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Batteries Other Electric Basics
   
BATTERIES--AND OTHER ELECTRIC
This starts with the basics then gradually gets more advanced, with more extensive tests and interpretations. Some topics are mentioned several times, usually with more details and explanation, sometimes for emphasis. Even experienced RVers should start with the basics -- there are a lot of misconceptions when it comes to batteries.
THE BASICS
First things first. A 12-volt battery is not a 12-volt battery. Twelve volts is just a nominal, convenient term used to distinguish one battery from another. A fully-charged 12-volt battery, allowed to "rest" for a few hours (or days) with no load being drawn from it (or charge going to it), will balance out its charge and measure about 12.6 volts between terminals.
When a battery reads only 12 volts under the above conditions, it's almost fully depleted. Actually, if a battery's resting voltage is only 12.0 to 12.1 it means only 20 to 25% of its useful energy remains. It's either a goner or it has been deep cycled, and a battery can only be deep-cycled a limited number of times before it is indeed dead.
12-volt batteries supply useful energy only through a limited range -- from over 14 volts (when fully charged and unrested) down to 10.5 volts in use/under load (when lights dim, pumps groan and TV pictures get small). No 12-volt battery will remain at over 14 volts for more than seconds unless it's being charged. The lowest limit is 10.5 volts (used in testing) and obviously unsatisfactory in practical use. Experienced RVers try to use no more than 20% to 50% of the energy available in a battery before recharging. That means they never let resting voltage get below 12.5. They never use more than 50% before recharging (resting volts of 12.3) except in an emergency. They know that, if resting voltage ever reaches 12.1, they have deep-discharged one cycle and that a battery is good for only so many cycles (from as low as 20 in an automotive battery to 180 in a golf cart battery, with the typical RV/marine battery good for no more than 30).
RESTING VOLTAGE--IMPORTANT TO UNDERSTAND
Resting voltage causes some confusion, although it shouldn't. It means with no charging and nothing drawing electricity. No night lights, clocks (to include microwave), radio or TV memories (some have circuits that remain "on" even with the unit turned "off" to recall preset channels, etc. -- so-called "phantom loads."). And don't forget the LP gas detector/auto shut-off -- a real energy user. No reefer on (the "brain" in a three-way uses 12 volts even when running on AC). Needless to say, testing voltage this way can be a real pain in the ass. You need to do it, though, so you can find the phantom loads and hidden users that will haunt you later. If you're just interested in checking the battery, there is an EZier way to do it: If you have two or more house batteries, disconnect one of them and charge it fully with a reliable manual charger. The other(s) will run your RV while the disconnected one "rests." Overnight is OK. A day or two is better. Then check the resting battery's voltage, reconnect it and repeat the test on another one. You only need to make this test once or twice a year -- it's a good time to clean your batteries and connections as well. (More later on converters, manual chargers, meters and correct voltage.)
MANAGEMENT
You need to know what you can get out of your battery without dropping it below an acceptable voltage, but it's not a daily exercise. Done a time or two (or three) with notes taken, you'll soon know how many hours you can run lights, TV, etc., before recharging is needed. This is how experienced RVers calculate how many days they can rally or park in the boonies before cranking up a smelly generator. It's a mix of conservation, intelligent charging and battery care.
It's best to practice the below test while you're on an electrical hookup so you can recharge faster when finished:
* Switch reefer to LP and turn off unnecessary 12V appliances.
* Fully charge battery(s).
* Record the time.
* Shut off converter or battery charger (or unplug your RV if it's a cheapo with no way to switch it off).
* Run off 12 volts for 24 hours (or some convenient time) using appliances and lights as you would normally.
* Occasionally check a digital volt meter (see later) and record time when meter drops to each % Of Charge setting on chart (later in this poop sheet). Don't, though, go below the 50% level in this test.
* Simultaneously record the appliances you have running and how many minutes they run (that's each appliance, including number and type of lights, if you really want to do this right).
* When you reach the voltage that approximates 50% Of Charge (on chart later in this sheet), turn your battery charger back on. You now know how long you can run the house, in "normal" use, without charging, before chancing a "deep cycle." Do some simple math (below) to refine this and calculate where you can conserve electrical use to extend your time.
NOTE: When measuring above, when the battery is under load, but not being charged, volt meter will read lower than actual battery state. For example: If TV and lights are on, the meter might read 12.4. Don't panic. Turn heavy loads like those off. Watch meter. If batteries aren't substandard, it will increase volt reading then stabilize. That's the point where you take your reading. (An easy, quick way to observe this is to watch the meter while you run enough water to make the pump start. You'll see a drastic drop in voltage. Shut water off. When pump stops you'll see meter reading start to creep back up.)
SOME SIMPLE MATH -- Either before or after the 8-step test above you need to calculate the amp hours you use:
Quick way: After the test above you found you could run (x) hours of lights, (y) hours of TV, (z) hours of etc., before reaching the point where you were about to deep-discharge the battery.
Proper way: Highly recommended that you do this at least once.
1. Record actual minutes any given appliance was running.
2. Convert to Amp Hours as follows: 2a) Read label on appliance for amp draw. Can't find it Look for Watts or VA (same thing for our purposes). Watts = Volts x Amps, so 60W = 12V x A and 60 ÷ 12 = 5 amps. (If this doesn't make sense you didn't pay attention in school.)
3. Whatever, your appliance draws 5 Amps. If it was running an hour, that's 5 AH (Amp Hours) because AH = Amps x Time (in hours). And if it didn't run an even hour Then use minutes ÷ by minutes in an hour (e.g., 60 minutes ÷ 60 minutes [the minutes in an hour] =1, so if the appliance was on 12 minutes and it was drawing 5 amps, then AH = 5 x 12/60 or 1AH (or 5 amps for 1 hr 15 min = 5 x 75/60 = 6.25AH).
TIPS: Almost any appliance will have a label or the same info in its booklet. The circuit boards on some reefers, water heaters and furnaces use 12 volts also. It's only a small amount, but should be considered because it's being consumed 24 hours a day and the total can add up. (On the other hand, the 12 volt fan on a furnace uses an enormous amount of electricity -- don't miss that one.) Doing it the proper way allows you to predict in advance how many Amp Hours you'll use. Don't be alarmed if your 105AH battery, when voltage indicates it's at 50% of capacity, has given you a lot less than the 50AH you expect. Most batteries are seriously overrated by the manufacturer, seldom yielding 80% of their stated rating. (See more on battery capacity later.)
CHARGING VOLTAGE
Charging voltage is different. Some more basics: If you read articles on how electricity flows, you see comparisons as to how water flows. This is okay up to a point, but water also flows by gravity. Electricity doesn't, it has to be "pushed" (just as water has to sometimes be pumped).
You have to have more "juice" at one end of a wire than you need at the output or electricity won't flow. The wire you pump electricity through and the connections in the lines resist the flow. You have to overpower it. Similarly, batteries have an inherent resistance to take a charge because of their chemical makeup. It's like "making a blivet" (a blivet is 10 pounds of poop in a 5 pound bag). You have to force more electricity into a battery than it would like to accept or it won't be fully charged. To charge a standard 12-volt battery, you have to bring it up to above 14 volts (amount varies with the type of battery).
The typical wet-cell battery (a tub of lead plates in a mixture of sulfuric acid and water) needs to be charged up to about 14.+ volts in order to adequately distribute those funny little things called electrons through the plates. Once that's done, the battery can rest. As it does, the electrons distribute themselves and eventually balance out at 12.6 volts (more or less, depending on the type battery and its condition). This is your starting point. Doesn't sound like much, does it
MORE AMPS AND VOLTS
I mentioned earlier that you should only draw a battery down to about 12.3 volts before recharging. Obviously, there's more to it than that. Amperes are the measure of actual power available. They're usually converted to amp(ere) hours (AH). Think of it as the amount of (nominal) 12-volt power you can draw out of a battery for a certain amount of time. It's not just three-tenths of a volt. It's 12 (nominal) volts for a certain amount of time. The three-tenths stuff is nothing more than a difference in measurement -- like the difference between three-fourths of a tank of gas and a half tank.
Look at voltage as two things: First, a force that pushes electrons -- Second, as a handy measurement.
Look at amperes as two things: First, a quantity of energy (like you would a gallon of gas) -- Second, as a handy measurement. From a (nominal again, don't forget) 12-volt tub of energy, you can extract just so many amperes of power.
Keep in mind that the laws of physics prevent you from getting more out of something than you put into it! Keep in mind that waste (those wires, battery contents and such) prevents you from taking out as much as you put in. Keep in mind that you're going to have to put in about 10% more electrical power than you use (high school physics). A battery bank is like a "money" bank or checking account -- if you repeatedly take more out than you put in, you'll eventually be in trouble.
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