Low water consumption is an indication of proper regulator setting. When a battery uses more than 30-60 ml of water per cell per 40 hours of operation the regulator is set too high. The ideal voltage setting for a regulator may be defined as that setting which will keep the battery at or near full charge with a minimum use of water.

The correct voltage setting at 26.7°C when using Rolls Marine Batteries is between 2.33 VPC and 2.36 VPC or 14 to 14.2 volts on a 12 volt system. Batteries that are not being exercised regularly the voltage setting should be reduced to approximately 2.17 VPC or 13.02 volts on a 12 volt system. This is common in a standby system or batteries left on charge over the winter months with the charger supplying the vessel's requirements.

Some boat operators will charge at a higher voltage for a short period of time. This is referred to as an equalize charge. This is an acceptable procedure for batteries that are being cycled daily but certainly not necessary will reduce the life of the battery.

In most instances the best trade off for maximum battery life and the absence of problems in marine applications is a multi-bank, dedicated correct fixed voltage system. This means heavy duty alternators designed for marine application and voltage regulation that does not drop below the desired level when the batteries approach full charge.

If charging voltage of 14-14.2 cannot be maintained the recharge time will increase. Voltage regulators with pronounced temperature voltage compensation curves as used in many automotive type systems are not usually suitable for recharging batteries in deep cycle applications. The alternator output voltage may fall below the necessary minimum long before the battery reaches full charge.

Multi-battery isolators are commonly used in charging systems. Beware of attempts to install isolators in systems using unmodified, internally mounted and internally sensed voltage regulators.

The charging voltage will be too low which will lead to battery sulfation and possible battery failure.

Avoid paralleling or placing batteries in series that are of a different brand, size and age. Premature battery failures will result.

A serviceable battery's ability to resist charging increases as it approaches full charge and decreases as it becomes discharged. This is due to the battery's higher counter voltage or resistance as it approaches full charge. As a battery becomes more nearly charged a higher voltage would be required to maintain the same charging current. On a properly regulated charging system the charging current approaches zero as the battery approaches a fully charged condition. A battery with a defective cell would have lower counter voltage and thus the charging current would be at a higher than normal level. Of course the good cells would soon fail due to overcharging and also consume more water than normal.

Do not completely discharge a deep cycle battery if it can be avoided. The deeper the discharge the less life you will obtain from the battery. The ideal method of operation in to charge and discharge the batteries through the middle range of their capacity (50% - 85%). The reason for this is that the charge acceptance rate is fairly high in the middle range of the capacity. From 85% to 100% requires a small charging current over a longer period of time. This is usually undesirable in a sail boat as it would require running the engine for a long period of time. No problem if you are connected to shore power using a high quality marine charger.

At least once a month the battery system must be fully charged when operating in the middle range of the battery capacity. During each discharge a little more lead sulfate accumulates. If this lead sulfate is allowed to remain for too long a period it will become very difficult to remove. The reason is that the lead sulfate will become hard and have a high electrical resistance. This is what is normally called a sulfated battery. The lead sulfate may become so hard that normal recharging will not break it down. This is the reason many sail boat owners complain at the end of the season that their batteries will not accept a charge.

See Information Bulletin 501.

The amount of recharge a battery needs can be determined by measuring the specific gravity with a hydrometer. The chart below shows the approximate "percent of charge" at various specific gravity values at 26.7°C.

On a manual charging system the charging current at less than 75% charged can be 25% of the 20 hour rate. The rate can even be higher below 50% charged. Once the battery approaches 75% charged reduce to 10% of the 20 hour rate. At 85% reduce the rate to 3% of the 20 hour rate. At 100% the charge is discontinued. A well regulated fixed rate system will do this automatically.