by Bill Moeller
BATTERY BASICS
Let’s start our discussion of batteries by considering the starting battery you find in any automobile. The standard automotive starting battery is a 12-volt wet-cell lead-acid battery—also known as a flooded-electrolyte or liquid-electrolyte battery—composed of six cells, with each cell having a potential when fully charged of 2.105 volts. Thus, the voltage of a fully charged automotive battery is 6 × 2.105 = 12.63 volts.
Each battery cell contains an odd number of lead plates that are either positively or negatively charged. These plates look like a grid with rectangular holes. The grid holds the material that will provide the electrons with which the battery generates power. Positive plates contain a lead dioxide paste; negative plates contain sponge lead. These plates are immersed in a solution of 25% sulfuric acid and water, which acts as the electrolyte (a solution that allows electric current to flow through it). The positive plates are connected to each other and to the positive terminal of the battery; the negative plates are connected to each other and to the negative terminal. A battery has one more negative plate than positive. Between the plates are nonconducting insulators, called separators, that prevent the negative and positive plates from touching and thus short-circuiting.
When you use the energy stored in the battery, a chemical reaction occurs. The acid reacts with the materials in the plates, producing lead sulfate and water, which dilutes the sulfuric acid solution. As the reaction continues, the voltage potential of the battery decreases. Once the electrolyte becomes mostly pure water, the battery is fully discharged.
When you recharge a battery, you reverse this process. The water and lead sulfate are converted back to lead and lead dioxide, and the electrolyte solution increases in strength as the water is removed.
A simple electrical circuit. (Reprinted with permission from Boatowner’s Illustrated Electrical Handbook, second edition, by Charlie Wing)
* * *
Volts, Amps, and Circuits
It’s useful to compare electricity flowing through a copper wire to water flowing through a common garden hose. The voltage potential that causes the flow of electricity is analogous to the water main pressure maintained by your city’s public works department, except that electrical pressure is measured in volts rather than pounds. And the resultant rate of electrical flow through the wire is analogous to the rate of water flow through the hose except that it’s measured in amperes instead of gallons per minute. Water flows in only one direction, as does direct current (DC) electrical power supplied from a battery or a converter. The analogy breaks down somewhat with alternating current (AC) power, which flows in both directions along a wire, first in one phase and then the other. AC power (like the 110- or 220-volt power in a house) is supplied from a campground hookup, a generator, or from a battery via an inverter.
In order to produce some form of productive work, all electricity must travel in a complete circle, going from the voltage source (the batteries when you’re boondocking) to the appliance or load and back to the source again. This completes a circuit(see illustration at left).
Electricity is also polarized, with a positive, or plus, side, and a negative, or minus, side. Therefore, most electrical circuits require two wires—one positive and one negative. The positive wire goes from the source of power to the appliance, and the negative wire returns to the source.
* * *
During discharging, the acid must remain in contact with the lead for the process to continue. To achieve this, the lead materials are porous so the acid can diffuse through the plates. This allows the water to move out and fresh acid to move in (much as tea diffuses through the filter material in a tea bag when you’re making a cup of tea). The reverse happens during charging.
The rate at which this occurs is called the diffusion rate, and it can vary. The electrolyte reacts quickly with the surface areas of the plates. But as these areas are discharged, the acid must now diffuse into the inner plates, and this happens more slowly.
For example, when you try to start an engine on a very cold morning, the battery operates the starter for only a short time and then seems to go dead. If you let the battery rest, it revives. The reason is that you’ve given the acid time to diffuse into the inner plates. Diffusion is also the reason for a battery’s slow acceptance of a charge during the recharging process. The surface areas of the plates receive their charge quickly, but it takes time for the electrolyte to reach the inner plates. The rate at which this occurs is the battery’s charge acceptance rate.
Cutaway view of a battery. (Trojan Battery Company)
* * *
A Few Electrical Terms
Most people have heard electrical terms and may even use them, but here are a few definitions for clarity:
ampere (A), or amp: the unit of measure for the flow rate of electric current
ampere-hour (Ah), or amp-hour: a measure of the quantity of current that has flowed over time; a current of 7 amps flowing for 3 hours would draw 21 amp-hours of electricity from a battery
current (I): the flow of electrons through a material (measured in amperes)
resistance: opposition to electric current (measured in ohms); a voltage drop of 1 volt across a device through which 1 ampere of current is flowing means a resistance of 1 ohm; a voltage drop of 6 volts across a device through which 2 amperes of current is flowing means a resistance of 3 ohms
voltage (V): the force that pushes current through a circuit
voltage drop: the loss of voltage between one point in a circuit and another caused by resistance
watt (W): the unit of electrical power; watts = volts × amps; a 12-volt light drawing 1 amp is consuming 12 watts of power
* * *
Gassing
During the recharging process, the water is broken down into hydrogen and oxygen, which are released as gases, resulting in the loss of electrolyte. With every discharge and recharge cycle, the battery loses more electrolyte. Checking the level of electrolyte is a standard maintenance task for wet-cell batteries (we’ll cover maintenance at the end of this chapter). If the level is low, add only distilled water; do not use tap or rainwater since they can contain minerals that will damage the battery.
You can also mitigate electrolyte loss by replacing the regular vent caps with special caps called Hydrocaps. These allow the hydrogen and oxygen gases to recombine to water, and the water returns to the cell. Hydrocaps require regular maintenance; if they get dirty and become plugged, the result can be disastrous.
Shedding
Shedding is a natural outcome of charging and recharging a battery. Over time, the bond between the lead materials and the grid is weakened, and some material falls off and accumulates at the bottom of the battery. As more and more material falls off, the battery capacity (the amount of energy the battery can store) diminishes because the battery has less material with which to supply the chemical reaction.
Shedding is part of a battery’s aging process and cannot be fixed. Eventually, one of two things will happen: (1) the battery will lose so much lead material that the chemical process can no longer occur, or (2) enough material will accumulate on the bottom of the battery to reach the plates and short-circuit the battery. Either way, the battery fails.
Sulfation
As we learned above, lead sulfate forms during discharging and converts back to lead and lead dioxide during charging. Lead sulfate is normally a soft material and easily reconverted. Under certain conditions, however, the soft lead sulfate hardens into crystals and is no longer able to reconvert. This phenomenon is called sulfation, and it will shorten the life of your battery.
Sulfation occurs under these conditions:
The battery is left in a discharged state for a long period of time.
The battery is consistently undercharged, leaving part of the battery uncharged and in the lead sulfate state.
The inner areas of a battery’s thick plates are not charged (a variant of undercharging).
The b
attery self-discharges because it has not been used.
To prevent sulfation, ensure that your battery is properly and completely charged after each discharge cycle.
Self-Discharging
A fully charged wet-cell battery, when it is left idle with no charging and recharging cycles, will self-discharge over time. This is due to electrochemical processes within the battery, equivalent to the application of a small external load.
Overcharging
As we just covered, the charging process coverts water and lead sulfate back to lead and lead dioxide. If a battery is overcharged, the lead is converted to lead oxide, which is basically a nonconductive material. With this loss of conductive material, both the battery’s capacity and acceptance rate are reduced. In effect, you’ll get less power from the battery and a shorter life span.
Additionally, overcharging increases gassing of the electrolyte. This produces more oxygen and hydrogen gases, which increase the danger of a battery explosion.
TYPES OF BATTERIES
Motorhomes use two types of batteries, the first of which is the standard automotive SLI (start, lights, ignition) battery that you find in any vehicle. The SLI battery starts the engine and supplies power to the headlights, taillights, and other automotive functions and is almost always a liquid-electrolyte lead-acid battery as described in the previous section. SLI batteries are constructed to supply large amperages for a short time. Since discharge rates are directly related to plate surface area and the resultant diffusion rate of ions through the electrolyte, SLI batteries have many thin plates. This construction is the reason they cannot tolerate a deep discharge without severe damage.
The other battery type is one or more house batteries for powering your domestic appliances—all those electrical needs not associated with the normal operation of the vehicle on the road. A house battery should be a deep-cycle battery, which has heavier lead plates than an SLI battery, giving it greater capacity (albeit at a lesser rate of discharge) and the ability to withstand repeated heavy discharges such as those that occur during boondock camping. For this reason, our discussion will focus on deep-cycle batteries. These are what trailer RVs use. (For a complete discussion of all types of batteries, their properties, construction, and use, see our book RVElectrical Systems.)
Rule 1. Always use deep-cycle batteries for the house batteries of an RV’s electrical system.
Deep-cycle batteries are further subdivided into traditional and sealed types as follows:
Wet-cell or flooded-electrolyte battery: A lead-acid battery with a liquid electrolyte. This is like a typical starting battery but with heavier lead plates, and like a starting battery it has removable caps to permit inspection and servicing.
Sealed immobilized-electrolyte battery: A lead-acid battery that is sealed and cannot be serviced. It is often called a "maintenance-free" battery.
Sealed batteries are further subdivided into two types:
Gel-cell battery: A lead-acid battery in which the electrolyte is a gel or paste with the consistency of toothpaste or butter.
Absorbed glass mat (AGM) battery: A lead-acid battery in which the electrolyte is a liquid absorbed into sponge-like plate separators.
Most RVs come equipped with marine/RV 12-volt deep-cycle wet-cell batteries. While these are better suited for our purposes than a regular SLI battery, the better choices for a house battery are gel-cell,AGM, or golf-cart wet-cell batteries. All three types have proven their suitability. We installed two gel-cell batteries in our old trailer about ten years ago, when they were new on the scene. The batteries lasted seven years, which included a lot of boon-docking, and they were still going strong when we sold the trailer several years ago. (We’re sorry now we didn’t keep them. We had fine-tuned the charging voltage to just the right amount, and they gave us great service.)
However, each type of battery has its own advantages and disadvantages, which we’ll cover next.
Wet-Cell Batteries
If you decide to use a wet-cell battery for your house battery, you can choose either a six-cell, 12-volt battery or a three-cell, 6-volt golf-cart battery.
A deep-cycle liquid-electrolyte 12-volt battery has several advantages:
relatively inexpensive
accepts high recharging voltages (see Chapter 9)
less likely to be affected by overcharging
good deep-cycle use with proper maintenance
lighter than other deep-cycle batteries
Its disadvantages are:
requires a battery box (so house batteries can be restrained in an upright position to avoid spills) and adequate ventilation
needs frequent maintenance (adding water and cleaning terminals)
has a higher rate of self-discharge than sealed batteries
is less rugged than golf-cart and sealed batteries
contains sulfuric acid, a dangerous corrosive that must be handled with extreme care
A golf-cart battery offers different advantages, which may better fit your needs:
Has heavy lead plates that allow for a higher degree of discharge than SLI batteries or even regular deep-cycle batteries. (The plates are usually four times thicker than SLI battery plates and much heavier even than 12-volt deep-cycle wet-cell battery plates.) These batteries can occasionally be discharged to as much as 80% of their charge capacity thus delivering power for a long period of time—an ideal characteristic for boondocking.
Has a long life and is reasonably priced.
Case is taller than other batteries, allowing for more battery capacity on the same "footprint" and more space under the plates to accommodate shedded material.
The disadvantages of a golf-cart battery are:
It requires regular maintenance (and subsequent risk of dealing with sulfuric acid).
The taller case may prove difficult to fit in a typical RV battery space, requiring a special battery box or compartment.
Its ultra-thick plates do not provide the surface area necessary for high-amperage discharge loads.
For the same reason, it requires longer recharging times than other batteries when deeply discharged.
Gel-Cell Batteries
Gel-cell batteries have large thin plates and either fiberglass or felt separators. The gelled electrolyte is pasted onto the plates and separators, which are then compressed tightly together during manufacturing. This technology results in a strong cell that will withstand vibration and shocks very well. Also because of the size and thinness of its plates, a gel-cell battery has a high rate of discharge and charge. Its charge absorption (acceptance) rate is likewise twice that of a deep-cycle wet-cell battery, thus allowing for a faster recharging time at a higher rate.
Gel-cell batteries are sealed batteries, meaning you can’t get inside them, so they don’t need maintenance. Although gassing is minimal with this design, overcharging can cause excessive oxygen and hydrogen to be produced. For this reason, they have valves to allow venting and thus are also called SVR (sealed valve regulated) or VRLA (valve regulated lead acid) batteries. When gassing does occur, it means the gel is drying out, and the battery’s life is declining. To prevent gassing, limit charging voltages to 14.1 volts and below (see Chapter 9).
The advantages of a gel-cell battery are:
minimal to no gassing and no risk of spills and corrosion
no maintenance
doesn’t require ventilation
good performance at low temperatures
shock and vibration resistant
excellent long life with many life cycles
low self-discharge
high discharge and charge-acceptance rates
no sulfation
A gel-cell battery does have some disadvantages:
high price tag
heavier than wet-cell batteries
requires accurate charge voltage regulation
must not be overcharged
AGM Batteries
The AGM battery is the newest kid on the block. Its liqui
d electrolyte is contained in a sponge-like material of glass fibers, which is packed between the positive and negative plates. Like the gel-cell, this battery produces minimal gassing. The small amounts of oxygen and hydrogen produced are recombined within the battery, thus allowing automatic replenishing of the battery’s water. This technology offers batteries that have both engine starting (SLI) capabilities and deep-cycle use.
An AGM battery has several advantages:
minimal to no gassing and no risk of spills and corrosion
no maintenance
can be installed at any angle
shock and vibration resistant
excellent long life with many life cycles
low self-discharge
high discharge and charge-acceptance rates
no sulfation
Its disadvantages are:
high price tag
heavier than deep-cycle wet-cell batteries
requires lower charging voltages than wet-cells when charging