by Bill Moeller
Rule 8. Always fully recharge the batteries after each deep discharge. Allow some resting time of the batteries between cycles if possible.
You should always fully recharge your batteries after each deep discharge. If you only partially recharge your batteries between discharge cycles, your battery life will be diminished. Cycles where the battery is discharged down to, say, 50% of amp-hour capacity and then recharged to only 80% of capacity should be avoided at all costs. If the charging system cannot fully recharge the batteries, such deep discharges should definitely be avoided.
We have mentioned previously the need to rest batteries as much as possible between charging and discharging. The human body can be compared with a battery. We humans work and play hard, using a lot of energy. We then refuel our bodies by eating, and after awhile we have to rest or sleep. Batteries work the same way: they put out a lot of energy during discharge, and are then recharged, but they need to rest too. Resting for a battery is just as important as recharging and the proper maintenance.
As matter of fact, most manufacturers recommend allowing batteries to rest for 24 hours after recharging. This is rather impractical for boondocking RVers, since they need to use their batteries more frequently than that. But a few hours of rest will work wonders in adding to the life of your batteries.
Rule 9. Rest your batteries as much as possible.
To explain further, as a battery is discharged, the cell nearest to the terminal through which the current flows will have a lower voltage than the terminal at the other end of the battery. Conversely, when a battery is recharged, the cell nearest the charging current terminal will develop a higher voltage than its neighbors, and the one farthest from the current will be the lowest. This higher recharging voltage is called surface voltage. Resting allows the surface voltage to dissipate throughout the other cells and equalize the overall voltage. So voltage readings will not be accurate unless the battery has been allowed to rest first.
We make it a habit to rest our batteries. To accomplish this we’ve wired our converter/charger and the charger in the inverter to the same AC circuit with a 30 amp, two-way, double-pole/double-throw switch. The switch allows us to select whichever charging device we want to use, and putting the switch toggle in the middle position turns off both devices. Whenever we leave the trailer, we turn off both devices, allowing the batteries to rest. Even when we’re home during the day, we shut off all charging devices if they are not needed. This has extended our batteries’ lives. We had a pair of cheap deep-cycle 12-volt batteries that lasted for five years, and they were still going strong when we got the chance to buy two gel-cell batteries at a good price. Those in turn lasted seven years with no sign of failure, at which point we sold the trailer.
THE RULE OF TWENTY-FIVES
Perhaps you’ve noticed we’ve mentioned 25% of amp-hour capacity frequently and in different contexts. Here’s a summary that we call the Rule of Twenty-Fives:
1. Do not apply a load to the batteries greater than 25% of amp-hour capacity—except for short periods. We recommend limiting loads to 25% because greater loads will reduce the amp-hour capacity of your batteries. This percentage also plays an important part in inverter size and operation.
2. Do not regularly discharge batteries by more than 25% of their amp-hour capacity. Holding to a 25% depth of discharge will allow faster recharging, protect the battery bank from overdischarging, and give the battery more life cycles. Occasional discharges to a depth of 50% are acceptable provided that you allow sufficient charging time to bring the battery back to a complete charge.
3. Always recharge at a rate of about 25% of amp-hour capacity. We recommend recharging at a rate of 25% amp-hour capacity (through the bulk charging phase) because this is the best overall rate of charge. It will give a fast recharge yet also allow the batteries to reach their fullest charge. If batteries have been deeply discharged, higher rates can be used for even faster times, as long as the charging method allows for an automatic reduction after 75% of the charge has been completed from the higher amperage rate to the 25% rate. If it doesn’t, stick to the 25% rate overall.
Working within these guidelines will result in longer battery life, higher battery capacity, faster battery recharging, and happier boondock camping.
CHARGING DEVICES
In the final chapters of the book, we will cover the various charging devices and technologies:
Chapter 10: Engine alternators
Chapter 11: Generators, converters/chargers, and inverters
Chapter 12: Solar panels and wind generators
If you recall our fictional couple from Chapter 1, Bob and Mary Jones used several methods throughout their two-week trip to charge their batteries: the alternator when driving, solar panels when camping, the generator when the neighbors were gone, and a wind generator on a bluff overlooking a beach. Each device has advantages and disadvantages, and no one method will fully supply all your charging needs all of the time. Understanding each method, identifying your family’s pattern of battery use, and taking into account how long you’d like to boondock will help you design a balanced system.
CHAPTER 10
Engine Alternators
An alternator is a machine that generates electricity by spinning a magnet (the rotor) inside a series of coils (the stator). The resulting power output is AC current, which is changed (rectified) to DC current via silicon diodes. It is coupled with a voltage regulator that controls the rate of electrical current into the battery or batteries to prevent over- or undercharging. (For a more detailed explanation of alternators, see our book RV Electrical Systems.)
ALTERNATORS AND RVS
In your everyday car, SUV or truck, the alternator keeps the engine, or SLI, battery charged. Its amperage output is just high enough to replace the current used to start the engine, then it tapers off quickly until it has little charging capability. When your truck is attached to a trailer or the vehicle is a motorhome, the alternator has the additional task of charging the house batteries. In fact, for many years the engine alternator was the only way to charge the house battery. This system worked reasonably well because the demand for electrical energy from the house battery was small; usually a few lights were used in the evening, along with a radio (which may even have had its own internal batteries), so the RV battery was only discharged by a few amp-hours. The alternator very likely, and probably without too much trouble, fully charged the battery the next day while the RVers were traveling to their next destination.
(RVIA)
Modern RVs, however, have more equipment and appliances that require more power and larger battery banks. They now have charging requirements that some stock engine alternators are not capable of handling. But today’s alternators and regulators have gone through their own transformation. So it is now possible to upgrade your charging system and improve the capabilities of its alternator.
One way to upgrade is to install a multistage voltage regulator on your alternator. The standard regulator is a constant-voltage regulator, and so provides a tapered charge. It will not do a good job of rapidly recharging deeply discharged batteries. But note that the alternator cannot have an internal regulator (which are usually found in stock alternators).
If one of these new regulators will not fit your engine, then it is possible to replace your stock alternator with a high-output alternator. A multistage alternator will certainly improve the charging rate of your batteries, and it may be a less expensive way to get this modern means of charging than some of the other high-powered charging systems.
A multistage regulator. (Balmar)
A modern high-output alternator. (Wrangler NW Power Products)
Another advantage of a high-output alternator is that it can handle running your onboard inverter during travel, allowing you to use high-wattage AC equipment. (Although this will probably not include an air conditioner, even though we have heard stories of RVers doing just this. It is just too large a load.)
The a
lternator will still have to be capable of handling other extra electrical loads. These loads can help steal away horsepower from your engine for hill climbing and such, but the alternator load is still a very small one for large pickup truck and motorhome engines—1 horsepower for every 50 amps of electrical power.
Installing a multistage regulator can present problems with the onboard computer on your engine, however, and it may even void the warranty on your vehicle. Also, it could interfere with the diagnostic computer in your local auto repair shop. The bottom line is that this is not an easy do-it-yourself project. You must be knowledgeable about automotive systems and have the necessary test equipment. We recommend hiring someone who knows how to wire this equipment. We have worked with Wrangler NW Power Products (www.wranglernw.com) for years, which specializes in solving these sorts of charging problems.
VOLTAGE DROP
If you have a standard tapered alternator, you may be able to improve its charging capabilities by checking that it’s wired properly. The use of an inadequately sized wire can result in voltage drop between the alternator and batteries, which can greatly diminish your alternator’s charging capabilities.
Voltage drop is a loss in voltage in a circuit; that is, voltage is lost between the power source—such as the alternator—and the load—for our purposes, the battery bank. The cause is high electrical resistance. Some materials conduct electricity very well (known as conductors), and some do not (known as insulators). All conductors resist the flow of current to some degree. Even copper wire, the metal found in all automotive and RV wiring, has some resistance. Resistance in a charge wire can cause voltage drop, especially over a long wire run. Also, as the diameter of a wire decreases, the amount of current it can carry (its ampacity) also decreases.
Voltage drop will not only affect battery charging but also the performance of your appliances and equipment because they won’t be receiving enough power to run properly. This is why choosing the correct wire size is so important when wiring a circuit. Basically, increasing the diameter of the wire will reduce voltage drop.
As to what size wire to use, fortunately there are tables that will help us with that process, such as Table 10-1 on page 108.
Here’s an example. At 50 amps, 14-gauge wire has a voltage drop of 5.16 volts over a run of 40 feet (20 feet for the positive wire and 20 feet for the negative wire). This is the usual combined distance of most charge lines for travel and fifth-wheel trailers. With this much voltage drop, the voltage reaching your batteries would be only 9.34 volts, lower than the voltage of a dead battery. Note that 14-gauge wire is suited only for lights and light loads. Its ampacity is only 20 amps, which means that loads over 20 amps will cause voltage drop.
Table 10-1. Wire Ampacity and Voltage Drop
Or say we increase the wire size to 12-gauge wire, keeping the same amperage and wire run. Now the voltage drop per foot would be 0.08100:40 feet × 0.081 = 3.4 volts, which is still quite a loss. It’s easy to see why battery charging can be greatly curtailed by voltage drop.
How much voltage drop is allowable? Table 10-2 lists some examples. At 14 volts, the usual minimum charging voltage for a 1% drop should not exceed 0.14 volt; at 12 volts the drop should not be more than 0.12 volt. A 1% maximum drop is the best for charging lines to get the greatest charge possible. For purposes other than charging, a drop of 3% is permissible—0.36 volt at 12 volts.
When we first became fulltimers, our alternator could not satisfactorily charge the batteries on our 23-foot travel trailer. We suspected that the problem was voltage drop in the charge line between the alternator and the batteries. After charging the batteries via another source, we did a voltage check with the engine running. The voltage at the alternator was 13.8 volts, but only 13.2 volts at the batteries, showing a definite voltage drop. With further checking, we found that the wiring harness, which was part of the manufacturer’s towing package on our truck, used 14-gauge wire for the charge line, while the rest of the line—that was installed by the dealer—was 12-gauge wire. The whole line was about 14 feet in length and used a ground wire to the chassis. No wonder the voltage was so low.
Table 10-2. Acceptable Voltage Drop
After we discovered our voltage drop problem, we had three options: increase the size of the charging line, switch to a high-output alternator with a multistage regulator, or do both. We chose to increase the size of the charging line. We thought 8-gauge would be about the right size. So we made the rounds of auto-parts and RV dealers looking for 8-gauge wire. One RV dealer asked what we wanted with wire that large. When we told him, he replied, "Any size wire will do the job." Later, we laughed at the man’s ignorance, but in retrospect, he was partially right because voltage will equalize eventually over many hours.
Since most stock alternators produce a tapered charge delivering its maximum amperage at low voltage, the high-amperage output never reaches the battery because of the voltage drop, but as the amperage output diminishes, eventually a charge will reach the battery. So smaller wire will charge the battery over a long period of time (although you should never use anything less than 4-gauge wire for charge lines), but the high-amperage output is lost in the process, particularly when fast charging is desirable.
On our present fifth-wheel trailer, we use 4-gauge wire (with welding wire plugs so we can break the connection when we unhitch the pickup truck). It has dramatically improved the charging rate. When the batteries are fully charged, we now find that the voltage is the same at both the alternator and the batteries—no loss. Admittedly, it still takes 4,5, or more hours of charging to bring the batteries to a full charge, but they will get charged when we are traveling. When we spend several days at a campsite, running the engine for 30 to 60 minutes at a higher rpm than idle helps speed up charging when the sky is cloudy and the solar panels need some help. It does come at a high price, considering the cost of gasoline and diesel fuel. However, the combination of these two charging sources (alternator and solar panels) will charge batteries faster than either will by itself because of the additive nature of the two processes. This is particularly true after the high-amperage alternator output has tapered down to a few amps.
Trailers usually present the most problems while charging due to the long wire runs between the alternator and the batteries, which can lead to voltage drop. Motorhomes do not have this problem because their batteries are usually very close to the alternator. However, we have seen some motorhomes with wiring problems as bad as those on trailers. Far too often manufacturers will install the batteries wherever there is sufficient room for them, not where they will be the most efficient or effective. We once saw a diesel-pusher motorhome at an RV show where the house batteries were located at the front of the rig behind the grille. We measured the distance, and it was more than 30 feet one way. So, figuring on twice the distance, that would mean a run of about 65 feet, and the wire was only 10-gauge, causing voltage drop of about 3.31 volts. RVers should investigate prospective RVs for wiring irregularities before buying them, or they may find they have a large rewiring job to do.
TESTING ALTERNATOR EFFICIENCY
Table 10-3 shows the results of a test we conducted with our 136 amp truck alternator. We discharged our batteries until they were down 47 amp-hours according to our amp-hour meter, which was a discharge of 29.3% of capacity. The battery voltage was 12.4 volts. The next morning we left on a trip of 122 miles to our next destination where we would be staying in a campground with full hookups. Here are some highlights of the test:
We started the engine and waited for the engine to reach operating voltage. Then we plugged in the separate trailer charge line. Immediately the charging amperage rose to 42.1 amps, the battery voltage to 12.9 volts.
After 10 minutes, the charging amperage had dropped to 15.8 amps, and the voltage had risen to 13.4 volts.
After 30 minutes, the charging amperage was 12.5, and the voltage had stabilized at 13.6 volts.
After 3 hours and 10 minutes of driving, we ar
rived at our destination. The charging amperage was at 7.5 amps, and the battery voltage had risen to 13.7 volts. But our amp-hour meter showed that we still had 15.5 amp-hours to replenish for a full charge.
At the campground, our converter/charger completed the charging cycle.
We projected the alternator would have needed another 2 hours of driving time to finishing charging the batteries, making a total of 5 hours and 10 minutes.
As you can see, alternator tapered charging can involve many hours of driving, although it will do a fairly good job, in part thanks to the 4-gauge charging wire we installed.
The charging time would have been much longer in our older trucks, which had smaller alternators than our present one. You may wonder why such a large alternator didn’t do a better job. First, the alternator has to provide all the electrical needs of the engine, including the dash air conditioner, lights, radio, and so forth, so there is less electrical energy left for charging. Second, most standard stock alternators are only about 50% efficient. And third, with the Dodge Cummins diesel, when the engine and outside temperatures are about freezing—the precise time when you wish to charge batteries as fast as possible—the alternator must provide the power to recharge the 90 amps the intake manifold heater draws from the truck batteries during start-up.
Table 10-3. Test of Our 136 Amp Alternator
But to sum up, the alternator as a charging device can do a reasonably good job if the amp-hour deficit is not too large and you use an adequately sized wire for the charge line.