Chuck Husick: Techno-Talk, March 2001 BoatUS Magazine - Updated September 2008

Charging Systems

The storage batteries on your boat must be properly recharged after use. So long as you use the boat fairly frequently and don't run the battery down too far by excessive use of lights or radio when the engine is not running, the engine's automotive-style alternator will do an adequate job of keeping the battery charged. With reasonable care, the battery will probably last about three years. However on some boats, especially those where the batteries are located at a distance from the engine, the standard charging system will fail to fully recharge the batteries. Batteries that are not fully and promptly recharged will gradually lose energy storage capacity and will have a shorter than normal life.

Electrical power for the boat and for battery charging is provided by an engine-driven, automobile-type alternator, controlled by a voltage regulator built into the alternator's case. The voltage required to properly recharge a storage battery varies with battery temperature. Cold batteries require a higher voltage, warm batteries a lower voltage. The built-in regulator's temperature sensor measures alternator temperature, not battery temperature. This charging voltage control system design assumes that the battery is close to the engine/alternator and its temperature is directly related to the "under the hood" temperature.

The system works well on boats where the battery is in the engine compartment and its temperature is influenced by engine temperature, as it is in a car. However, when batteries are located away from the engine's heat, as is frequently the case for house batteries and especially on sailboats, the batteries will be much cooler than the voltage regulator's temperature sensor assumes them to be. The charging voltage delivered from the alternator will then be too low, slowing the recharge process and often preventing a full recharge even after a long period of engine operation. Repeated failure to fully recharge batteries can result in a loss of 20% or more of energy storage capacity and shorten the useful life of the battery.

The charging problems created by inaccurate sensing of battery temperature can be eliminated by replacing the alternator temperature sensing voltage regulator with one that measures the actual temperature of the battery being charged. The external voltage regulators sold for marine use include a temperature sensor that is clipped onto the terminal post of a battery. Using this actual battery temperature information, the regulator will ensure that alternator voltage matches the battery's charge voltage requirements. By sensing battery voltage at the positive terminal of one of the batteries in the boat's battery bank the remote voltage regulator will eliminate any charging voltage error that may arise due to the resistance of the wires between the alternator and the battery.

A "smart" microprocessor-controlled external voltage regulator will also sense the charging voltage at the terminal of the battery being charged. Direct sensing of battery voltage eliminates the low battery charge level problems that result from the use of diode type battery isolators. In addition, a smart regulator can periodically "equalize" flooded cell (wet) deep cycle batteries by applying a controlled, higher than normal charging voltage that brings all cells to an equal and optimum state of charge, thereby assuring maximum performance and battery life.

Most external voltage regulators can be adjusted to match the charging voltage requirements of the type of battery being used. This especially important with gel cell and AGM batteries that require a lower charging voltage than flooded cell batteries. Gel and AGM batteries can be seriously damaged or destroyed if charged at the voltages appropriate for flooded cell batteries. Each type of battery must be provided with an appropriate charging system.

External regulators equipped with an alternator temperature sensor will automatically control the alternator's power output as necessary to prevent damage from overheating. The temperature of an alternator depends largely on how much power it is being asked to produce. Large DC/AC inverters can impose very large power demands on the alternator, as can the high charge currents that AGM batteries will accept.

The multi-step battery charging program managed by a smart, external regulator charges batteries as rapidly as possible while protecting them from being damaged by excess current or overheating. Programmed charging also ensures the most complete recharge possible, maintaining maximum battery energy storage capacity for the longest possible time. The $150 cost of an external regulator will be recovered by improved battery performance and life in a few boating seasons.