by Bill Moeller
does not tolerate extended overcharging
Many solar panel dealers recommendAGM batteries. Relative to other deep-cycle batteries, they seem to offer superior longevity and trouble-free service (this is particularly true of the Lifeline brand, www.lifelinebatteries.com). AGM batteries are usually charged at a slightly higher voltage than the gel-cells, which means it is easier to find charging equipment that will safely charge these batteries.
A word of caution: If you plan to buy a sealed, maintenance-free battery, be sure it is really a gel-cell or AGM battery. Some manufacturers produce wet-cell batteries containing extra electrolyte in partially sealed cases; such batteries should not be used as deep-cycle batteries.
Battery Sizes
Four basic 12-volt battery sizes serve most RVers’ needs:
1. Group 24 batteries, with capacities ranging from 70 to 80 amp-hours (Ah). Singly, these are too small for practical RV use; they are best used in a bank of two or more.
2. Group 27 batteries, with capacities ranging from 80 to 105 Ah (the most common size).
3. 4D batteries, with capacities of 150 Ah and up (used in large Class A motorhomes).
4. 8D batteries, with capacities of 200 Ah and up (also used in large Class A’s).
Table 8-1 gives battery specifications, including dimensions, for a selection of battery groups.
It is possible to buy 6-volt gel-cells and 6-volt AGMs with the same footprint or base size as 6-volt golf-cart wet-cells. (This footprint is, incidentally, the same size as a 12-volt Group 24 battery, the smallest 12-volt deep-cycle battery made for RV use.) The advantage to this is that more batteries will fit into a smaller space, and will still have the higher amp-hour rating of the golf-cart batteries.
Note that Table 8-1 includes two 6-volt golf-cart batteries with amp-hour ratings of 220 and 225 Ah, while the L16 6-volt battery has an impressive 370 Ah capacity. This battery has roughly the same footprint as a Group 27 12-volt battery but is 16 inches high, so it presents an installation problem.
Table 8-1. Battery Specifications
The 220 Ah golf-cart battery is about the same physical size as the Group 24 12-volt battery, except that it is taller by about 2 inches. Other battery sizes and groups may apply, but the above are the most common.
BATTERY CAPACITY: THE ELUSIVE AMP-HOUR
You’ve seen what deep-cycle batteries are available, and Table 8-1 assigns capacities to the various types, but how can the boondock RVer use these capacities to make informed choices? Before we can answer that question, we need to know what battery capacities mean in practice.
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There Are Always Choices
We have always believed that a 12-volt battery system wired in parallel was better than a 6-volt system wired in series or series/parallel. (Series and parallel wiring are discussed later in this chapter.) The reason is that if a cell goes bad in one battery in the 12-volt system, you just disconnect the battery with the dead cell and use the other 12-volt battery until the dead battery can be replaced. If a cell in one of the batteries of a 6-volt two-battery bank goes bad, however, you lose the service of the whole bank, because the remaining 6-volt battery wouldn’t have enough voltage to run the system.
Further, we have always believed that the most cost-effective choice of battery for this setup would be wet-cell batteries, and that the higher cost of sealed batteries wouldn’t be justified by the benefits.
Recently, however, a friend pointed out something we had overlooked: golf-cart batteries have approximately the same footprint as the smaller Group 2412-volt batteries. Thus, two 6-volt golf-cart batteries in series provide as much capacity as two larger Group 2712-volt batteries in parallel while fitting into a smaller battery compartment (although you must account for the added height of about 2 inches). Making these 6-volt batteries the more expensive gel-cell or AGM type then makes sense, conferring the advantages of sealed batteries with their high amp-hour ratings in a smaller space.
The bottom line is that you have options when designing your own battery bank. Play around with different battery configurations (on paper) to determine what batteries will provide the best setup for your needs and space.
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There are three main ways to determine battery capacity: amp-hour rating; cranking amps (cold cranking amps, CCA; or marine cranking amps, MCA); and reserve capacity rating. We focused on amp-hours in the above discussion of battery types because, for deep-cycle batteries, the amp-hour rating is far and away the most telling, but here we’ll go over all three.
Amp-Hour Rating
As we learned earlier in the chapter, an ampere is the unit of measure for the flow rate of an electric current, and an amp-hour (Ah) is a measure of the total current that either has flowed or is capable of flowing from a battery or other power source for an hour. Put another way, the supply capacity of a battery is measured in amp-hours.
Say you have a water pump in your RV and you measure its current draw (using an ammeter) at 5 amps. If you run the water pump for 1 hour, you will have consumed 5 amp-hours of battery capacity:
5 amps × 1 hour = 5 amp-hours
If you run the pump for 15 minutes, you will have consumed 1.25 amp-hours:
5 amps × 0.25 hour = 1.25 amp-hours
Think of amperes as analogous to the instantaneous rate at which a vehicle is consuming gasoline, and think of amp-hours as the cumulative quantity of gasoline consumed over time. Most people know how many gallons of gasoline their fuel tank holds, and they have a fuel gauge to tell them when they need to fill up the tank. Similarly, we should know how much "gas" (in amp-hours) is stored in our batteries. There is, however, a difference between replenishing gasoline in a tank and amp-hours in a battery. Gasoline is replaced gallon for gallon, but due to battery inefficiencies, amp-hours must be replaced at a ratio of 1.2 amp-hours for every 1 amp-hour consumed.
Additionally there is likely to be a difference between the actual amp-hour capacity of a battery and the amp-hour rating assigned to it by the manufacturer. The actual capacity at any given time depends on the battery’s state of charge as well as its temperature, its age, the size of the load imposed on it, and even the method used to measure its capacity. No wonder the average RVer can get very confused by the elusive amp-hour.
Load Size
While you can safely draw 1 amp from a 100 amp-hour battery every hour for 100 hours, you cannot draw 100 amps for 1 hour. The battery would be dead in about 20 minutes and would probably self-destruct. Heavy loads should never be applied to a battery continuously; as the load increases, the capacity decreases. For this very good reason, never apply a load greater than 25% of the amp-hour capacity of your battery. To do so greatly reduces the capacity rating.
Rule 2. Never apply a load greater than 25% of the battery capacity.
Temperature
Temperature often affects chemical reactions. In the case of batteries, as the air temperature drops, battery capacity decreases. For example, a battery that is at 100% of its rated capacity at 68°F would only retain about 85% of its capacity at 32°F, and only 70% at 0°F. This would mean that a 105 Ah battery would become a battery with a capacity of 89.25 Ah or 73.5 Ah respectively at the lower temperatures.
Age
As a wet-cell battery ages, it loses capacity because the unconverted sulfate forms clumps on the lead dioxide plates. When battery capacity has dropped to 75% to 80% of its capacity when new, it is time to replace it. You can tell a battery has reached this stage when, after a recharging and a 24-hour rest, the voltage is below 12.63 V or the specific gravity of even one cell is below 1.265 V (see Table 8-6 and pages 86-87).
Determining Amp-Hour Ratings
The most common and accurate way manufacturers determine the amp-hour rating of a battery is with a test called the amp-hour rate, or the 20-hour rate, which is usually performed at 80°F. A specific load of 5% of the estimated amp-hour capacity, usually from 4 to 11 amps, is applied to the fully charged battery until the ba
ttery voltage drops to 10.5 volts. If this takes 20 hours and the load is 5 amps, the battery will have a 100 amp-hour rated capacity:
20 hours × 5 amps =100 amp-hours
The 20-hour rate was the standard for years, but now many manufacturers use test durations ranging from 5,6, or 8 to 18,25, or even 100 hours, with lower-amperage loads at the higher intervals, and at different temperatures.
Table 8-2. Reserve Capacity at Various Amperage Rates
When purchasing a battery, our best advice for determining amp-hour capacity is to look for specifications that are based on as long a time period as possible. These tests will probably be at a current-discharging rate similar to what you will normally be using to discharge your batteries.
It is possible to rate your own batteries when they are new, as follows:
1. Apply a load of a known value, say 5 amps (see Table 8-2).
2. Constantly check the battery voltage with a good-quality DC digital voltmeter (covered in Chapter 9).
3. Observe the time it takes for the voltage to drop to 10.5 volts.
4. Multiply that time by the amperage to get the amp-hour rating.
Cranking Amps
Two other battery ratings used by manufacturers are cold cranking amps (CCA) and marine cranking amps (MCA). These ratings refer to the amount of amperage available for starting an engine at either 0°F (CCA) or 60°F (MCA) and are strictly for rating engine starting batteries. (Some manufacturers use 32°F instead of 60°F in their MCA ratings.) These ratings have no relationship whatever to the deep-cycle batteries needed for house batteries in RVs. If a battery is rated in either CCA or MCA ratings, don’t even consider buying it unless an amp-hour rating is also given (however, the chances are the battery will not be a true deep-cycle battery).
Reserve Capacity Rating
Another rating that has come into popular use by manufacturers is the reserve capacity rating, which gives the rating in reserve minutes (see Table 8-2). This method determines the number of minutes it takes for the battery voltage to reach 10.5 volts (a dead battery) with a 25 amp load applied. For example, a battery with a reserve capacity of 160 minutes would have a capacity of 66.67 amp-hours:
160 minutes × 25 amps = 4,000 amp-minutes ÷ 60 minutes = 66.67 amp-hours
This example shows how a high-amperage load can diminish the amp-hour capacity of a battery. The battery in this example would normally have an amp-hour capacity of 105 Ah, a common rating for a Group 27 wet-cell deep-cycle battery. Yet the equation above tells us the battery only has 66.67 amp-hours. An approximate way to convert reserve capacity to amp-hours is to multiply the reserve capacity by a factor of 0.65.
The reserve capacity rating is not suitable for rating an RV deep-cycle battery. Boondocking RVers seldom discharge their batteries at a steady 25 amp rate. Also, they certainly would not normally discharge them until the battery was completely dead at 10.5 volts. So the reserve capacity rating is not very useful for our purposes.
BUILDING A BATTERY BANK
By now it should be clear that amp-hour ratings are your best standard when selecting deep-cycle batteries. You can usually find these by searching the specification sheets from dealers and manufacturers. But one battery—regardless of type—will seldom be enough to satisfy all your boondocking power needs. More likely you will have to wire together two or more batteries of a given type to form a bank. But how do you determine the number and size of batteries you will need? Here are some guidelines:
1. Determine your daily use. Estimate the total amp-hour consumption you think you might use in a typical night of battery use. You can use Table 8-3, and total your average night’s usage of various appliances and lights.
2. To determine the battery storage capacity you’ll need, multiply your amp-hour estimate from step 1 by 4. (We use 4 because, as explained in Chapter 9, we’re considering the daily use estimate as 25% of the battery bank’s amp-hour capacity.) This total will give you the approximate amp-hour battery capacity you may need.
3. Choose a type of battery. Based upon the information you’ve learned about the different types of batteries, choose one that will meet your needs: physical RV space, capacity, budget, etc. Do not combine batteries of different types or ratings in a bank.
4. Wire the batteries together to form the battery bank (see pages 81-82).
5. Monitor the electrical system to control amp-hour consumption (see Chapter 9 for more on monitoring batteries).
Determining Daily Battery Needs
To accurately determine the battery capacity you will need, you must first determine how much power you use on a daily basis. This exercise may seem a bit tedious, but is important to identify how much power you will be using, on average, when boondocking. It is also a good way to become familiar with the amount of power different items use so you can make informed choices about power consumption and battery charging. Your electrical needs won’t be the same every night. For example, one night everyone may watch television for 1.5 hours, but the next night, the whole family wants to watch a 2-hour movie—except for Dad, who wants to surf the Internet. Can the batteries handle both activities? Will you be able to recharge the batteries the next day?
1. Beware of the amp-hogging equipment such as HDTVs, large-screen computer monitors, etc. Check the wattage before going boondocking with such items. We have a 25-inch TV that draws about 15 amps when run on an inverter. When boondocking, we use a 9-inch, 35-watt TV.
2. These ratings are approximate. Ratings vary between manufacturers and from product to product.
Table 8-3. Wattage and Amperage of 120 VAC and 12 VDC Appliances1
To calculate your daily usage:
1. Identify the equipment and appliances you would generally use on a daily basis.
2. Find the amperage of each item using Table 8-3.
3. Estimate the amount of time you’ll use each item, either in hours or percent of an hour. If you need a percent of an hour, you can either use Table 8-4 below for decimal equivalents, or convert hours and minutes to all minutes and divide by 60; for example:
47 minutes ÷ 60 = 0.78 hour
2 hours, 33 minutes = 153 minutes ÷ 60 = 2.55 hours
4. Multiply amps by time to get amp-hours.
Table 8-4. Time/Decimal Equivalents
Table 8-5. Daily Usage Example
Let’s illustrate the above with an example of items you might use over the course of an average day:
12-volt lights; the two most popular 12-volt lightbulbs are the #1003 at 1 amp and the #1141 at 1.5 amps (see Table 8-3)
TV and satellite dish receiver, both running on 12 VDC off an inverter
water pump
phantom load, such as the refrigerator—even on propane, the refrigerator draws some amperage and accumulates some amp-hours (we’ll cover phantom loads in detail shortly)
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Large TVs in New RVs
The RV industry currently seems to believe that all RV buyers want large TVs—19-, 21-, or 24-inch or even larger models—in their new units. These sets may be nice to look at, but they exact a huge toll in battery consumption. Larger sizes can consume up to 175 watts. When on inverter power, this amounts to a draw of up to 14.6 amps (175 watts * 12 volts) on your battery bank; 3 hours’ use would equate to 43.75 amp-hours, a hefty discharge for only a few hours of television viewing. If you have one of these behemoths, we suggest using it only when you have campground power and getting a small 9-inch AC/DC set for boondocking. That is what we have used for over twenty years, and we’ve been happy with it. The newer ones even have built-in DVD players.
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Given these appliances, a corresponding daily usage is shown in Table 8-5. (Converting watts to amp-hours is explained in the following section.)
Eliminating the color TV and satellite receiver would reduce the daily consumption to 15.83 amp-hours, thus allowing for a much smaller battery bank capacity.
Calculating Amp-Hours
in a DC/AC System
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br /> One of the problems when calculating amp-hours in a system that supplies both DC and AC loads is that you are mixing apples and oranges. You must convert AC amps to DC amps to calculate amp-hours. Here is an easy conversion method:
1. Convert all 120-volt AC and 12-volt DC appliances and equipment into watts (watts = amps × volts). Watts are watts, whether AC or DC.
2. Multiply each item by the hours used to get the number of watt-hours.
3. Divide the total watt-hours by 12 volts to get amp-hours.
As an example, let’s look at the color TV and satellite receiver from Table 8-5. The color TV draws 0.45 amp at 120 volts AC, which is 54 watts, and the satellite receiver draws 0.28 amp at 120 VAC, which is 34 watts. For 3 hours of usage:
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Ohm’s Law
Ohm’s Law explains the mathematical relationship between current (amps), voltage (volts), and resistance (ohms). It is expressed as:
I (current) = V (voltage) * R (resistance)
There are several variations of this formula:
I = V ÷ R
V = R × I
R = V ÷ I
Now let’s add a fourth element, power, or the rate at which work is done, measured in watts. Just like volts, amps, and ohms, watts can be calculated easily: