There's always a sense of the super-natural associated with lead-acid batteries. Old wives' tales prescribe aspin tablets, Rolaids or acid transfusions as cures for any number of battery illnesses, the cause of which is invariably attributed to several days of storage on a concrete floor. Mysterious mail-order potions hold forth the promise of bringing long-since-expired cells back from beyond the grave. Shades of Frankenstein!!

Even without all the myths, there's much to learn about batteries. Knowing how to select, install and maintain your batteries can pay big dividends in peace of mind ... not to mention saving you some money over the long run.

Types Defined

Deep-cycle batteries are designed to deliver moderate amperage over long periods of time between recharges. They are designed to be cycled (discharged and charged) hundreds of times, with plates that are more robust than engine-starting batteries to withstand the frequent deep discharging they must endure.

Engine-starting batteries, on the other hand, must deliver high amperage for relatively short bursts during cranking. Their design includes more numerous but thinner and more porous plates (compared to deep-cycle units) to present more surface area for higher current output over short periods. Starting batteries used in deep-cycle service will typically last only 50 or fewer cycles.

There are several major types of lead-acid storage batteries: flooded, "low maintenance", sealed non-gel and gel cell. The oldest and most commonly used design is the conventional flooded cell. Flooded- cell batteries contain blends of lead-antimoney plates with liquid sulphuric acid as an elecrolyte. Flooded-type batteries are the least expensive for a given storage capacity, but require regular addition of water, and they give off corrosive and potentially explosive vapors.

Thermoil "low maintenance" batteries might be considered a subgroup of flooded-cell batteries. They containe an oil that floats on the electrolyte which reduces acid mist and "out-gassing" that corrodes battery terminals. This design makes them better suited to withstand overcharging and also decreases the need for adding water. Thermoils solve some of the problems of conventional flooded cells without the cost penalty of more exotic batteries.

Sealed non-gel batteries, which are flooded-cell calcium batteries (also known as "maintenance-free"), contain a special lead-calcium plate material with liquid acid electrolyte. These batteries can be sealed because they don't "off-gas" as much and use less water. However, the main advantage of flooded-cell calcium batteries is that they require no maintenance. They typically don't withstand deep-cycling quite as well and don't recharge as easily as conventional wet-cell batteries.

Gel-cell batteries also use lead-calcium plates, but the electrolyte is a thick, pasty gel-type acid that doesn't splash and flow like conventional electrolytes. Gel cells are more expensive than conventional cells, but offer several advantages. They're maintenance- free, can be tipped over without leakage, cause very little corrosion, self-discharge more slowly during storage, and are less susceptible to deterioration from sulphation if left discharged for a while. However, their reserve capacity ratings usually are slightly lower thanb those of equilvant-size flooded batteries.

The newest type of deep-cycle battery, the "absorbed electrolyte" design by Optima, uses recombinant technology that allows the cells to be completely sealed. These batteries are presently the most expensive type based on rated output. They are completely sealed and maintenance- free and offer excellent resistance to vibration and leakage. Optima has offered engine-starting batteries for several years and recently introduced deep-cycle versions.

Rate Batteries by the Numbers Among engine-starting battiers, the Cold-Crankning Amp (CCA) is probably the most common specification for comparing different products. This rating describes how many amps the battery will deliver for 30 seconds at 0 degrees F. Sometimes a Marine Cranking Amp specification is used instead, in which the test temperature is increased to 32 degrees F. Both ratings are intended to convey some idea of how well the battery will spin the starter on a cold engine; the higher the number, the better the cold-weather performance.

A good yardstick for characterizing deep-cycle performance is the amp-hour (AH) rating, which describes how much current a battery can continuously deliver over a specific time period (usually 20 hours) before the voltage drops to 10.5 (which is considered to be fully discharged). The battery's AH rating is determined by multiplying the load current by the length of time it lasts. For example, a battery capable of deliving 11 amps for 20 hours would earn a 220-AH rating.

Reserve Capacity is an similar, more modern specification that describes how long the battery will deliver 25 or 75 amps before going dead. The idea behind reserve capacity is to provide an indication of how long a vehicle can be driven with a "dead" alternator. To convert 25-amp reserve capacity to a roughly equivalent AH rating, multiply it by 0.6. Larger deep-cycle batteries are rated at 75 amps; golf-cart batteries are usually rated in this manner. A 105-minute battery is rated to sustain a 75-amp load for 105 minutes at 80 degrees F. A high-minute rating indicates high reserve capacity. To convert 75-amp reserve capacity to a roughly equivalent AH rating, multiply the minutes by 2.0.

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Figure 1 - Approximate Lifetime Ownership Costs
           For Some Typical Batteries
------------------------------------------------------------------------
           Battery       CCA      AH    Cycle    Price     Lifetime Cost
            Type        Rating  Rating  Life     Range     per 1000 AH
         .............  ....... ......  .......  ......... .............
*******  Group 24       520-550   85    100-350  $50-$90    $2 - $11
Wet      Group 27       550-600  105    100-350  $60-$100   $2 - $10
Cell     Group 29/30    665-675  130    100-350  $80-$120   $2 - $9
Deep-    GC-2           1025     220    500-750  $80-$200   $0.50 - $2
Cycle     (Golf Cart)
*******  Group 8D       1200     220    200-350  $225-$275  $3 - $6

*******  Group 24       400       70    200-325  $135-$170  $6 - $12
Gel      Group 27       490       85    200-325  $175-$200  $6 - $12
Cell     Group 30       550       95    200-325  $215-$300  $7 - $16
Deep-    GC-2           5850     180    350-500  $400-$470  $4 - $7
Cycle     (Golf Cart)
         Group 4D       1100     180    200-325  $375-$400  $6 - $11
*******  Group 8D       1250     225    200-325  $450-$490  $6 - $11

*******  Group 24       165-625   50    10-25    $30-$50    $24 - $100
Engine   Group 27       270-700  110    10-25    $40-$80    $14 - $73
Starting Group 30       380-685  130    10-25    $75-$120   $23 - $92
Battery  Group 4D       490-950  160    20-50    $120-$175  $15 - $55
*******  Group 8D      850-1250  200    20-50    $150-$190  $15 - $48

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Figure 2 - Prolonging Battery Life How far you cdrain your house batteries before recharging them has a dramatic effect on how long they'll last. Run them completely down on a regular basis, and you'll also be buying replacements on a regular basis ... Up to six times as often as for a 25 percent discharge/ recharge regimen.

------------------------------------------------------------------------
Frequently      Frequently      Frequently       Frequently
Discharged      Discharged      Discharged       Discharged
25 percent      50 percent      75 percent       100 percent
or less

RESULT:         RESULT:         RESULT:          RESULT:

Maximum         Battery Life    Battery Life     Battery Life
Battery Life    Reduced         Reduced          Reduced
                50 to 70        90 to 200        200 to 600
                percent         percent          percent

------------------------------------------------------------------------
Figure 3 - Battery State of Charge
------------------------------------------------------------------------
   Battery         Specific       Depth of
   Voltage**       Gravity**      Discharge

12.66 - 12.75   1.265 - 1.280     0 percent   **NOTE:
12.45 - 12.51   1.225 - 1.240    25 percent     Valid only if battery
12.25 - 12.27   1.190 - 1.200    50 percent     has been idle for
12.03 - 12.05   1.145 - 1.160    75 percent     several hours.
11.79 - 11.90   1.100 - 1.120   100 percent

------------------------------------------------------------------------
Battery Discharge & Charge Chemistry
------------------------------------------------------------------------

During discharging of a lead-acid battery, lead (Pb) from the plates combines with sulphate (SO4) from the sulphuric acid (H2SO4) electrolyte to form lead sulphate (PbSO4) in the plates. Water (H2O) is given off during discharge and this lowers the specific gravity of the electrolyte. During charging, the reactions are basically the reverse of discharging. Sulphate (PbSO4) in both plates splits into hydrogen (H) and oxygen (O). As the sulphate leaves the plates, it combines with hydrogen and becomes sulphuric acid (H2SO4) again. Meanwhile, the oxygen combines with the lead of the positive plates to form lead dioxide (PbO2). The specific gravity of the electrolyte increases during charging because sulphuric acid is being formed and is replacing water in the electrolyte. A battery will give off gas as it's being charged. Excessive water use occurs when the battery is charged to a higher rate than it can accept.

Selecting A Starting Battery

As a rough rule of thumb for gasoline engines, the CCA rating of a replacement battery should at the very least match the cubic-inch displacement (cid) of the engine it will be starting; e.g., at least 350 CCAs for a 350-cid engine. However, more CCAs are better. For diesels, the CCA rating is typically three to four times the engine displacement, and is ually spread among two identical batteries. Normally, both batteries should be replaced at the same time.

A top-quality starting battery will generally last at least four or five years. Some vehicles go through batteries much sooner, usually due to high engine-compartment temperatures and water loss or sulfation from of use. Since a location next to an engine exhaust or in a poorly ventilation corner can quickly kill a new battery, it's sometimes worthwhile to either move the battery to a cooler location or install a heat shield and/or air ducting.

Comparing Coach Batteries

The Chart in Figure 1 includes the approximate cycle lives for a variety of battery types and sizes, along with long-term costs of ownership; in other words, each battery's purchase cost divided by the approximate number of amp-hours it can be expected to deliver over its lifetime.

From this chart, it becomes apparent that golf-cart batteries generally provide the lowest ownersip cost. Since these are 6-volt batteries, they must be used in series-connected pairs to obtain 12 volts. This is an advantage, since series-connected batteries share currents equally, thereby distributing the wear and tear equally among all batteries. Note that unlike connections in parallel, wiring two batteries in series doesn't increase the AH capacity.

The same chart shows that starting batteries are hideously expensive to use in deep-cycle battery service, even though their up-front cost is sometimes less. Consequently, it's best to restrict their use to the application for which they are optimized ... that of starting an engine.

How Much Battery Is Enough?

Estimating 12-volt electrical requirements in your motorhome is easy; simply multiply the current consumption of each 12-volt light or appliance by the anticipated number of hours you'll be using it between battery recharges to find the AH requirements for that light or appliance. For example, if a reading lamp draws 2 amps and you use it for three hours, that's 6 amp-hours that will be drained out of your batteries. Do the same thing for all the other lights and appliances you'll be using, add them all together, and you'll know what your total AH requirement is. You could buy a battery rated for that same number of amp-hours, but it's wise to add a considerable safety margin. Here's why:

• Depth of discharge: Battery life expectancy is seriously shortened by deep discharges (see Figure 2). For a reasonable lifetime, no more than 50 to 60 percent of a battery's rated AH capacity should be drained on a regular basis.
• Aging: Battery capacity tends to drop with age, sometimes long before the end of the battery's useful life. Furthermore, most new batteries require a break-in period (typcially 10-15 cycles).
• Temperature: The effective capacity of any battery drops considerably at low temperatures (typically 35 percent at 32 degrees F). Also, extensive furnace operation places high demands on batteries.
• Internal resistance: At high discharge currents, the effective AH capacity of any battery falls off. For example, a 220-AH battery that would last 20 hours with an 11-amp load might last only two hours with a 75-amp load, a cant 150 AH. This effect is particularly important for users of inverters and other high-current 12-volt appliances.

Since any one of these factors can leave you with fewer amp-hours than you thought you were buying, it pays to "go large" when shopping for a battery.

Upgrading House Battery Capacity

If your 12-volt electrical demands exceed the present capacity of your motorhome's batteries, it's time to consider upgrading your battery bank. If a second battery (or one or more larger batteries) will fit inside your existing battery compartment, then you're home free. In this vein, note that Group 29, 30, or 31 deep-cycle batteries can sometimes be substituted for the more common Group 27 size. They're only a tad larger, but pack considerable extra punch.

either to enlarge the existing battery compartment or to add a second one. Remember that any compartment alterations or additions must provide adequate ventilation, corrosion resistance, ease of maintenance, and structural support for up to several hundred pounds of additional weight. Note that installing sealed batteries can greatly simplify every one of these requirements except weight.

Ideally, all house batteries should be the same size, brand, and age. This ensures that each will share the load and recharging current equally. For this reason, it may be advantageous to delay upgrading your battery capacity until your present batteries are worn out.

Full or Ready to Recharge?

The chart in Figure 3 shows the approximate state of battery charge, based upon measurements with both a voltmeter and a hydrometer (or specific gravity meter). These readings will vary somewhat among various brands and types of batteries. Note that all voltage readings should be taken with a digital voltmeter, since most analog-type meters lack enough precision to be useful.

Several recently developed products make it possible to directly display state of charge, using a microcomputer to continuously measure the amp- hours flowing into or out of the batteries. By taking into account such factors as battery-bank size, age, electrolyte type, ambient temperature and previous discharging/recharge history, these instruments can provide quite accurate readings, considerably better than traditional voltage or specific gravity measurements. With prices currently starting at around $200, these products aren't for everyone, but nonetheless may, in some cases, be a worthwhile investment for properly maintaining an expensive bank of batteries.

Recharging Them

For non-sealed coach batteries, a good recharging technique limits maximum charger current to no more than 10 to 25 percent of the battery bank's total AH rating (e.g., up to 22 to 55 amps for a 220-AH bank), keeping maximum battery voltage under approximately 14.4 to 14.8 volts. This allows the batteries to absorb a charge as rapidly as possible, without excessive heating or plate stress. As the batteries reach a full state of charge at this voltage, the current will gradually decrease to around 2 to 4 percent of the bank's total AH rating (e.g., 4 to 9 amps for a 220-AH bank). At this point, it is desirable to either shut off the charger, or apply a long-term maintenance voltage of approximately 13.3 to 13.5 volts.

For gel cells, maximum charging current should be limited to 20 percent of total AH capacity, with charging voltage limited to 13.8 to 14.4. Any long-term maintenance voltage should be in the range of 13.5 to 13.9 volts.

Note that in the interest of simplicity and economy, the battery-charger circuits in many DC power converters use a single voltage setting (approximately 13.8 to 14.0 volts) for both recharging and long-term maintenance purposes. This can result in considerably longer recharge times and greate water loss. If gel cells are installed, it may be necessary to reduce this setting to prevent eventual battery damage.

Maintaining the performance of most non-sealed deep-cycle batteries is aided by applying an equalization charge at least once a month. In this process, low-amperage charging voltage is allowed to rise to approximately 15.5 - 16.0 volts for a short period of time. This overcharge serves to remix the electrolyte and helps in removing sulphate deposits. During this procedure, care should be taken to disconnect any sensitive 12-volt loads, ensure adequate ventilation and maintain battery water levels. Gel cells and other sealed batteries should never be equalized due to the amount of gas generated and becaue their electrolyte does not need remixing.

Note that all the above recharging and maintenance voltages are valid for batteries at 80 degrees F. At considerably warmer or cooler temperatures, some adjustments to these values usually will be necessary.

Maintaining Them

Prompt, full recharging is vital to battery health. Most batteries shouldn't be left in a partially discharged state for more than several days; to do otherwise permanently reduces battery capacity and life expectancy. Periodically checking water levels and cleaning terminal connections on non-sealed versions is also essential. Only distilled or de-ionized water should be added.

Now, regarding those potions and old wives' tales: Few battery additives provide enough benefit to justify their cost, and adding anything else to a battery (besides distilled water) generally does consideralbly more harm than good. Finally, self-discharge due to storing batteries on concrete hasn't been a problem for years ... not since back when battery cases were made out of tar and rubber.

NOTE: Be sure to use special care when working around batteries. Flooded-cell versions produce explosive hydrogen gas, so avoid sparks and provide adequate ventilation. Also, battery electrolyte can be extremely corrosive; always wear protective clothing and face gear.

Finally, be careful when using hand tools, wearing jewelry and handling other metal objects around battery wiring. An accidental short circuit can easily generate temperatures high enough to melt metal and cause severe burns.