Ampere-Hour (or Reserve Capacity) Rating

The most important consideration in buying a deep cycle battery is the Ampere-Hour (Ah) or Reserve Capacity (or Reserve Minutes) rating, sufficient to meet or exceed the requirements of your application and how much weight you can carry. Most deep cycle batteries are rated in number of hours it takes to discharge a fully charged battery to 10.5 volts. Discharge rates of 100 hours, 20 hours, or 8 hours are common ratings. The higher the discharge rate, the lower the capacity due to the Peukert Effect and the internal resistance of the battery. (Section 11.). RC (Reserve Capacity) is the number of minutes a fully charged battery at 80°F (26.7°C) is discharged at 25 amps before the voltage falls below 10.5 volts. To convert Reserve Capacity (RC) and Ampere-Hours, check the battery manufacturer's specifications. Generally, the battery with higher ampere-hours (or RC) will tend to have a longer life and weigh more because of thicker plates and more lead. The best buy will probably be the battery capacity that best suits your application and has the lowest cost per life cycle.

If more ampere-hours are required, two new and identical six-volt batteries can be connected in series (positive terminal of Battery One to the negative terminal of Battery Two). Two (or more) new and identical 12-volt batteries can be connected in parallel. If you connect two 12-volt batteries in parallel and they are identical in type, age and capacity, you can potentially double you original capacity. If you connect two that are not the same type or capacity, you will either over charge the smaller of the two, or you will under charge the larger of the two.

When connecting in series or parallel and to prevent recharging problems, do not mix old and new batteries or ones of different types. Cable lengths should be kept short and cable must be sized large enough to prevent significant voltage drop; there should be a maximum of 0.2 volts (200 millivolts) drop between batteries.


Car batteries are especially designed for high initial cranking amps (usually 200 to 400 amps for a few seconds) to start a car and for shallow (3% or less) discharges. They are not designed for deep cycle discharges. Deep cycle batteries are designed for prolonged discharges at lower current and not for high current discharges. The plates in a car battery are more porous and thinner than in deep cycle batteries and use sponges or expanded metal grids instead of solid lead. A deep cycle battery will typically outlast two to ten car batteries when used in deep cycle applications. Deep cycle batteries are used for applications that will consume between 20% and 80% of the battery's capacity.

Deep cycle batteries are broadly divided into cycle and float applications. Cycle applications are where the battery is discharged in applications that will consume between 20% and 80% of the battery's capacity before being recharged. Examples of cycling applications are RV, marine, wheelchair, golf cart and folk lift trucks. Float applications are where the battery is used to provide back-up power during loss of the primary source of power such as uninterruptible power supplies (UPS), emergency lighting, telecommunications systems, etc. Generally, cycling batteries have shorter lives, fewer cycles and cost less than float batteries. The chargers for cycling and float batteries are different as well.

A "dual" or starting marine battery is a compromise between a car and a deep cycle battery that is specially designed for marine applications. A deep cycle or "dual marine" battery will work as a starting battery if it can produce enough current to start the engine, but not as well as a starting battery. For saltwater applications, sealed AGM or gel cell batteries should be used to prevent the formation of DEADLY chlorine gas.

For RVs, a car battery is normally used to start the engine and a deep cycle battery is used to power the RV accessories. The batteries are connected to a diode isolator. When the RV's charging system is running, both batteries are automatically recharged. An excellent and easy to understand free booklet on multi-battery applications, "Introduction to Batteries and Charging Systems", can be down loaded from http://http://www.surepower.com/pdfs/intro%20book.pdf.

The two most common types of deep cycle batteries for cycle applications are flooded (also known as wet or liquid electrolyte) cell and valve regulated (VR). There are 50% average Depth-of-Discharge limits and sponge lead plate batteries. The more expensive deep cycle (stationary) batteries used for float applications typically have an 80% average Depth-of-Discharge limitation, solid lead plates, and longer lives.

Flooded (Wet) Cell

Flooded cell deep cycle batteries are divided, like their car battery counterparts, into low maintenance (the more common) and maintenance free (or sealed), based on their plate formulation. Low maintenance batteries have lead-antimony/antimony or lead-antimony/calcium (dual alloy or hybrid) plates; the maintenance free batteries use lead-calcium/calcium. The advantages of maintenance free batteries are less preventative maintenance, up to 250% less water loss, faster recharging, greater over charge resistance, reduced terminal corrosion, and up to 200% less self discharge. However, they are more prone to deep discharge (dead battery) failures due to increased shedding of active plate material and development of a barrier layer between the active plate material and the grid metal. Finally, maintenance free batteries are generally more expensive than low maintenance batteries.

Valve Regulated Lead-Acid

Gas-recombinant VRLA (Valve Regulated Lead-Acid) or SLA (Sealed Lead-Acid) batteries are generally divided into two groups, gel cell and AGM (Absorbed Glass Mat). VRLA batteries are spill proof, so they can be used in semi-enclosed areas, are totally maintenance free, and have a longer shelf life. Their greatest disadvantage is the high initial cost (two to three times that of wet low maintenance) but arguably can have an overall lower total cost of ownership due to a longer lifetime and no "watering" labor costs.

Size and Terminals

In North America, a Battery Council International (BCI) group number (e.g., U1,24, 27, 31, 8D, etc.) is based on the physical case size, terminal placement and terminal polarity. In Europe, the EN, IKC, Italian CEI, and German DIN standards are used and in Asia, the Japanese JIS standard is used. Within a group, the ampere-hour or RC ratings, warranty and battery type will vary between models of the same brand or from brand to brand. You can also find BCI size information online at http://www.exidebatteries.com/bci.cfm. Generally, batteries are sold by model, and some of the group numbers are sold for the same price. This means that for the same money you can potentially buy a physically larger battery with more ampere-hour or RC than the battery you are replacing. Be sure that the replacement battery will fit, the cables will connect to the correct terminals, and that the terminals will not touch anything else.

There are six types of battery terminals: SAE Post, GM Side, "L", Stud, combination SAE and Stud, and combination SAE Post and GM Side. For deep cycle applications, the stud terminals are the most popular. Terminal locations and polarity will vary.


Determining the "freshness" of a battery is sometimes difficult. Never buy a non-sealed wet lead-acid battery that is more than three months old or a sealed wet lead acid battery that is more than six months old. This is because it has started to sulphate, unless it has periodically been recharged or it is "dry charged". The exceptions to this rule are AGM and gel cell batteries that can be stored up to 12 months before the State-of-Charge drops below 80%. Please see Section 12. for more information on sulphation.


As with tyre warranties, battery warranties are not necessarily indicative of the quality or cost over the life of the battery. Some dealers will pro-rate warranties based on the list price of the bad battery, so if a battery failed half way or more through its warranty period, buying a new battery outright might cost you less than paying the difference under a pro-rated warranty. The exception to this is the free replacement warranty and represents the risk that the manufacturer is willing to assume. A longer free replacement warranty period is better.