by Bill Moeller
Many AGM batteries require yet another level of voltage, 14.2 to 14.3 volts. This allows some mild gassing to occur, which will be recombined into the sulfuric acid electrolyte as the battery charges. Table 9-2 summarizes charging voltages for each battery type.
CHARGING METHODS
There are three main charging methods—tapered, two-stage, and multistage—which we’ll cover in the order of their development and prevalence.
Constant-Voltage, or Tapered, Charging
The most common charging method—but by no means the most suitable for RV deep-cycle house batteries—is called constant-voltage, or tapered, charging, and it is used with the following devices:
automotive alternators
portable generators with DC output
many converters with charging capability
most ordinary portable battery chargers
Tapered charging has been the standard charging method for years because it is simple and inexpensive. In your automobile and in its many other applications, it works like this:
1. The voltage source (alternator, charger, etc.) produces a specific voltage, ranging from 13.8 to 14.5 volts, which is applied to the battery. This voltage varies with the manufacturer (see the sidebar on charging rates on page 99).
2. Initially the discharged battery accepts a high-amperage current, which makes the battery voltage rise quickly. The current at that point can be as high as the maximum amps allowed by the charging device’s rating (such as a 100 amp alternator, a 40 amp converter/charger, or a portable 50 amp charger).
3. Once the battery voltage reaches about 13.5 volts, the high-amperage output of the charge rapidly tapers off and stabilizes.
4. A voltage regulator on the charging device then allows the amperage flow to continue to taper off in order to maintain a constantly applied voltage until the battery is fully charged.
The battery voltage rises quickly at first because of the high voltage differential between the discharged battery and the charger. The surfaces of the battery plates are charged first, and this happens quickly. Then the charge has to diffuse to the interior of the plates, which happens at a much slower rate. Acceptance of the charge slows down, and the regulator starts to taper off the charging rate. So although the charging device has high-amperage output, it is only operating at this rate for a short time—basically until the surfaces of the plates have been fully charged. Given enough time, as the charge diffuses to the interior of the plates, the battery will eventually reach the applied voltage of the charger or alternator and be considered fully charged.
We have found that under the best of conditions, a large alternator of 100 amps or more may only charge at a maximum rate of 50% of its rating (in this case, 50 amps) for a few minutes or so during the beginning of the charge cycle. We once timed the charging rate of our Dodge truck’s 136 amp alternator. With the batteries discharged to about 22%, the alternator initially put out 42.1 amps for about 10 minutes; then the charging rate started dropping. After another 30 minutes, the rate was down to 12.5 amps—not enough to quickly replace the 40 amp-hours the batteries had been discharged to (see Table 10-3). This charging rate would eventually charge the battery, but it would take 5 or more hours of charging and driving to do so. Naturally, these results will vary with different alternators.
Tapered charging works well for most normal automotive use. Usually the starting battery is only slightly discharged during the initial cranking of the engine, and tapered charging will quickly recharge it. Further electrical needs such as the ignition system, lights, radio, etc., are met by the alternator directly while the engine is running, not from the battery. However, this method is not really suited for recharging the deep-cycle house batteries of an RV, which (1) have thicker plates and a correspondingly slower diffusion rate, and (2) may be depleted by anywhere from 25% to 50% or more of the batteries’ capacity. Tapered charging would likely result in undercharging or overcharging, depending on the charging time. Either way, it would shorten battery life. Tapered charging can be more efficient if the regulator holds the voltage higher for a longer period of time. See the results of the Iota charger/converter test, pages 118-19.
Two-Stage Charging
Another method that is becoming popular and is superior to tapered charging for recharging deep-cycle batteries is the two-stage process called constant amperage/constant voltage charging.
Studies have shown that heavily discharged deep-cycle batteries can accept a high-amperage charging rate of as much as 40% of the batteries’ amp-hour capacity for a reasonable time. This is particularly true for gel-cell batteries. A 200 amp-hour gel-cell battery bank can handle an initial charging rate of up to 80 amps (200 Ah × 0.40 = 80 amps).
A two-stage regulator directs the charging device to maintain its maximum amperage output longer than it would in a constant-voltage process. For example, the regulator on a charger designed for this process and rated for 80 amps of constant output will deliver that 80 amps to the battery by letting the voltage begin low and slowly rise to the lower limits of charging.
By the time the charging voltage of any deep-cycle battery reaches the charger’s maximum set-point output of between 14.0 and 14.4 volts, the battery has received approximately 70% of its full charge. Often this takes just minutes instead of the hours required with regular tapered charging. This first stage, the constant-amperage part of the charge, is called the bulk charge stage.
The second part of the process is called the absorption, or acceptance, stage. In this stage the set voltage that has now been reached is held at a constant rate, and the amperage is allowed to taper off as in the constant-voltage process. However, this whole process has been speeded up because of the high-amperage rate continuously applied at the beginning of the charge. The battery then accepts the charge at its own internal diffusion rate until it is fully charged.
Rule 5. For fast recharging of deep-cycle batteries, the constant amperage/constant voltage two-stage charging process is the best method.
Several types of charging equipment have two-stage charging options:
specially designed alternators and regulators
some converter/chargers
some solar panel regulators
the built-in chargers in most inverters
The equipment for two-stage charging is specifically designed to maintain the necessary high-amperage output. Standard alternators and chargers, even if equipped with a two-stage regulator, may not be able to handle the constant high-amperage output.
Multistage Charging
Multistage charging is also called smart charging because it is controlled by a regulator with a microprocessor that oversees the three (sometimes four) stages of this charging process, which are as follows:
bulk stage (constant amperage)
absorption, or acceptance, stage (constant voltage)
float stage (a lower maintenance voltage)
equalization stage (not used in all applications)
Many multistage regulators let you choose the specific type of battery you will be charging—flooded wet-cell, gel-cell, or AGM. Charging devices that are offered with multistage regulators include alternators, generators, chargers, and solar panels. Balmar (www.balmar.net) and Xantrex Technology (www.xantrex.com) are two leading suppliers of multistage regulators and battery chargers.
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Charging Rates-What’s Best
According to manufacturers, the very best way to charge batteries absent any considerations of time or effort would be at a low amperage rate of 5% of amp-hour capacity at 13.8 volts. This would allow the battery to accept the charge at its natural diffusion rate with light gassing, only slight heating, and minimal depletion of its electrolyte. The problem with this method—which is close to the traditional tapered-charge approach—is that it is simply impractical. You might need 10 to 20 hours of charging time to restore a battery to full capacity, and rarely do you have the luxury of that much charging time.
Another axiom
is that the lower the rate of charge and the longer the length of time to reach the gassing voltage, the higher the state of charge in the battery. For example, if any completely discharged battery is continuously charged at an amperage that is 25% of its amp-hour capacity, it will take 2.75 hours to reach gassing voltage, at which time the battery will be charged to 70% of its capacity. But if deep-cycle wet-cell batteries (including 6-volt golf-cart batteries in series) are charged at a 40% rate, they will reach the gassing voltage level sooner, and at only 55% of capacity. But why is this so?
Because of the thicker plates in deep-cycle batteries, the diffusion rate of the acid through the battery is slower than the charge rate. This ultimately results in a lower state of charge. If charging is continued at the higher rate of 40%, excessive gassing and heating will occur with little additional charging. The charge rate must be reduced to about 25% of capacity in order for charging to continue to completion.
Gel-cell batteries can be charged at the higher rate of 40% because there is no diffusion of acid in the electrolyte to contend with, and these batteries have a high acceptance rate. However, gel-cell batteries often reach the charge voltage too quickly and thus trip a two-stage charger into its absorption stage before the battery has received the full bulk charge.
The best all-around compromise bulk-stage charging rate is 25% of the battery amp-hour rating. A lower rate will take too long, and a higher rate will result in a lower state of charge.
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The first two stages proceed exactly as with the two-stage charging covered above. But then the regulator automatically switches to the float stage, which reduces the charging voltage to a constant low of 13.2 to 13.7. This level of charging voltage maintains the battery at a full charge without causing gassing or overcharging. It also compensates for self-discharging by keeping the battery topped off.
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Golf-Cart Batteries and Float Charging
A little-known problem with charging 6-volt golf-cart wet-cell batteries is that most golf-cart battery manufacturers do not recommend float charging, stating that these batteries are not designed for this type of continuous charging. Golf-cart batteries require a rest period between the charging and recharging cycles.
One manufacturer told us that although its golf-cart batteries could be float-charged, it preferred that floating not be used and that the batteries be rested after charging. But if the batteries were floated, it should be done at no more than 13.5 volts. Golf-cart batteries also have a high rate of self-discharge during such rest periods.
This requirement is something that does not necessarily fit in with the RV lifestyle, particularly that of fulltimers and snowbirds. These RVers may be at a campground for long periods of time with their RVs plugged into the campground’s electric outlet, and using their converter/chargers or inverter/chargers to supply 12-volt power and to keep the batteries fully charged. It appears that only a few manufacturers of chargers and solar panel regulators are aware of this golf-cart battery requirement. We know of one multistage charger manufacturer and one solar panel regulator manufacturer that provide settings that will eliminate the float stage from the charging process. Until this Catch-22 is resolved, however, RVers recharging 6-volt golf-cart batteries with multistage regulators would do well to terminate charging after the absorption phase.
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Note: Although the lower voltage of 13.2 maintains the charge in the battery, it cannot handle additional loads—such as lights, water pumps, etc.—without discharging the battery. Maintenance devices, such as converters, usually operate at the slightly higher voltage of 13.7 volts to handle these loads.
The equalization, or conditioning, stage included on some charging devices is designed to rejuvenate wet-cell batteries. This stage applies a controlled high-voltage overcharge (up to as much as 16.2 volts) at low amperage (5% of amp-hour capacity or lower) to the battery for no more than 4 hours to bring the cells back to their fully charged states. The purpose is to equalize the voltage in all the battery’s cells and remove excess sulfate from the cells’ plates, and ideally it should be done every month or so.
This stage is only for golf-cart wet-cell and 12-volt deep-cycle wet-cell batteries. (You will have to top off with distilled water after this procedure.) Do not perform this stage on sealed AGM or gel-cell batteries. Check the manufacturer’s instructions on how to perform this function. The only sure way to test for proper equalization is with a hydrometer (see Chapter 8), checking the cells repeatedly until all cells are up to full charge.
Multistage charging offers the speed and completeness of two-stage charging together with the possibility of maintaining float voltages and periodically equalizing the batteries.
BATTERY LIFE CYCLES
All batteries have a life span based on the number of times the battery has been discharged and recharged; each discharge/recharge (D/R) represents one life cycle.
To illustrate, let’s say we have an automotive SLI battery with an approximate life span of three years. In the course of a day, while driving around town, we start the engine four times, and the alternator recharges the battery after each start. This represents four D/R cycles per day. Multiplying the four cycles by 365 days per year for three years gives the battery a projected life span of 4,380 life cycles (4 × 365 × 3 = 4,380 life cycles).
Now let’s look at deep-cycle battery usage. Usually a deep-cycle battery on an RV is discharged during the evening and recharged during the day, equaling one life cycle per day. With this in mind, a three-year deep-cycle battery would have a theoretical life span of 1,095 life cycles (365 × 3 = 1,095 life cycles).
Unfortunately, other factors also affect the number of life cycles. The depth of discharge (DOD) and the frequency of deep discharges can drastically shorten the life of the battery. Here are some examples:
Discharging a standard deep-cycle wet-cell battery to 50% of its amp-hour capacity will give it a life span of about 1,000 cycles if it is well maintained.
Limiting the depth of discharge to 10% of amp-hour capacity will increase the battery’s life span to as much as 20,000 cycles.
Occasional discharges to a depth of 80% can reduce the life span to as little as 200 to 300 life cycles.
Golf-cart, gel-cell, andAGM batteries have a longer life span than ordinary deep-cycle batteries. Six-volt golf-cart batteries can occasionally be discharged to a depth of 60% to 80% because of their heavier, thicker plates without too much ill effect. Good-quality gel-cells can go to a DOD of about 60% without damage. The effects of DOD on AGM batteries are still not yet completely known. We must, however, ask ourselves if we really want—or need—to discharge our batteries to a depth as great as 60% to 80%, pushing them to their limits.
It is easy to see why it is smart to keep the DOD as low as possible, because it will definitely increase the battery’s life span. It is wiser, and perhaps cheaper in the long run, to have a larger battery bank to handle large amp-hour demands than it is to overdischarge a smaller bank of batteries. If you regularly discharge your battery bank by about 100 amp-hours, your 200 amp-hour bank would be down 50%. If you increased your bank size to 400 amp-hours, then the same amount of discharge would only bring the batteries down by 25%.
It is good practice never to discharge batteries by more than 50% of capacity. We know of one independent authority and several manufacturers who suggest limiting DOD to only 20%, and another authority who recommends only 33.3%. Our own practice and recommendation is to try to hold DOD to no more than 25% of amp-hour capacity as much as possible.
Rule 6. Do not discharge batteries to more than 25% of Ah capacity if at all possible.
Even though golf-cart and gel-cell batteries can handle deep discharges, the fact that you must be able to recharge them the next day is a compelling reason to restrict the depth of discharge as much as possible.
Rule 7. Never discharge your batteries to more than your charging equipment’s capability to recharge them within the next daily charge
period. (Unlike the other rules, this one is based more on common sense than empirical evidence.)
If you discharge a 200 amp-hour bank by 80% of its amp-hour capacity the batteries will be down 160 amp-hours. Your charging equipment must be capable of putting back this many amp-hours and more during the next charge period. Because of inefficiencies in the charging process, for every amp-hour consumed, 1.2 amp-hours must be returned to the battery. A battery bank that is down 160 amp-hours must have 192 amp-hours put back the next day for a full recharge.
If you do a little math, you can easily see how difficult or even impossible it would be to achieve that level of charging the next day. Here are some options:
Suppose you have two 100-watt solar panels capable of delivering about 10 amps in full sunshine. It would require 19.2 hours of continuous charging at full panel output to completely recharge the batteries (10 amps × 19.2 hours =192 amp-hours). But you will rarely if ever have 19 hours of full sunshine. You could get more panels, but they would probably be more than could fit on the roof of your RV.
A 100 amp engine alternator with a standard regulator working at 50% efficiency would require an equal duration of driving time (19 hours) to recharge, and that doesn’t sound like much fun!
An alternator with a multistage regulator or a multistage charger powered by a 120 VAC generator would probably do the job in 4 hours or more. The question then becomes, do you really want to run your engine or generator for such a long period just to charge your batteries?
To us, it makes more sense simply to limit the size of your discharges to no more than 25% of amp-hour capacity and be able to comfortably recharge your batteries the next day.
