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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:

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 ( ), 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.
Someone said "Making good decisions comes from experience, experience comes from bad decisions." I say there are three kinds of people: those who learn from their mistakes, those who learn from the mistakes of others, and those who never learn.
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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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