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Old 10-07-2008, 07:15 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, 08:08 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, 08:21 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, 08:55 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, 10:07 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, 11:50 AM   #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, 05:32 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, 05:45 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, 05:52 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, 02:44 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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Old 10-23-2008, 02:08 AM   #11
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Re: More Electrical Stuff... (BZZZZT!)

OK, all the numbers seem to look good - I didn't run them through a calculator. One point though:

Quote:
Re: More Electrical Stuff... (BZZZZT!)

Post by SeanF on Wed Oct 22, 2008 2:44 pm

CHEESE_WAGON wrote: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.
Actually, the more amperes you draw, the less capacity you can get out of any battery. I checked the Optima web site, and didn't see the exact battery (45 AH) that you were looking at. They didn't have multiple discharge rates to compare anyway, only C/20 (see below).

I presume the 225 AAH Trojans are the common T-105's. Let's look at the specs:

In the Trojan Product Spec Guide ( http://www.trojanbattery.com/Product...Guide_1008.pdf ), you will see several columns. The 225 AH figure is in the "20 hour" column. This is the common standard for deep-cycle capacity rating. Other batteries manufacturers are more cryptic with the "C/20" spec. This abbreviation stands for Capacity divided by 20 hours. What this means is the battery is rated to provide 225 Amps divided by 20, or 11.5 Amps for 20 hours until the battery reaches a state considered "empty." That's 11.5 amps for 10 hours to reach a 50% discharged state to extend battery life.

Put two batteries in series for 12 volts, and you have 11.25 A x 12 V = 135 watts continuous at the standard draw rate. Run 135 watts for 10 hours and you get the 1350 watt-hours previously quoted for 50% discharge.

The column to the left shows 185 AH at a 5-hour (C/5) rate. 185/5 = 37 amps or 444 watts at 12 volts for 5 hours, or 2.5 hours to reach the 50% point. At 444 watts the usable storage drops to 1110 watt-hours (444 x 2.5).

Going back to the "Reserve Capacity" columns, at 25 amps constant draw the rating is 447 minutes. 447 minutes divided by 60 minutes/hour = 7.45 hours. 25 amps is 300 watts at 12 volts. Half capacity is reached in 3.725 hours for 1117.5 watt-hours usable. Reserve capacity in the 75 amp column is 115 minutes. Half capacity gives us 900 watts for 57 minutes and 30 seconds. Half capacity is now only 862.5 watt-hours.

In summary, two 6-volt T-105 batteries in series for 12 volts, discharging to 50% to prolong battery life, gives:
11.5 amps or 135 watts for 600 minutes (10 hours) totaling 1350 watt-hours,
25 amps or 300 watts for 223.5 minutes totaling 1117.5 watt-hours,
37 amps or 444 watts for 150 minutes totaling 1110 watt-hours, or
75 amps or 900 watts for 57.5 minutes totaling 862.5 watt-hours.
Lots of battery power gets expensive and heavy. Bummer.

Let's look at the solar panels. Like batteries, the panels are made up of individual low-voltage cells in series. Assuming for a minute you can get 135 watts out of each panel, and you have 3, that's 405 watts in "standard sun." There are tables that show how much "standard sun" an area gets on a given day. Up here in the north, it adds up to about 4.5 hours per day in winter. So parked in my yard, you could theoretically refill 405 x 4.5 or 1822.5 watt-hours in the batteries, not counting efficiency losses. This would refill one pair of batteries with "standard sun" every day.

Solar panels have three basic ratings, all measured in "standard sun": the maximum amperage into a short circuit, the maximum voltage into an open circuit (no load connected), and the maximum power rating, which is what gets published. Let's say for a minute that you are correct that the "maximum power point" for these panels is 16 volts x 8.125 amps. Lets also say for this example the short circuit amps equal 10, and the open circuit volts equal 18. Bear with me for a minute.

Maximum short-circuit current is 10 amps, multiply by volts shorted to zero equals zero usable watts provided. Maximum disconnected voltage is 18 volts times zero amps drawn is also zero usable watts provided. Maximum power occurs at a little less than maximum amps and a little less than maximum volts, for instance the 16 volts x 8.125 amps. If you chart the amperage at each voltage, it would probably make a steady 10 amps up through 12.5 volts, then taper off to zero at maximum voltage. This is good news and bad news.

If your batteries are discharged, at 10 volts the panels load down and produce the 10 amps, or 10 x 10 = 100 watts each. Charge the batteries to 11 volts and the panels now put out 11 volts x 10 amps or 110 watts each. At 12.3 volts they put out 123 watts. The lower the batteries are and the more charging you need, the less power you get. Batteries discharged 50% at 12 volts would receive 120 watts from each panel. Multiply this times 3 panels times 4.5 hours of "standard sun" and you still get 1620 watt-hour per day, enough to fill the batteries if nothing is running.

A simple solar charge controller just reads battery voltage and "unplugs" the panels when the batteries are full. There are now also "Maximum Power-Point Tracking" (MPPT) controllers. They act like a DC-DC inverter, allowing the panels to run at the 16 volt maximum point even when the batteries are low. This provides the 135 watts consistently, and gives you the full 1822.5 watt-hours each standard day, less efficiency losses. I calculated I can get about 30% more power from the panels at two radio sites I care for, just by replacing the old-style disconnects.

What this all means is that going completely off-grid is a lifestyle change. I'm preparing to do that, and don't know if we can make it work yet. We have a lot of loads to consider and calculate.

You might run a few efficient lights and an energy star or special DC refrigerator, but air conditioning is usually a no-no. Better insulation comes first. In dry climates, an evaporative ('swamp') cooler might be OK. A fan blows air through it to have water draw heat from the air as it evaporates.

Learn to shut off lights, computers, and TV's when not actually in use. Unplug cell phone chargers when not in use. Forget electric heaters. You might be able to run a 1200-watt microwave a couple of times per day for maybe 10 minutes total (200 watt-hours), but most other cooking would be with propane, etc. You could carry a generator, and use the batteries to tide you overnight between runs, but an electric from a meter is probably cheaper than fuel and maintenance for a genny in an inefficient lifestyle where it HAS to run every day. If you can get your usage down to where the genny only runs for special devices, or after a long spell of bad weather, you are doing OK.
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Old 06-19-2009, 07:51 AM   #12
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Re: More Electrical Stuff... (BZZZZT!)

6v trojans are not a a cheaper alternative to crappy rv batteries. However they are a far superior battery. I used 6v trojans in two different buses, then i switched over to 8D starting batteries. In my application, i find a single 8d out performs a pair of trojan's.

The easiest way make switching between shore power/battery power is to buy a fancy inverter/charger. Many of these automatically switch to shore power when they are plugged in. The cheaper alternative is to run all of your bus electrical through a single circuit (assuming you don't have a/c or other high draw electrical needs) This single circuit can be wired to a standard 15 amp 110 volt plug. Near the plug you can wire 3 standard household outlets, each hooked to a different power source: genny, shore, inverter. you can simply plug the bus power into the appropriate electrical outlet. This gives you a zero percent chance of mixing the electrical outputs from 2 different sources which would be catastrophic and expensive.

making your genny automatically start when your batteries get low....i don't have a good solution to that. although....i installed a remote starter on my truck many years ago and it had a function that would automatically start the engine if/when the batteries reached a certain voltage. Perhaps something like that could work. The ideal setup for a genny that is told to auto start would be a separate battery just for starting the generator so you don't end up with a scenario where your genny is called for and there isn't enough juice in the batteries to turn the starter.
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