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As crucial as battery power is to the operation of most modern vessels, it remains one of the most baffling subjects most boaters have to contend with. Ensuring that you have adequate electrical energy really isn't terribly complicated, and it doesn't require a degree in electrical engineering. But if you don't happen to have an abundance of "smarts" when it comes to managing your charging needs, it helps to have a system with the "smarts" built-in. Fortunately, there are a variety of modern "smart" charging options available that make proper care of batteries almost idiot-proof.
Marine deep-cycle batteries will last the longest and charge the fastest if they are charged in distinct phases, rather than in a single phase as with archaic charging systems and simple regulators. The method of charging described below is recommended by virtually all marine battery manufacturers. In the description that follows, remember that recommending precise voltages for batteries is subject to at least two caveats:
Rick Proctor, founder of Cruising Equipment Co., is one of the leading innovators in the field of marine electrical systems. He coined the term "Ideal Charge Curve" to describe the best way to restore energy to your batteries. Here's how each phase of the process works:
Bulk Phase: Charge at a rate up to 20%-40% of C to a voltage of about 14.4 volts (gel: 14.1V). For example, a 200 amp-hour battery would be charged at 40-80 amperes. This will bring the battery to about 75% of full charge, and is efficient (more amp-hours per hour of charge time) since the battery accepts more current when it is discharged. AGMs require slightly different voltages, and unless there is an AGM setting, should be charged using lead-acid settings.
Acceptance Phase: Maintain battery at 14.4 volts (Gel: 14.1V) while the amperage is steadily reduced. This will restore the next 25% of capacity at a declining rate. Your battery can be considered fully charged if it will accept current equal to 2% of C at 14.4 volts; e.g. a 200 amp-hour battery will only accept 4 amps.
Float Phase: When the battery's acceptance declines to 2-4% of C, the voltage is reduced to 13.3 volts (Gel: 13.7V) to maintain the battery without losing electrolyte from the cells. This is a maintenance phase, not a charging phase.
Equalization: An optional (and frequently omitted) stage is equalization. It is used to prevent
flooded lead acid batteries from aging prematurely. After the battery reaches the end of acceptance phase, the battery continues to be charged at 4% of C until the voltage stops rising-usually around 15.5-16.2 volts. This forces the battery to its highest possible state of charge, and dissolves the crystals of lead sulfate that have collected on the battery's plates. In applications where maximum energy storage is important, this phase is done every charge cycle. In the marine environment, it is more likely to be done every 20-50 cycles to extend the life and capacity of wet batteries. Gel batteries should not be equalized. Since electrical equipment and light-bulbs can be damaged by high voltage, the battery should be disconnected from all loads during equalization.
This type of battery charging, consisting of multiple stages, is not possible with automotive-type regulators, unregulated solar panels, ferroresonant chargers, or taffrail generators. We strongly encourage the use of efficient charge devices that use modern multiple-step regulation, such as Balmar and Xantrex Regulators, Xantrex, Vector or Guest battery chargers.
If you have to replace lots of amp-hours, it may be tempting to use a massive charger or alternator and really pour on the current. This works up to a point, but batteries cannot accept unlimited amounts of current without getting hot or gassing excessively (when the electrolyte disassociates into hydrogen and oxygen), which shortens battery life. Plus, you'll soon confront the voltage limits mentioned above, where charge rates must be reduced. In fact, adding battery capacity may actually decrease the time needed to recharge to levels adequate to meet your energy needs more than any other change you can make to your system. Battery banks with greater acceptance can use more of the available charging current before reaching a voltage where the regulator begins to reduce the current.
If you operate your house bank between 50%-85% state of charge, as many experts recommend, and charge once daily, you should be able to return the 35% of battery capacity by operating a properly-sized alternator for slightly over an hour. More deeply discharged batteries, or smaller alternators, will require more time.
Most cruisers would like to have fully charged batteries but settle for something more practical perhaps 85% charged-to reduce engine running time. There are several ways to decide when to stop charging, but we feel that the following methods are the best:
1. You can use an amp-hour meter, and stop charging when it reads a certain value. You might select 30 amp-hours on a 200 amp-hour bank, corresponding to 85% charged.
2. You can do what many sophisticated cruisers do, and stop charging when the acceptance of the batteries reaches a predetermined level. Assume that you've run your batteries down 50% before you start charging. When the alternator kicks in, the charge current jumps to 100 amps, and the system voltage begins to climb. Assuming that you have a multi-step regulator, this amperage should remain constant until the voltage limit is reached, when it should start to be reduced. The question becomes "How little current am I willing to accept?" If you turn off your engine as the voltage peaks and the current is about to drop, then you will get the greatest number of amp-hours returned per engine hour, but your batteries will be only 75% charged or so, since they will not have undergone an acceptance stage. If you allow the current to taper off to 5 measly amps, you will have fully-charged your batteries, but you'll spend a long time doing it. We think that letting the amperage decline to 10% of C is a reasonable rule for long trips.