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12 Volt Design and Safety Considerations.

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  • 12 Volt Design and Safety Considerations.

    Welcome RVer’s, Campers, Small Cabin Owners, Tinkerers, and Toy Meisters to the 12 Volt Forum. This Sticky is here to assist people with design guidelines, limitations, and safety concerns with respect to 12 volt battery systems. We hope you find this information useful and will read it before asking questions.

    12 Volt Limitations

    First thing to address is 12 volt systems like all low voltage systems are very limited in the amount of power that can be generated and used practically, safely, and economically. Lower the voltage, the lower the power. In theory there is no limit, but in reality and practice 12 volt battery systems are very limited. The limiting factors are equipment available, tooling, hardware, economics, safety, and the special limitations of the 12 volt world. 12 volt systems are going to be limited to lighting, entertainment, very small water pumps like those found in RV/Campers, phones, laptops, very lower power cooking and refrigerators. Cooking, heating, refrigeration, cooling and other medium power is going to require LPG, diesel, gasoline, or shore power connections.

    Power

    Watts = Volts x Amps

    We know voltage is fixed at 12 volts, so the only question is Amps required to make watts. Only way to get more watts is Amps. Voltage is usually thought of as being the dangerous component of electricity, and while it is, at 50 volts or less is insignificant and of not much concern. Voltage only kills if you come into contact with it and anything 50 volts or less is considered safe to touch. Current or Amperage is the dangerous component in all Low Voltage systems as that is what causes fires, not the voltage. In high voltage systems both voltage are current levels are dangerous. The higher the current , the larger the wiring and contact surfaces must be and greater attention to detail and skill is required to work with it.

    Charge Controllers today and for the foreseeable future is 80 amps maximum. That is where the industry draws the line and is a safety issue above that not to mention cost and tooling required. With that 80 limit on a MPPT controller operating into a 12 volt battery means panel wattage is limited to 1000 watts input. However that does not mean you should run 1000 watts on a 12 volt battery that would economically foolish and greatly increase your risk. So 1000 watts is the limit on panel wattage, but do so at your own risk. This author recommends 600 watt input limit and use a much less expensive 45 to 50 amp Charge Controller. That would be 3-200 watt panels Grid Tied in series, or 2-300 watt GT panels in series.

    Econimcs

    A word or two about Controllers and Panels. I am not going to address systems under 200 watts and PWM controllers as that implies using very expensive battery panels, wired in parallel, and losing a minimum 30% power. Waste of my time and your money. For example to make a 600 watt system with 100 watt battery panels and a 50 amp PWM controller is going to require a expensive combiner with 7 ports, 7 fuses, 6 Panel mounting kits, and very large expensive feeder conductor to the CC requiring special tooling with a higher risk of failure and fire. Battery panels cost 2 to 6 times more than Higher Voltage Grid Tied Panels. Battery panels are limited to about 160 watts, GT panels up to 350 watts. To use Battery Panels for 600 watts with a PWM system you are talking in excess of $3000. Do it with 2 or 3 Grid Tied Panels wired in series you do not need any combiner, fuses, less panel mounting kits, much smaller wiring, much lower risk, and fewer failure points. A 600 watt system with GT panels and MPPT controller with everything cost less than $1600. One last word about 600 watt limits, going higher than 600 watts means 2 strings of panels which can require an expensive combiner. As I said it would be economically foolish to go over 600 watts on a 12 volt system. If you need more than 600 watts, move up to 24 or 48 volts. You will save money, be simpler, and safer.

    Workmanship and Skill

    Let’s talk wiring for a minute because it has a lot to do with safety, expense, and being able to work with at an amateur level. 8 to 6 AWG copper wire is about as big as most of you can handle with affordable tools. 8 and 6 AWG copper cable has current limits. Dang that current limit thing again. 8 AWG copper cable with 90 C degree insulation (use only 90 C degree insulation or higher) has a free air limit of 80 amps, and 6 AWG is 100 amps. Where have we heard 80 amps before? Of course larger wire can be used, but you have crossed a big line in tooling and expertise required to terminate it properly. If you find yourself in that situation buy terminated cables.

    12 volt has another huge pitfall. In any low voltage system we want to limit voltage drop to 3% total or less. Well 2% at 12 volts .24 volts. All cables and wiring has resistance. More Math:

    Voltage = Current x Resistance

    So we know the voltage can be no more than .24 volts at 12 volts. Not much to work with. To control voltage we only have two things to work with: Cable size and length. Resistance in wire is a function of size and length. So if we have 80 amps out of the controller, or going from the batteries to Inverter and using the minimum 8 AWG copper we have to limit the 1-way distance to 5 feet, and 8 feet with 6 AWG.

    If your idea of a Crimping tool is a hammer, vice grips, pliers, or something you buy at a box or automotive store, you have no business terminating wire. A good compression crimper for 6 AWG is going to cost you around $100 up to thousands, and the connectors cost $1 to $3 each for decent UL ones tough enough for high current and mechanical stresses. The author advice is buy your wire already terminated. Most places you buy the equipment have them for sale in various gauges and lengths to fit your application. The pre-made cables is about the cost of the tools alone. Use your heads. This goes for 24 and 48 volt systems too. Marine and Golf Cart shops are good resources for cabling and have the tooling and experience. Or even some electricians have the tooling. DO NOT CHEAP OUT on wiring, it is too important.

    Batteries

    First thing get out of a 12 volt box, they make batteries in 2, 4, 6, and 12 volts. Point here is if you need more than 100 AH, you need to be using 2, 4, or 6 volt batteries. Once you get over about 300 panel watts input you are looking at 6 volt batteries. Above 600 watts you are looking at 4 and 2 volt batteries. So get out of that 12 volt box mind set, once you get above 200 panel watts 12 volt batteries are not the right choice. You do not want to parallel batteries if not necessary, and at 12 volts it is rarely ever necessary except to accommodate the area you are installing them. If you must install in parallel, no more than 2 strings If needed. If you need 225 AH, buy 2- 6 volt 225 AH batteries.

    Battery type and size as related to Panel Wattage input must be maintained. You can find more information elsewhere in this forum, So I will limit the types that best fit mobile and low voltage applications to Hybrid and AGM batteries. Hybids have many marketing names like GOLF CART AND RV. True deep cycle batteries generally do not work well with 12 volt systems because their internal resistance is too high to supply the large C/4 discharge rates some 12 volt systems demand.

    You must at a minimum supply a C/12 charge current where C = the total battery capacity Amp Hour, and the number 12 is a 12 hour charge rate. So for a 120 Amp Hour battery you must supply at least 120 AH/ 12 H = 10 amps. That means a 120 watt panel minimum because 12 volts x 10 amps = 120 watts.

    You must not ever charge or discharge them more than C/4. So that means if you have a 120 AH battery panel wattage and Inverter wattage cannot exceed 360 watts in this example. In a perfect world C/10 is the sweat spot. So a very simple rule can be applied 1 Watt of panel, to 1 AH of battery, to 1 watt of Inverter. That does not mean you can use a 4000 watt Inverter at any time. That is asking for a fire.

    Inverters

    OK many will not like this, but never use more than a 1000 watt Inverter on a 12 volt system. A 1000 watt inverter at 12 volts requires 100 amps of current. Largest wire you should be using is 6 AWG and 6 AWG has a current limit of 100 amps. You do not want to go 1 watt over 1000 watts on a 12 volt system and a 1000 watt inverter demands a minimum 400 AH battery. But know this; a 1000 watt Inverter on a 400 AH will be a strain on the battery. Limit full power discharges to brief usage like a cup of coffee water.
    They do make 2000 watt and higher 12 volt inverters. That does not mean you should buy them. They make cigarettes, booze, bongs and sell marijuana too legally. That does not mean you should use them. Do so at your own risk, you have been warned.

    Additionally be aware of the battery maximum discharge rate not to exceed C/4 and a warning you are asking for trouble going that high with VOLTAGE LOSS which can easily cause your Inverter to trip off-line from under voltage. Remember the 2% limit on cables. Run a battery at C/4 and you add another 5% on top of that 2% cable losses. Your Inverters will trip at around 11 volts. So what can and will happen from time to time your battery can still have plenty of charge left in it and not be able to run at full Inverter power.

    OK that Is it for now. This thread should be locked so if you have any questions start a thread. To answer questions not covered and more details check out the Stickies in OFF GRID and BATTERY FORUMS

    Battery Tutorial
    Off Grid Design.
    Batteries.
    Last edited by Sunking; 04-03-2016, 11:46 PM.
    MSEE, PE
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