by Bill Moeller
2. After charging, allow the battery to rest for several hours so the charge can equalize throughout the battery.
Table 8-6. Battery Capacity by Specific Gravity and Voltage
3. Before adding water to the cells, turn off all loads and the battery charger to prevent acid from bubbling up.
4. Remove each cap separately and check the electrolyte level. Wear goggles or a face shield when doing this to protect your eyes. Otherwise, we recommend using a mirror and flashlight. Direct the flashlight beam into the mirror to bounce the light into the cell. You’ll see the inside of the cell in the mirror without placing your face over the cell.
5. If the level is low, add ONLY distilled water. (Spring water or any other non-distilled water has mineral deposits that will ruin your battery.) Do not overfill or you will not leave enough space under the cell cap for gassing.
6. Replace cell caps and wipe off any spilled liquid.
The above may sound like a lot of work—initially. However, after you’ve done it a few times, it really isn’t that bad. The advantage is, of course, a long life of good service from your wet-cell batteries.
CHAPTER 9
Monitoring and Charging
Your Batteries
So far we’ve looked at the basics of batteries, as well as how to calculate daily power consumption and build a battery bank. In this chapter we’ll cover how to monitor the state of charge in your batteries, which will tell you when you need to charge them, as well as battery-charging methods.
MONITORING YOUR BATTERIES
It is never a good idea to go into the wilderness without a system in place that allows you to easily and regularly monitor your batteries’ state of charge. This not only helps you manage your resources properly while boondocking, so you get the most comfort and pleasure out of the experience, but also protects the life span of your batteries. Improper discharging and charging will guarantee that you’ll be buying new batteries frequently. And unfortunately, the typical battery condition meter installed in an RV’s monitor panel is not accurate enough, so you’ll probably need to get some other equipment. The most common monitoring meters used for measuring current and voltage are ammeters and voltmeters.
(RVIA)
Ammeters
Ammeters measure the flow of amps within an electrical system. They can be very useful for determining how big an amp load the system is drawing from your batteries at any one time and for monitoring your battery-charging system. There are two types of ammeters used in RVs, series and shunt.
Series Ammeters
A series ammeter is inserted into the positive wire between the battery and the fuse panel. Because it is directly in the circuit, the full current flows through the ammeter, allowing it to measure both load currents from the battery and charging currents from the various charging devices to the battery.
To monitor battery usage with this device, take meter readings every hour over the course of the evening, then add them up at the end of the night. This figure will give you an approximate amp-hour consumption. If you used about 6 amps per hour for 6 hours, you would have depleted your batteries by roughly 36 amp-hours.
Although the series ammeter is a bit outdated today, it is still probably the simplest way to monitor battery usage. We used a series ammeter for years on our older RVs. These meters have several advantages: they are available at most auto parts stores, are inexpensive, and are simple to wire. And they work well if you use a little common sense.
The series ammeter does have a few disadvantages. Because it is installed in the circuit between the battery and the fuse panel, it requires a very heavy wire to handle the current flowing through it. In most cases, you’ll need 4 AWG or larger wire. This location usually means the meter is in an inconvenient place for frequent monitoring, and routing the heavy wire to a more convenient location may be both expensive and impractical.
Another disadvantage relates to its scale. The most common series ammeter has a scale ranging from -60 amps to 0 to +60 (−60−0−+60) amps (the negative readings are discharges and the positives are charging). With this scale range, the numerical markings are in 5 or 10 amp increments, making it too large for accurate battery readings. Such a meter can only show approximate readings.
You can get ammeters with smaller scales, although they may be harder to find. In our first fulltiming trailer we mounted a −20−0−+20 meter on the front of the dinette seat, which was where the fuse panel was installed. This meter measured 1 amp increments but only went up to 20 amps, which is small by today’s standards.
My shunt ammeter has a knob that lets you choose between volts and two ammeters, allowing you to monitor both charging and discharging amperages.
Shunt Ammeters
A shunt ammeter is really a sensitive voltmeter that measures amps instead of volts. It has two components. The shunt is a conductor of known resistance that is inserted directly into the circuit through which the full current flows. The meter is connected to the shunt by a lightweight wire. A small portion of the current is diverted from the circuit to flow through the meter.
As current flows through the shunt ammeter, it causes voltage drop (see Chapter 10). The ammeter determines the amperage by measuring this voltage drop. For example, if a shunt has 0.5 ohm of resistance and the measured voltage drop is 1 volt, the current flow (from Ohm’s Law—see Chapter 8) is 2 amperes.
The primary advantage of a shunt ammeter is that you can locate the meter wherever it is convenient for you to view. Because only a small portion of the current flows to the meter, you can use lightweight 16 to 20 AWG wire for this connection, making it easy to route and inexpensive.
Shunt ammeters can be analog or digital. The digital models provide a higher level of accuracy, with readings to either a tenth or a hundredth of an amp, making them more useful than analog meters. Needless to say, however, the digital models are more expensive.
Another disadvantage of most digital meters is that they do not identify the readings as either positive or negative. A reading is usually the difference between the negative and positive readings, and it must be interpreted correctly.
Our shunt board for different instruments measuring amperage, voltage, and amp-hours. A setup like this is not normally needed on an RV, but we use it for the tests we run.
Here are a few examples:
Your solar panel is delivering a charge of 6 amps, and you have a light on and several other phantom loads creating a discharge of 2.5 amps. The meter will show a net reading of 3.5 amps, and without a plus/minus sign you must interpret which way the current is flowing. (A charging current would register as a positive value if your meter gave plus/minus signs.)
You have a discharge of 5 amps and a charge of 2 amps. The meter will show 3 amps, which would be a negative figure.
You must always figure out from what you know or can infer from the reading whether it is a positive or negative value.
One solution to this problem is to have two ammeters, one in the positive wire and another in the negative one. This way you can measure both discharging and charging currents at the same time. However, this also increases the expense.
Voltage Meters
Voltmeters measure the voltage present in the system, and it would seem logical that you could use this to ascertain a battery or battery bank’s state of charge. A fully charged battery at 77°F should measure 12.63 volts across the two battery terminals. A completely discharged battery has a voltage of 11.82 volts. A battery that has been discharged to 50% of its capacity will, in theory, show a voltage of 12.18. (As mentioned earlier, most battery manufacturers consider 10.5 volts the reading at which a battery is really dead, but any battery with a voltage reading of 11.8 or lower isn’t going to run much of anything.)
In practice, any load (lights, etc.) applied to the battery will cause a temporary voltage drop proportional to the size of the load, and the reading under this load will be less than the battery’s resting voltage. By the same token, if the b
attery bank is being charged, the meter will show a surface charge voltage caused by the charging process, not the true resting-state charge, and the result is a falsely high reading. A fully charged battery will give a highly accurate voltage reading only when the battery has rested for 24 hours—without load or charge—to allow the voltage to equalize between the cells.
A 24-hour rest is impractical in most cases, of course, so we have found by experimentation that an approximate state of battery charge can be measured after resting the battery bank (by turning off all loads and charging sources) for about 5 minutes.
This is enough time to allow the voltage to partially equalize across the cells, thus giving a somewhat truer reading. Longer resting periods will give more accurate readings, at least up to 3 hours. After that you may have reached the point of diminishing returns.
Checking battery voltage with a panel-mounted analog voltmeter or an analog multimeter won’t give you what you need to know, because there aren’t enough incremental markings for accuracy. Most analog (needle-type) voltmeters have a possible error greater than the 0.81-volt difference between a fully charged battery and a fully discharged one. A digital meter that reads to only one decimal place isn’t accurate enough either.
The so-called battery condition meters installed in most RV monitoring panels are therefore worthless. There are, however, expensive analog battery condition meters that have an expanded scale between 10 and 15 volts. These meters, which usually read as a percentage of charge, will help you interpret the voltage more accurately than a regular voltage meter, but they still fail to give a completely reliable picture.
Panel-mounted voltmeters and multimeters with digital readings to at least two decimal points can give very accurate readings and will have 81 increments between the fully charged and fully discharged states. The resultant readings must be interpreted properly, however.
Such readings can be used to monitor state of charge if taken hourly as current is drawn from the batteries. Allow the batteries to rest (no load, no charge) at least 5 minutes before each reading is made. Soon you will have a good idea of the differences between readings made with and without loads, and by applying this knowledge you can get a good estimate of your battery bank’s state of charge. Most authorities believe batteries should never be discharged to voltages below 12.1 volts if you want long battery life.
Note: It will be easier to use a multimeter if you install a special 12-volt outlet where you’ll be using the unit. Then make up special wire leads with a 12-volt plug on one end and meter jacks on the other. We used this method for years before fancier meters came along. And once you have a 12-volt DC source—DO NOT USE A CIGARETTE LIGHTER PLUG—use any plug as long as it is not a standard household plug. Check marine supply catalogs and stores; they offer a variety of 12-volt plugs.
Voltmeters do have other uses. For example, they will tell you if your batteries are being charged (by virtue of the high surface voltage reading you will see), and the voltage reading mode of a multimeter is useful for troubleshooting electrical problems.
This multimeter gives digital readouts to two decimal points.
The control panel of the Xantrex Battery Monitor, which measures amp-hours. (Xantrex)
Amp-Hour Meters
The best way to monitor your RV batteries’ state of charge is with an amp-hour meter. With this meter, you can know for sure how many amp-hours you’ve consumed, when it is time to recharge the batteries again, and when the batteries are completely recharged. It also provides amp readings and other useful features.
Amp-hour meters are usually adjusted to read 000, or slightly higher, when the battery is fully charged. As amp-hours are removed from the battery, a counter shows a negative number, indicating the cumulative amp-hours consumed. So if you have a 105 Ah battery, and the meter shows 26 Ah, you’ve used 25% of the battery’s capacity. If the meter shows 52 Ah, you’ve discharged 50% of the battery and it is definitely time to recharge. (If at all possible, never discharge batteries more than 50%.) As you recharge the battery, the meter will add back amp-hours; when the meter again reads zero, the battery has been recharged to 100% of capacity. (These meters take into account the charging inefficiency of the 1 to 1.2 Ah charging factor.)
Rule 3: Always monitor battery consumption; if possible, use an amp-hour meter.
Today there are amp-hour meters that incorporate other monitoring features: ammeters, voltmeters, time left to total discharge, and a light signal indicating when the batteries are fully recharged. Many of these fancier meters are incorporated into the monitoring panels of inverters and chargers, so that you can get all functions into one panel. These panels also have the capability of turning the inverter or charging device on and off as needed.
The readout panel of our Truecharge battery charger.
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Our Monitoring System
Over the years, we have developed a sophisticated monitoring system incorporating several different meters.
an amp-hour meter to record the amp-hours being consumed or charged
ammeters to read the charging amps of each of our four different charging devices
an ammeter located so as to read the total amps being used at any time
a voltmeter to monitor the AC voltage output of the inverter or generator
You may not want and probably don’t need a system as complicated as ours, especially since most of this data is available from a good-quality amp-hour meter.
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If you have problems finding suitable meters from RV stores, Camping World (www.campingworld.com), or RV supply houses, try marine catalogs or look online. West Marine is a good source (www.westmarine.com); it has a good selection of meters—both analog and digital—as well as plugs and sockets.
CHARGING YOUR BATTERIES
As we learned in Chapter 8, a chemical reaction takes place within a battery as electrical energy is removed from it. To recharge the battery, we pass an electrical current through it in order to reverse the reaction and restore the battery’s original composition.
One big misconception concerning battery charging is that amperage charges a battery. Not so—voltage charges a battery. This confusion is understandable since battery chargers are rated by their amperage (such as a 50 amp charger or a 100 amp alternator). Amperage in this case is a measure of the rate of current flow from the charging source to the battery and thus a measure of the speed of recharging, but it is not the cause of recharging. You could hook a 50 amp charger to a discharged battery and throw the switch, but unless the charger has a higher voltage potential than the discharged battery, there will be no charging current at all, much less 50 amps’worth.
Rule 4. Voltage recharges a battery, not amperage.
A battery that has a voltage lower than its fully charged level of 12.63 volts can be recharged by applying a charging source at a higher voltage. Within reason, the higher the voltage applied, the greater the charge-current amperage and the faster the recharge (there are limitations to this, as we will see). In other words, when a source of higher voltage is connected to a battery of lower voltage, a current will flow between the source and the battery until the battery’s voltage rises to equal the source voltage. In practice the charging voltage should be at least 1 volt higher than the battery’s fully charged voltage level of 12.63. This establishes a minimum charging voltage of 13.63.
But there is also a maximum acceptable charging voltage, and it varies according to the type of battery. Charging voltages higher than 14.5 are bad for wet-cell batteries because the liquid electrolyte boils vigorously, causing severe gassing and creating heat. If allowed to continue, this reduces the electrolyte volume and results in severe overcharging, both of which will damage the battery. Yet wet-cell batteries will not fully charge unless the charging voltage is maintained at the gassing level of 14.3 to 14.5 volts. Many charging devices do not charge to these high levels of voltage.
1. Although the incremental voltage variations ma
y not seem very important, they can greatly affect the charging rate.
Table 9-1. Wet-Cell Voltage Variations1 Due to Temperature
Table 9-2. Voltages Needed to Recharge Various Battery Types
Temperature affects both voltage and gassing. As the temperature increases, the voltage at which gassing occurs decreases. Many high-quality charging devices have temperature sensors that vary the voltage to match the ambient temperature. Putting all this together, we find that charging a wet-cell battery to its fullest requires the voltage to reach 14.4 volts, the gas point, at 77°F. Table 9-1 shows how temperature changes affect the gassing voltage.
Gel-cell batteries, however, require a lower charging voltage level because gassing must be minimized in these sealed batteries. Thus, they are only charged to a level of 14.0 to 14.1 volts, depending on the battery manufacturer’s specifications. Higher voltage will cause overcharging, drying out the paste or gel electrolyte. Several years ago when we had two gel-cell batteries we adjusted the charger voltage to a set point of 14.1 volts, and the batteries served us well for over seven years.
