12, 24 or 48 volt
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firefly
Solar Expert Posts: 66 ✭✭
This is a question I have wanted to ask for some time. When you have a battery bank, in my case 3 4volt 1104 amp/hr batteries for a 12 volt system, my useable amperage is approx 552 amps. If I where to add 3 more batteries of equal amperage and maintain 12 volts, I increase my useable amperage, to 2208 amps. Now if I where to change this to a 24 volt system, what is my advantage other than a possible down size in wire. By going to a 24 volt system I lose the increased amperage. Two other examples I know of are, a 12 volt system of 9 4 volt batteries, storage 3312 amps and the other is a 24 volt sysem made up of 12 4 volt 1104 amp/hr batteries, storage 1104 amps. I just don't understand, can someone explain?
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Re: 12, 24 or 48 volt
The end available watt/hours are the same:
1104 A/hr * 12 V = 13248 W/hrs
552 A/hr * 24 V = 13248 W/hrs
Thus the total available power is theoretically equal with either system. (The experts here will probably post an explanation of the minor differences).
Going with the higher voltage system reduces the DC current, good if you use a lot of power at once (e.i. consistent high wattage draws). But it is easier to recharge the 12V from solar because the panel array is more likely to be above battery voltage under any given light conditions. 12 V inverters tend to be less expensive, but also less wattage capacity*.
Others will post their own points of view with advantages/disadvantages. You get a lot of info off this board!
* I have seen an 8 kilowatt 12 volt inverter. Requires four sets of 1/0 cables to connect the DC. Sounds dangerous to me! 
Re: 12, 24 or 48 volt
Yes the overall power would be equal. A great analogy that works for me would be if you were trying to spin a waterwheel to say run a grinding mill or something. Basically picture running a pipe of water into the wheel, the faster you can get the wheel to spin the more power that represents (think watts). So you have two options: one is a tank of water with a large (maybe HUGE) pipe that is about 12' above the wheel itself, the other a tank of water that is raised up to say 48' above the wheel, but with a smaller pipe of water. The pressure from that second tank would be considerably higher, and it's easy to see that to spin that wheel it would take accordingly less water (flow/current).
In this analogy the water volume would be the amps, the pipe would be the cable needed to deliver this flow, and the height of the tank would equate to your voltage. The lower the voltage (pressure), the higher the flow rate would have to be in amps in order to put out the same amount of power.
There are several factors that might play into which voltage to use in a given system: size of the system and the loads (ie how much power will be needing to travel through the wires, imagine the size cable that you'd need to run a large central air conditioner at say 12v versus 240v. Then there is the distance factor to consider, think of all those amps trying to race down your wires as quickly as possible to provide power to your lights, there can be quite a lot of friction that would really drag things down (lost power, voltage drop). So the greater the distance between the panels, charge controllers, inverters, etc... the bigger a factor this becomes; running a system at 24 times the voltage would drop the current/amp rate down to 50% or 25% greatly reducing this issue, and/or allowing use of smaller wires. A very rough "typical" set up would probably use 12v in small systems and 24v (at least) for mid size systems, large systems would probably be most simple and efficient at 48v or higher.
Often the big deciding factor will be what inverter is wanted and/or available, as the inverters out there are designed around a set voltage.
Hope that helps a bit 
Re: 12, 24 or 48 volt
If you add three more 4V batteries in series with your current three in series, your voltage will double from 12 volts nominal to 24 volts nominal, but your deliverable amps (amperage), your amphour rating (total capacity), and your usable amphours (usable capacity, i.e. rating ÷ 2) won't increase. They will remain at 1104 and 1104 ÷ 2 = 552 amphours respectively.
If you add a series string of three 4V batteries in parallel with your current series string of three 4V batteries in series, your deliverable amps (amperage) and your amphour rating will DOUBLE (NOT quadruple) to 2208 total and 1104 usable, but your voltage won't increase. It will remain at 12 volts.
In both cases, you are doubling the wattage, the power to do work. By increasing voltage and not increasing amperage and amphour capacity, you can use the same wire, but you have to replace all your 12volt chargers and loads with 24volt versions (or use 24 to 12 volt converters). In other words, you need to series double your solar panel voltage to match.
By leaving the voltage at 12volts and doubling the amperage and amphour capacity, you get to keep the original loads (and add even more loads), but you need to upgrade some of the wiring, particularly the battery cabling, as well as probably increase your charging capability, if it isn't up to par. 
Re: 12, 24 or 48 volt
Thanks to all for the clarification, this is a great help. Also, thanks to NAWS for this forum.
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