by Joel Donaldson
BATTERY BASICS (from Trailer Life, May 1994) by Joel Donaldson Inadequate battery reserve power has long been the Achilles' hell of RVers who like to get away from the usual trappings of civilization, including hookups. While an AC generator can be used to supply auxiliary power, it can't be operated continuously, and RVers who lack both a generator and campground electrical hookups are very battery- dependent. Beyond conventional 12-volt appliances, owners who have discovered the benefits of power inverters (see "Inverters" - April 1994) to operate 120-volt AC appliances often find their previously adequate auxiliary batteries lacking. To power all these newly added luxuries, batteries must provide adequate output and must be kept in excellent condition. The lead-acid battery types that are most common in successful RV auxiliary-power applications are all of deep-cycle design. This is important because a deep-cycle design stands up to repeated heavy discharge-recharge usage much better than an ordinary automotive battery. An automotive battery is designed to deliver very large bursts of current for short periods (when starting an engine) and then is immediately recharged (by the vehicles' alternator). Most RV 12-volt DC and inverter power applications require the battery to provide current for extended lengths of time before receiving any recharge. An automotive battery will lose a significant percentage of its full storage capacity after being heavily discharged just one time. It will typically lose half of its capacity after 50 discharge-recharge cycles. (A heavy discharge is one that removes all but 20 percent of the battery's original full charge.) By contrast, even the lightest-duty deep-cycle battery will typically toleratre 200 to 300 such discharge-recharge cycles before reaching a similar state; some of the heavier deep-cycle designs can exceed 10,000 such cycles. In short, no matter how "heavy duty" a battery is claimed to be, if it isn't a deep-cycle design it won't last very long in most inverter applications. The only battery in an RV that needn't be of deep-cycle design is the one that starts the vehicle's engine. When a battery becomes too old and weak to sustain a usable charge, sulphation is most frequently the culprit. Every time a battery is discharged, its sulfuric-acid solution is gradually broken down, leaving deposits on the battery's lead plates. If the battery is promptly recharged, most of this sulphation is driven back into solution, leaving the plates in an essentially unchanged state. Leaving the battery in a discharged state for extended periods, however, allows the sulphation to harden into a form that permanently embeds itself within the plates. Suplhation deposits permanently reduce the battery's storage capacity. Chronic undercharging or excessive discharge also lead to plate shedding, in which some of the active solid-plate material flakes off and accumulates in the bottom of the battery. This accumulation eventually sorts out the plates, resulting in a dead cell. Consequently, if full storage capacity over a long service life is to be realized, it is important to fully recharge a battery promptly and to avoid over- discharge. Figure 1 - Battery State of Charge Charge Voltage Voltage Specific Level (12v) (6v) Gravity ------ ------- ------- -------- 100% 12.7 6.3 1.265 75% 12.4 6.2 1.225 50% 12.2 6.1 1.190 25% 12.0 6.0 1.155 0% 11.9 6.0 1.120 The maximum storage capacity of a deep-cycle lead-acid battery is usually rated either in amp-hours, or in minutes of reserve capacity. The amp-hour value refers to the number of amps a battery will deliver over a specified period of time (generally implied to be 20 hours if not specifically stated), before the battery has discharged to a useless level (10.5 volts for a 12-volt battery). The reserve capacity value specifies the number of continuous minutes the battery can last while delivering 25 amps before dropping to this same 10.5 volts. As a rule of thumb, for the smaller batteries you can multiply the number of reserve minutes directly by 0.6 to arrive at an approximate equivalent amp-hour rating for the battery. Therefore, a 50 amp-hour battery (or a battery with approximately 83 minutes of reserve capacity) can be expected to deliver at least 2.5 amps for 20 continuous hours, or at least 1 amp for 50 continuous hours. Note that at current drains much higher than those specified at the 20- hour rate, however, the capacity of the battery starts to decline due to internal losses and chemical inefficiencies at high currents. Consequently, this same battery might only be able to deliver 5 amps for nine hours (45 effective amp-hours), instead of the 10 hours (50 theo- retical amp-hours) implied by the battery's amp-hour rating. In general, bigger batteries can deliver higher currents without incurring this effect. The life expectancy of a deep-cycle battery, like all lead-acid batteries, is directly dependent upon how heavily the battery is routinely discharged before being recharged. Batteries that are regularly discharged until only 10 percent of their rated capacity remains have a much short life expectancy than identical batteries that are rarely discharged below 50 percent. Therefore, you should not buy a 100 amp-hour battery if you plan on routinely using all 100 amp-hours between recharges. A good rule of thumb is that a deep-cycle battery should not be depleted beyond 80 percent of capacity, with 50 percent being even better. A 50 percent discharge represents a good compromise between battery life and reasonable battery-bank size. Therefore, you would do well to buy at least 200 amp-hours worth of batteries to meet an anticipated 100 amp- hour discharge "budget". Ambient temperature also has a strong effect on battery performance. Performance of most batteries is rated at around 80 degrees F. At higher temperatures, they have greater capacity, but their life span is shortened, due to the acceleration of detrimental chemical reactions. At lower temperatures, they last longer than normal (provided the electrolyte is not allowed to freeze), but their capacity drops. At 32 degrees F, typical capacity is reducted by 35 percent; at zero degrees F, it is reduced by 60 percent; and at minus 20 degrees F, it is reduced by better than 80 percent. A battery's ability to accept a charge also drops along with the thermometer. In general, the best trade-off between efficiency and long life occurs when the battery is maintained at around room temperatures. For RV owners, this means that batteries in a compartment that is insulated from extreme cold and heat will last longer and deliver more consistent power. As a battery is discharged, the sulfuric-acid solution inside each cell is gradually converted to water. Consequently, the specific gravity of this solution also drops as the battery discharges. This change can be easily measured with a hydrometer in order to determine the battery's state of charge. A good battery hydrometer includes a temperature- correction scale (specific gravity versus battery charge varies somewhat with temperature) and will often yield readings that are more precise than those obtained with a voltmeter. Using a voltmeter is usually more convenient, however, and is the only accurate method of checking sealed batteries. Consult Figure 1 when determining the state of charge of a battery, using either a voltmeter or a hydrometer. Specific gravity readings should be taken by inserting the hydrometer suction pipe into the battery cell, squirting the electrolyte into and out of the hydrometer several times (electrolyte agitation improves accuracy), and then reading the hydrometer while the suction tube is still inserted into the cell. Keeping the suction tube in the cell while taking readings minimizes the chance of spilling the electrolyte, which could cause burns or destroy clothing. Read the hydrometer scale at the center of the fluid inside the tube, not at the edges. Note that any heavy battery charge or discharge currents drawn just prior to taking specific gravity or voltage measurements will have an adverse effect on the accuracy of the readings. The greatest accuracy is obtained after the battery sits idle for at least 24 hours prior to taking hydrometer or voltmeter readings. Specific gravity readings are also helpful in determining the overall health of a battery. For example, differences in specific gravity of more than 0.050 between any two individual cells in a battery generally indicate that the battery is headed for problems. By taking specific gravity readings every month or so, owners can catch battery problems before they cripple the entire system. WHAT TO BUY Regardless of what type of battery is selected, all the house batteries in an RV should ideally be the same age, size, and brand. This is because unsimilar batteries tend to charge and discharge at differing rates, leading to some of the batteries in the group being consistently undercharged during recharge and overstressed during discharge. Matching batteries will ensure maximum life for the entire battery bank. If the bank is diligently maintained, all batteries will wear out at about the same time, allowing the entire bank to be changed out after a long service life. In buying batteries, look for similar date codes stamped on each one. If the batteries have sat on the dealer's shelf for more than a month, use a hydrometer or voltmeter to ensure that the state of charge has been maintained. Don't buy old or partially discharged batteries. If in doubt, ask the dealer about the date of manufacture and shelf storage procedure. Among the deep-cycle variants, the most common type is the RV/marine, typically sold by hardware and department stores and by RV-parts counters in automotive package (or group) sizes 24 and 27. Typical ratings for this class of battery are approximately 80 amp-hours (110 minutes) for size 24 and 105 amp-hours (170 minutes) for size 27. These batteries represent a reasonable value in smaller systems that are equipped with inverters, or in installations where space is at a premium. As deep-cycle designs go, however, they are lightweights, with relatively short life expectancy in heavy service (typically two to three years). This deficiency is primarily due to the use of thin lead plates in their construction and the low antimony content of the plates themselves. The next most common deep-cycle version is probably the golf cart/electric vehicle, typically sold through battery-supply houses, some wholesale clubs, and occasionally department stores (frequently by catalog only). These batteries are all of 6-volt design (connection of two in series produces 12-volt output) and typically cost a tad more per pair than a single size 27 RV/Marine battery. They provide superior service in most RV applications (due to thicker plates and higher antimony content) and probably represent the best value for installations that can accommodate their large size (10-1/4 inch width, 7-inch depth, and 11-inch height). Typical ratings are 220 amp-hours, or 400 minutes of reserve capacity. Expected life is typically three to five years. Note that connecting two 6-volt batteries in series does not double the amp-hour or reserve capacity ratings, but connecting two of the resulting 12-volt battery banks in parallel (a total of four golf-cart batteries) does. Gelled-electrolyte ("gel-cell") batteries are becoming cheaper and more popular among Rvers. Available in group 24, 27, 4D, 8D, and 6-volt golf-cart sizes, they offer very good performance with virtually zero maintenance. Where ordinary "wet-cell" batteries require monthly checks of electrolyte levels, the gel-cells are sealed, using an electrolyte that is jellied with nothing to replenish. They also offer higher charging efficiency than ordinary batteries and provide slightly higher output voltage down to complete discharge. Expected life is two to three years, although some models may better this estimate by a great margin. Examples of this class of battery are the Interstate, Dryfit Prevailer, Sonnenschein, Deka, Johnson Dynasty, and Exide Nautilus Megacycle brands. Don't confuse these batteries with the "maintenance-free" wet-electrolyte RV/marine batteries being sold in some department stores under brand names such as Delco Voyager and GNB Stowaway. Unlike the true gel-cells, these batteries are basically sealed RV/marine batteries with slightly altered plate chemistries that reduce battery gassing (and, consequently, water loss). To determine how much battery capacity your application requires, add up the total anticipated amp-hours of all the 12-volt DC appliances you will be operating between recharges, including the demands of an inverter if you have one. Select batteries that meet or exceed this amp-hour value, plus a considerable safety margin. As an example, assume you will be recharging the batteries every day adnd your appliance use habits are as shown in Figure 2. Figure 2 - TYPICAL POWER CONSUMPTIONS AC Current Daily Total Daily Appliance Consumption** Use Consumption ------------ -------------- ---------- -------------- TV set 5 Amp-hr 6.0 hours 30.0 Amp-hr Microwave 85 Amp-hr 0.1 hours 8.5 Amp-hr Hair Dryer 125 Amp-hr 0.1 hours 12.5 Amp-hr VCR 3 Amp-hr 3.0 hours 9.0 Amp-hr 120-v Light 1 Amp-hr 3.0 hours 3.0 Amp-hr 120-v Light 1 Amp-hr 4.0 hours 4.0 Amp-hr Blender 3 Amp-hr 0.1 hours 0.3 Amp-hr Toaster 90 Amp-hr 0.1 hours 9.0 Amp-hr ----------------------------------------------------------- Total AC appliance usage: 76.3 Amp-hr ** Measured at the 12-volt input to the inverter. DC Current Daily Total Daily Appliance Consumption** Use Consumption ------------ ------------ ---------- ------------- Refrigerator 0.25 Amp-hr 18.0 hours 4.5 Amp-hr Propane Alarm 0.35 Amp-hr 24.0 hours 8.4 Amp-hr Water Pump 4.00 Amp-hr 0.2 hours 0.8 Amp-hr Cassette Player 2.00 Amp-hr 4.0 hours 8.0 Amp-hr Porch Light 1.80 Amp-hr 3.0 hours 5.4 Amp-hr Interior Light 1.80 Amp-hr 4.0 hours 7.2 Amp-hr ------------------------------------------------------------ Total DC appliance usage: 34.3 Amp-hr Total Battery Usage: 76.3 + 34.3 = 110.6 Amp-hr In this case, figuring a 50 percent safety margin, you would need at least 221.2 amp-hours worth of batteries. Consequently, installing a pair of golf-cart batteries would meet your needs, with no power to spare. likewise, three group-27 batteries would suffice, with some reserve power. HOW TO KEEP THEM HAPPY Although routinely overlooked in battery manufacturers' literature and in many reference, most deep-cycle batteries (with the excpetion of the gel-cell and other sealed varieties) are benefited by a periodic controlled overcharge, which is often referred to as an equalization charge mode. To equalize a battery, the charging is allowed to continue well after the point at which the battery is normally considered to be "full", taking care to avoid excessive battery heating or electrolyte boil-off. In a typical equalization cycle, the battery voltage is allowed to rise to approximately 16 volts, where it is maintained for up to eight hours by adjustment of the charging current. This process helps to mix up the electrolyte, which otherwise tends to "stratify" (i.e., separate into overlappying layers of acid and water), and is also useful in removing some sulfate deposits. When performed properly, equalization doesn't make the battery boil over, but does produce fairly vigorous bubbling. At the end of this cycle, you can expect to add some water. Most battery manufacturers consider one equalization charge per month to be appropriate for batteries that are in a continuous state of charge and discharge; less often is adequate for batteries that see a lot of standby service. Due to the generation of considerable gas that accompanies this process, equalization shoud NEVER be performed on a sealed or gel-cell battery. Also, most 12-volt DC appliances will not tolerate the 16-plus volts, so remember to disconnect everything or detach the battery cables before you equalize. Refer to Figure 3 for the suggest maintenance charge and equalization voltages for various batteries. Obviously, a charger with equalization capability is needed; there is no way to alter voltage output on most RV converters. Figure 3 - BATTERY VOLTAGES Charge Cutoff Maintenance Equalization Voltage Voltage Voltage Wet-Cell Battery 14.4 13.5 16.3 @ 80 degrees F Wet-Cell Battery 13.9 13.3 15.8 @ 100 degrees F Gel-Cell Battery 14.4 13.8 NA @ 80 degrees F Gel-Cell Battery 14.1 13.8 NA @ 100 degrees F The "charge cutoff voltage" is the battery voltage at which heavy recharging should cease; the "maintenance voltage" is the voltage at which the battery can be safely maintained for long periods of time without excessive water loss. As a final thought, remember that lead-acid batteries generate highly explosive gases. The larger the battery bank, the more gas is produced. Do not mount any battery in an unvented location, and avoid any sparks or open flame around the battery (particularly during and shortly after recharging). Making or breaking electrical connections at the battery terminals is particularly dangerous. Battery explosions often shower large areas with acid. Wear eye, face, and skin protection, and give the bank plenty of time to "air out" before attempting any maintenance or inspection.