Sierra Nevada Airstreams

Enjoyment of the whispering winds, the zephyrs, the airstreams of the Sierra Nevada and Great Basin areas of the United States in a recreational vehicle.

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Understanding Battery Charging

After you take electricity out of a battery, you need to get electricity back in so you can use it once more. This is the process of charging the battery. You should only charge batteries designed for re-use. Most RV lead acid batteries are designed for re-use. Carbon zinc flashlight batteries are an example of a battery that is not safe to charge. Charging batteries means reversing a chemical reaction by running an electrical current through some materials. This generates heat due to the resistance of the materials to electricity flow and chemical reactions. The chemical reactions may also produce products that can be hazardous. If a battery is not designed for charging, attempting to charge may can cause its container to burst or even explode spreading caustic material on anything nearby.

Discharge considerations

Batteries should not be discharged too fast or too far. Taking electricity out of a battery or putting electricity back into a battery requires both chemical and electrical processes. Chemical processes require that two (or more) materials come into contact with each other and change into something else. Electrical processes move electrons through materials. Neither process is 100% efficient and both suffer degradation through use cycles.

"If you operate your house bank between 50% to 85% state of charge, as many experts recommend, and charge once daily, you should be able to return the 35% of battery capacity by [properly charging] for slightly over an hour." (West Marine)

A twelve volt battery is about half discharged at 12.2 volts and this is about as far as you should discharge it in normal use.

Charging versus maintenance

It is one thing to get a battery charged and yet another to keep it charged. When batteries sit, they will slowly discharge even if there is nothing connected to them. This means that battery maintenance needs to accommodate three states. One is usage. Another is charging. And the third is keeping the charge. Each of these states has its own particular demands on a battery charger.

If a battery charger is left on-line when appliances are connected to the battery (the usage sate), you need to be careful that the charger is designed so as not to damage the appliances. This characteristic is one of the primary considerations of an RV converter.

The state that kills most batteries is that of keeping the charge when the battery is not otherwise in use. A trickle charger (low current charger at a float voltage) can prevent discharge. The other problem is called sulfation ( see Az Wind Sun - http://www.windsun.com/Batteries/battery_desulfator.htm ) which happens when a battery isn't stirred up every now and then.

Maintenance device brands: Battery Pal, Battery Tender,

Basic Charging Devices

The most basic type of battery charger is called a "constant voltage, current limited" type. This type of battery charger can be made with only three components. A transformer converts house voltage to battery voltage. A rectifier converts the household alternating current (AC) to battery direct current (DC). A resistor limits the current as a protection for both the charger and the battery. They are very good at providing a bulk charge but not so good at finishing a charge or maintaining a battery.

All a basic charging device needs to know about its battery is its voltage. This knowledge is usually built into the design of the device.

Source: most 'under $100' automotive battery chargers at department or automotive stores.

RV Converters

The converter in an RV is intended to provide a nominal battery voltage for RV lights and appliances as a first priority. Charging batteries is a second priority. These are similar to basic charging devices except that they may have some extra protections as well as some filtering to minimize noise and interference in RV appliances and to prevent excess voltage.

A converter should not be confused with an inverter. The 'con' takes AC and converts it to DC. The 'in' takes DC and inverts it to AC so you can run household appliances when not connected to the AC grid. Many of the better quality inverters will include a multi-stage battery charger as well.

RV Converters are not good at battery maintenance so, if yours is left connected for any period of time, be sure to closely monitor battery water level.

Manufacturers: Intelli-Power, Magnetek,

Multi-stage Charging Devices

Somewhat intelligent, these battery chargers will tailor charging current for rapid restoration of battery capacity. They incorporate controls to separate battery charging into several stages. These stages can include the following.

  1. Bulk - full current to bring battery voltage up - usually ends when the voltage gets up to a pre-determined point.

  2. Absorption or acceptance - maintain a rather high fixed voltage to complete the battery charge - usually ends when the current gets down to a pre-determined point.

  3. Float - a reduced constant voltage to provide maintenance without boiling off electrolyte.

  4. Equalization - a controlled periodic overcharge to equalize and balance the battery cells and reverse the build up of chemical effects resulting from a battery sitting for a long period.

A multi-stage charger needs to know not only the voltage of the battery to be charged but also its charging current limitations. The charging current for lead acid batteries is usually assumed to be one fifth of its amp hour capacity. (e.g. a 100 amp hour battery should be charged at no more than 20 amps).

Brands: Truecharge, Charge Pro, Intelli-Power with Charge Wizard,

Intelligent Charging Devices

Intelligent devices have a means to learn about the battery so charge restoration can be carefully controlled for a specific battery and conditions. These are often programmed with switches and dials and may have a temperature probe in order to consider battery temperature while charging. The programming tells the device what kind of battery (AGM, Gel Cel, Wet Cell, NiCd, etc) it is charging and the battery capacity. With this knowledge, they can carefully shape charging characteristics for fastest charging and best battery life.

Automotive Alternators

The alternators in common car systems are essentially a basic charging device. They have some temperature compensation to adjust charging voltage for under the hood temperatures but are otherwise constant voltage current limited devices. The common problems with these types of chargers is not at issue because automobiles are not operated continuously. The downside is that, if you leave the vehicle unused for a while, you may need to pay attention to maintenance charging and sulfation.

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