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" Spring has come and gone. With the yard finally whipped into shape, you&amp;#039;ve managed to schedule a long weekend up in the mountains--just you and your family and a few days of a lot of nothing. The motorhome starts with the aid of jumper cables, and a few hours of freeway driving should fully charge the battery. The sad part is, when you finally reach the campsite, the kids can&amp;#039;t watch TV, the microwave won&amp;#039;t work and, horrors, the blender is incapable of making your favorite frozen concoction. The battery hasn&amp;#039;t charged much at all. You could run your generator or the motorhome engine for power, but you hate to disturb the untrammeled wilderness by doing that. And besides, the campground has rules against generators after sunset. Too Close To Home? The problem you have is common to many RVs, and there are many possible reasons for your predicament. The RV&amp;#039;s deep-cycle house battery was left uncharged over the winter, froze and is now useless except as an anchor for your boat. Or, you simply forgot to turn on the battery switch. Or, the alternator on the RV was maxed out by running the a/c fan, GameBoy and stereo and trying to recharge the battery all at the same time.  How Batteries Charge A conventional lead-acid car battery is filled with interleaved plates of spongy lead and lead dioxide soaking in sulfuric acid and water electrolyte. The sulfuric acid is diluted to a specific gravity of about 1.26 to 1.30, depending on the type of battery. Charging the battery combines some of the sulfuric acid and lead, reducing the concentration of sulfuric acid down to about 1.12 and coating the plates with a layer of spongy lead sulfate. Reversing this process gives you back most of that electricity. This means that checking the specific gravity of the electrolyte is a very good way of finding out the actual state of charge of a battery. It&amp;#039;s also fussy and potentially dangerous if the battery electrolyte ends up someplace it&amp;#039;s not supposed to be, like in your eye or on the leather upholstery of your wife&amp;#039;s vehicle. Also, some maintenance-free batteries have no filler caps and there&amp;#039;s no way to check or add electrolyte. So, we&amp;#039;re left to estimate the battery&amp;#039;s state of charge by reading its voltage. A fully charged 12-volt battery should have an open-circuit voltage of 12.6 volts. A discharged battery will have a voltage of about 11.2 volts--but these numbers need to be checked with no load and with several hours of rest after charging. And this is at a normal battery temperature of 80\u00B0F--colder batteries have a slightly lower voltage. A battery left discharged for even a few hours will convert some of its soft lead sulfate deposits--the kind that readily change back into lead oxide and sulfuric acid--into hard permanent deposits. These deposits interfere with normal battery operation, and can damage the plates. Be careful when storing batteries in lower temps. A discharged battery contains mostly water and can freeze solid, fracturing the lead plates. Always store a battery fully charged and in an area that stays above freezing.  With hard-to-access batteries, look for a remote jumper cable post.  An RV parked outdoors is a good candidate for a solar battery maintainer, such as this type. HOW IT WORKS: There Are Batteriesm And Then There Are Batteries We commonly think of car batteries as lead-acid flooded wet-cell batteries that haven&amp;#039;t changed substantially since the earliest electric cars used them (the early batteries&amp;#039; cases were made of glass, not plastic). The lead in these is alloyed with antimony to make it hard enough to withstand the vibration it receives in a moving vehicle. In the last few generations, maintenance-free batteries have become the standard. These use an alloy of lead and calcium. In contrast, the antimony used in older batteries promotes the electrolysis of water, lowering the electrolyte level and requiring replenishment. Maintenance-free batteries generally have a higher level of electrolyte covering the plates, and they commonly lack filler caps. Totally discharging one of these types of batteries even once can permanently compromise its ability to take a full charge. Deep-cycle batteries use a purer lead in thicker plates, and are capable of many cycles of near-total discharge. They need periodic replenishment of the water in their electrolyte. The newest lead-acid battery technology uses a gel electrolyte that won&amp;#039;t run out if the battery or vehicle is upended. Another type becoming popular is the absorbed glass mat (AGM) cell. The electrolyte in this battery is held in a spongy mat and can&amp;#039;t move around. AGM and gel-cell batteries internally recombine the hydrogen and oxygen from electrolysis so you never have to add water to them. Uphill, Downhill As a battery is charged or discharged, electrolyte needs to diffuse from one plate to another. At high or low discharge or charge rates, the acid in the electrolyte develops a gradient, becoming more concentrated next to one plate and less concentrated at the other. Only time will even this out, and that&amp;#039;s why you can&amp;#039;t quick-charge a battery to any more than about 50 percent of its capacity. To charge a battery, a charger or your car&amp;#039;s alternator needs to raise the voltage across the battery posts. Normal charging voltage needs to remain below about 14.4 volts, which is the level at which gassing of hydrogen and oxygen starts. Your car alternator was never intended to charge a totally discharged battery. Here&amp;#039;s what happened on that fateful trip to the mountains. The battery was totally discharged after being left unattended for months. A totally discharged battery needs very high voltage (over 16 volts) or a long charging time at 14 volts to even begin to accept a charge. Once that battery started to accept a charge (but in this case it did not), it would have made the alternator work very hard to pump current into the battery, shortening the alternator&amp;#039;s life. This type of situation calls for an external battery charger. The traditional charger is a simple transformer and diode affair that charges a battery to a constant voltage just below the gassing voltage of 14.4 volts. It will start out charging at a good clip, and then taper off, taking as much as a full day to fully charge a large, dead battery. It&amp;#039;s not a candidate for keeping a battery charged, because the voltage it generates will still make the electrolyte disappear as gases--an especially bad idea for a maintenance-free battery that can&amp;#039;t be replenished. Even a small trickle charger will damage a battery over a few months if left on. The more modern battery chargers are smart. They will raise the current flow high enough to bulk charge the battery rapidly, then taper off until the battery is fully charged. This reduction is necessary to keep below the 14.4-volt gassing point. The chargers then throttle back to a somewhat lower voltage that won&amp;#039;t boil the electrolyte, and periodically check the voltage, occasionally raising the voltage for a few hours to maintain a full charge. The latest crop of smart chargers even have a battery-reconditioning function that is supposed to break up hard sulfate deposits by blasting them with pulses of high-frequency voltage spikes, returning a badly sulfated battery to near-new condition.  A smart charger will rapidly recharge a dead battery without boiling off electrolyte.  Voltage is a good indicator of a battery&amp;#039;s state of discharge, but not during the charging phase. "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.popularmechanics.com/cars/how-to/a108/1272526/>