Using Deep Cycle Batteries

Do not mix battery types, sizes or manufacturer. Failure to use the correct battery could cause batteries to short circuit and/or spark(s) that could cause electrical component failure or an explosion.

A cycling battery has a service requirement which is quite different than that of an automotive battery.

The automotive battery must deliver high cranking currents at a satisfactory voltage for a few seconds and a portion of the accessory load (10-25 amperes) for a minute or two at a time in city or heavy traffic. Therefore, the automotive battery is designed with maximum plate area and low internal resistance to provide high cranking performance. Power taken from this battery is immediately replaced by the alternator or generator. Therefore, the battery is subjected to shallow discharge cycles (2-3% of the battery capacity). These batteries operate in the 90-100% state of charge.

Cycling batteries supply all the motive power and power for the accessories for the vehicles in which they are used. The rate of discharge varies with the type of service. In electric vehicle service, the battery normally provides a nominal intermittent discharge of 75 amperes, with a typical range of 40-350 amperes. The depth of discharge varies with the length of time it is used before being recharged. Once the battery is discharged, it must be recharged to continue operating the vehicle. The batteries used in this utility vehicle usually receive “deep” discharge (60% or more of their capacity).

New cycling batteries do not have their full capacity until they have been cycled several times (usually between 20 and 50 cycles). Therefore, they can be excessively discharged early in their vehicular life, thereby shortening their service life.

Cycling batteries are designed to have good life performance in “deep” cycle service. The major cause of battery failure in “deep” cycle service is poor maintenance:

• Dirt or corrosion on battery tops or terminals.
• Water too much or too little.
• Excessive discharge (lack of charging).

A battery is a perishable item that requires periodic maintenance. With a reasonable amount of care, the life of a battery can be significantly extended.

The life of a cycling battery is determined not only by the number of cycles (a discharge and recharge) it receives, but also by the depth of each cycle. Assume a set of batteries is used four hours per day; lets call that one cycle. If they are used for eight hours, this is a much deeper discharge and is equivalent to approximately three cycles. A battery set used eight hours per day has a life span approximately one-third that of one used for four hours per day.

Electrolyte and Specific Gravity

The electrolyte in a lead-acid storage battery is a dilute sulfuric acid solution. A battery with a fully charged specific gravity of 1.250 corrected to 26.7°C (80°F) contains an electrolyte with approximately 33.9% sulfuric acid by weight or 23.5% by volume. The remainder of the electrolyte is water. Pure (concentrated) sulfuric acid has a specific gravity of 1.835.

The sulfuric acid in the electrolyte is one of the necessary ingredients for the chemical reactions taking place inside the battery. It supplies the sulfate ions (SO4) which combine with the active material in the plates. It is also the carrier for the electric current as it passes from plate to plate. When the battery terminals are connected to an external load, the sulfate combines with the active materials of the positive and negative plates forming lead sulfate (PbSO4) on both and releasing electrical energy.

Specific gravity is a unit of measurement for determining the sulfuric acid content of the electrolyte. The recommended fully charged specific gravity of most batteries today is in the range of 1.250 - 1.280 corrected to 26.7°C (80°F). The information in this manual will assume a fully charged specific gravity of 1.250 or higher.

On the specific gravity scale, water by definition is 1.000. Therefore, electrolyte with a specific gravity of 1.250 means it is 1.250 times heavier per unit volume than pure water.