy source, it was traditionally recommended to have a minimum of 5 days battery storage with the battery bank still retaining a minimum of 50% charge after the end of those 5 days. A single battery bank available will provide X amp-hours over a 100 hour period to be 50% discharged at the end of that period. It is not recommended to increase storage capacity by connecting two or more battery banks side by side (in parallel). However, by doubling number of cells in the battery the battery voltage is doubled, therefore the current (amps) from the loads is halved, so doubling the voltage has the same effect as doubling the amp-hour storage capacity of the battery bank without having the battery bank connected in parallel.

The battery voltages commonly used for stand alone power systems are 12V, 24V, 48V, 120V DC.

Solution

More cells may be placed in series to increase system voltage for greater efficiency. If lower voltage supply is required, it is possible to use a DC to DC converter.

Inverter Size

Limitations

For any particular battery voltage there is a limit as to how large an inverter is available. With higher battery voltages larger inverters are available. So if you expect big 230VAC loads choose a higher voltage for your stand-alone system

Inverter Power – Battery Voltage

• 1-1500 watts = 12 volt system
• 1500-3000 watts = 24 volt system
• 3000-10000 watts = 48 volt system

Solutions

If your demands increased over time, and a higher voltage for your system is not a feasible option, you may be able to overcome the inverter shortcoming by having several inverters, or having inverters that can operate in tandem.

Cable Length & Size

Limitations

The lower the battery voltage, the higher the current draw from the battery bank to supply a given load (measured in watts). There is an acceptable limit in the voltage drop in the cable before the voltage drop becomes excessive with the resultant output voltage becoming too low. A more serious limitation of the cable is its "current carrying capacity" (ccc). If the ccc is exceeded the cable will melt and/or catch fire.

Solutions

Doubling the voltage effectively halves the DC loads and halves the voltage drop.  Because the battery voltage is doubled, the percentage of the voltage drop in relation to the battery voltage is only a quarter of the percentage drop with the lower battery voltage. Hence, with a 24 volt system the cable need only be one quarter of the diameter as it does with a 12 volt system. Unless the cable runs are exceptionally long or the power draw (amps) of the loads is exceptionally high this consideration would not be an issue.

Instead of opting for a higher voltage, an increase in cable size could also have solved the problem. Both the battery voltage and the Amp-Hour storage capacity of your battery bank should be appropriate to your needs. Avoid placing many small batteries in parallel. Battery cells connected in series is OK.

Please see our Cabling/Wiring Chart.

Number of Required Solar Panels

Limitations

Solar regulators are generally limited to 100 amps maximum. With a large 12 volt system you may require twice as much cabling and twice as many regulators as with an equivalent 24 volt system.

Solutions

This limitation can be overcome by having several solar arrays separately wired through separate regulators. It must be remembered that maximum charging rate of most lead acid battery banks is 10% of their amp-hour capacity; more for Lithium batteries (see Max Charge Rate).

Maximum Charging Rate

Extract from

Limitations

Traditionally, the maximum charging rate for a battery bank is usually 10% of its amp-hour capacity for Lead Acid battery types measured at the 10 hour rate (C10). A 600 Ah battery should therefore not be charged at more than 60 amps. Capacity is usually referred to as amp hours (Ahr) but can equally be described in kilowatt hours (kWh).

Lithium based batteries generally have a higher charge capability, often the 1 Hour rate (C1), although this varies considerably with the differing lithium chemistry configurations. Capacity is usually referred in watt hours (Wh) or kilowatt hours (kWh).

Solutions

To increase charging rate it is necessary to increase the overall amp hour / kilowatt hour capacity of the battery.

Voltage of Charging Source

Limitations

If a large wind turbine with a DC output or large DC generator is incorporated into the system, the system voltage will be dictated by the availability and voltage of these charging sources.

Solutions

Place your cells in series with separate charging sources, regulators and loads.

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Source: https://www.rpc.com.au/information/faq/system-design/12v-or-24v.html

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