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Old 10-07-2008, 08:15 PM   #1
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Re: More Electrical Stuff... (BZZZZT!)

As far as I know there are no 24 volt batteries. My bus has a 24 volt system but it is really just 2 large 12 volt batteries hooked to provide 24 volt.
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Old 10-07-2008, 09:08 PM   #2
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Re: More Electrical Stuff... (BZZZZT!)

I don't know if there are 24 volt batteries (12 cells in a single case). I'm sure it's possible. The biggest I've worked on were 8-cell batteries in locomotives. Two of these giant 16-volt batteries cranked the engines over at 32 volts.

Voltage DOES affect amperage - if the AC load on the inverter was 120 watts, the output at 120V is 1 amp {120 V x 1 A= 120 W}. With a theoretical "lossless" (100% efficiency) inverter, the input draw from a 12 volt battery would be 10 amps {12 v x 10 A = 120 W], but on a 24 volt battery it would only be 5 amps {24 V x 5 A = 120 W}.

The advantage is that at the higher "pressure," the same work gets with less current flow, or more work gets done with similar current flow. That "pressure" is how the Peterbuilt and locomotive batteries can spin the big engines. Less current flow means smaller wires for a given load (saves $$$) and less heating of the wires and therefore less power lost between the source and the load. The disadvantage to higher voltages is better insulation is needed to contain the extra "pressure."

If you go this route, you MUST get an inverter rated for 24 volt input. They are out there, most cell sites and many "off-grid' homes use 24 volts. There are also ones that run on 48 volts. They are not as common as 12 volt inverters. You can pick up the smaller wattage 12 volt units anywhere. There may be a few high quality 12 volt ones with some regulator circuit that would throttle back a 24 volt input. Unless the range of input voltages is listed in the spec, I would expect the common 12 volt ones might only produce smoke at 24 volts.

As far as comparing batteries of different voltages for inverter use, compare watt-hours. Multiply the amperage rating by the voltage. The Peterbuilt batteries may or may not not store more energy.

A 75 AH battery at 12 volts has 900 watt-hours. A 40 AH battery at 24 volts would have 960 watt-hours, and would be the battery with more juice. A 100 AH battery at 12 volts would beat it with 1200 watt-hours.

Also, don't use cranking amps for a house battery, if it doesn't have an amp-hour rating, use the reserve rating to figure slow draw capacity.

A good place to start looking to understand electricity for buses, trailers and motorhomes is "The 12-volt Side of Life" at:
http://www.ccis.com/home/mnemeth/12volt/12volt.htm It's written pretty clearly without getting unnecessarily technical.
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Old 10-07-2008, 09:21 PM   #3
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Re: More Electrical Stuff... (BZZZZT!)

^^^^ That was an EXCELLENT post. Like he said...if you're going to run 24 volt batteries (they do exist, but not commonly) you will need a 24 volt inverter. The only way you're going to drop the 24 volts to 12 for use with a 12 volt inverter would be to use one badass ballast resistor. You would gain nothing but a ton of heat from the resistor. I guess maybe it's possible to somehow put two 12 volt inverters in series, but I doubt it and I'm certainly not trying it with mine.

The advantage with running a 24 volt system isn't on the consumption side, but rather on the generation side. Alternators putting out 100 amps at 24 volts are making twice the power (watts) as alternators putting out 100 amps at 12 volts without any appreciable difference in wear and tear.
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Old 10-07-2008, 09:55 PM   #4
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Re: More Electrical Stuff... (BZZZZT!)

some guy posted a link to cheap 24 volt inverters. they were the same price as cheap 12 volt ones. ie: $200 for a 2,000 watt inverter.
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Old 10-07-2008, 11:07 PM   #5
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Re: More Electrical Stuff... (BZZZZT!)

6V Golf Cart Batteries are the cheapest bang for the buck, this has been proven time & time again. AGM's are probably best, but who has the money for the best, when good enough (golf cart batteries) will do.
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Old 10-15-2008, 12:50 PM   #6
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Re: More Electrical Stuff... (BZZZZT!)

Regarding your last question: ("what can 450 AH power? And for how long?")

You really don't want to discharge your batteries past 50%. They can do it, but they won't last as long, and will take longer (more solar panels & sunshine, more generator run-time, etc) to recharge. So you really have 225 AH usable from a 450 AH bank. To calculate how much usable power this is, it may help to convert to watt-hours.

Watts = Volts x Amps (x hours)
Watts = 12 x 225 (x hours)
Watt hours = 2700

Now you can take the wattage of each of your electrical loads, and multiply that times the # of hours of use (or fraction thereof), and sum them up for an average day. If you end up using more power on a certain day of the week (more power tools on weekends, for example) use the day with the highest usage as your baseline. Also bear in mind that sending the power through an inverter for AC loads will include a loss of efficiency, so tack on an extra 10% for any AC current draws. If in doubt, round UP in your load calcs so there is a margin of safety. Most people end up using more than they calculate.

I'll only add that I was able to get big Trojan batteries through a local Batteries Plus at a good price (420AH @ 6 volts for $200 each, shipping included). And doing the watering maintenance for the batteries isn't really a huge hassle. Once a month, peek in the cells & top off if needed. It also gives one a chance to check the tightness of the connections and clean them if necessary. I'd rather do this on a few big batteries than several smaller ones.

HTH
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Old 10-15-2008, 06:32 PM   #7
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Re: More Electrical Stuff... (BZZZZT!)

2 batteries with 420Ah @ 6 volts each, wired in series, make the equivalent of a 420AH 12v battery. Then you'd have 210AH (at 12 volts) usable per pair.

I have two pairs of L16H batteries (each pair wired in series, then the two pairs wired in parallel), and they run everything fine for me.
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Old 10-15-2008, 06:45 PM   #8
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Re: More Electrical Stuff... (BZZZZT!)

I'm presuming the Optimas are 12-volt batteries - you didn't say for sure.

The Trojans are 6 volts each, so you can't add the 225 AH capacity twice because with series wiring you are adding up the voltage, not the capacity . Parallel wiring adds to the capacity.

Two 225 AH 6-volt Trojans in series equal 225 AH @ 12 volts, FIVE 45 AH 12-volt Optimas in parallel also equal 225 AH @ 12 volts

To extend battery life, multiply the 225 AH x 50% = 112.5 AH usable. 12 volts x 112.5 AH = 1350 watt/hours usable.
So, to get 1350 usable watt/hours at 12 volts, you need two Trojans or five, not ten Optimas.

If you need more capacity, you would have to parallel PAIRS of the 6-volt batteries, but you could add 12-volt batteries in parallel one at a time.

Look at pages 7 & 8 of the Trojan Batteries Users' Guide: http://www.trojanbattery.com/pdf/Use...08_English.pdf
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Old 10-22-2008, 06:52 AM   #9
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Re: More Electrical Stuff... (BZZZZT!)

could someone explain the whole batterybank thing....and what all is involved as far as materials and supplies go?

im only needing power for the weekends.
thanks!
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Old 10-22-2008, 03:44 PM   #10
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Re: More Electrical Stuff... (BZZZZT!)

Quote:
Originally Posted by CHEESE_WAGON
Couple things I need clarified -- Does "amp hour" mean......

A) Up to 225 amperes for up to an hour

B) Up to 225 hours at a 1 ampere load

If neither, please explain this, I'm confused. And how does an amp hour relate to a watt-hour?
Yes, 225 amp-hours will accommodate either of those scenarios. There is a little more to it than that, but you have the concept.

Quote:
Originally Posted by CHEESE_WAGON
Also, how is the required amp hour capacity for a given load determined?
If you specify amp-hours, you must also specify the voltage.
A 1000-watt appliance running for an hour uses 1000 watt-hours.
Which is 83.3 amp-hours at 12 volts DC.
And is also 8.33 amp-hours at 120 volts AC.

Another example: My laptop PC uses 4.62 amps at 19.5 volts (according to the power converter brick).
watts = volts x amps
watts = 90
I use it 10 hours per day, so that's 900 watt-hours per day.
or 75 amp-hours per day at 12 volts DC
7.5 amp-hours per day at 120 volts AC

Again, any load that is drawn through an inverter gets bumped up by 10% to account for inverter losses. That is not reflected in the above calcs.

Quote:
Originally Posted by CHEESE_WAGON
A) Typical AC amperage draw multiplied by 24 hours per day? (In this case 40 x 24 = 960, if this is correct, 1350 amp hours would give me a nice buffer for power lost through the inverters, about 40% if my calculations are correct)

B) Inverter's current draw on a battery bank?

C) Both?
Make a list of the stuff that uses electricity, actually, two lists, one for AC loads, one for DC loads.
Multiply the wattage of each item by the time of use for a typical day to get watt-hours. See my examples above.
Everything that plugs in has its wattage indicated somewhere. If not wattage, then amps. If only amps are provided, multiply that by 12 if it's DC item, and by 120 if it is an AC item. You'll end up with X watt-hours of DC loads, and Y watt-hours of AC loads. Multiply the the AC watt-hours by 0.1 and add that to the original AC watt-hours number, then sum that with the DC watt-hours figure. The result is some number of watt-hours. Convert that to amp-hours @ 12volts (divide by 12), and the result x 2 is your battery bank amp-hour target.
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