4.1. 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 that will meet or exceed your requirements and how much weight you can carry. Most deep cycle batteries are rated in discharge rates of 100 hours, 20 hours, or 8 hours. The higher the discharge, the lower the capacity due to the Peukert Effect and the internal resistance of the battery. Reserve Capacity (RC) is the number of minutes a fully charged battery at 80 o F (26.7 o C) is discharged at 25 amps before the voltage falls below 10.5 volts. To convert Reserve Capacity (RC) to Ampere-Hours at the 25 amp rate, multiple RC by .4167. More ampere-hours (or RC) are better in every case. Within a BCI group size, the battery with higher ampere-hours (or RC) will tend to have longer lives and weigh more because of thicker plates and more lead.

The following graph shows the effects of temperature on the capacity of a battery:


[Source: Concorde]

If more ampere-hours are required, you can connect two (or more) new and identical 12-volt batteries in parallel. You can also connect two larger new and identical six-volt batteries in series by attaching the negative terminal of the first battery to the positive terminal of the second battery. If you connect two 12-volt batteries in parallel that are identical in type, age and capacity, you can potentially double the total capacity. If you connect two that are not the same type, you will either overcharge the smaller of the two, or you will undercharge the larger of the two.

The recommended parallel and series connections are as follows:


[Source: Interstate Batteries]

When connected this way, the batteries will discharge and recharge equally. 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) or less drop between batteries.

4.2. Type

Car batteries are especially designed for high initial cranking amps (usually 200 to 400 amps for five to 15 seconds) to start a car and for shallow (10% or less) discharges. They are not designed for deep cycle discharges. Deep cycle (and marine) 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. In warm weather, starting an engine will typically consume less that 5% of a car battery's capacity. In contrast, deep cycle (or marine) batteries are used for applications that will consume between 20 and 80% of the battery's capacity.

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 car battery. For saltwater applications, AGM or gel cell batteries are highly recommended to prevent chorine 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 downloaded from or obtained by calling (800) 845-6269 or (503) 692-5360.

The two most common types of deep cycle batteries are flooded (also known as wet or liquid electrolyte) cell and valve regulated (VR). These types are divided into Marine and RV batteries. There are 50% depth-of-discharge limits and sponge lead plates batteries, and there are the more expensive Deep Cycle (traction and stationary) batteries with 80% depth-of-discharge limits, solid lead plates, and longer lives.

4.2.1. Flooded (Wet) Cell

Flooded cell deep cycle batteries are divided, like their car battery counterparts, into low maintenance (the most common) and maintenance free (or sealed), which is 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 preventive maintenance, up to 250% less water loss, faster recharging, greater overcharge resistance, reduced terminal corrosion, up to 40% more life cycles, 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. Further, if sealed, they tend to have a shorter life in hot climates because lost water cannot be replaced. Automobile industry liability lawyers prefer this type of battery because consumers are less likely to be injured. Finally, maintenance free batteries are generally more expensive than low maintenance batteries.

4.2.2. Valve Regulated

Gas-recombinant Valve Regulated Lead Acid (VRLA) batteries are generally divided into two groups, gel cell and Absorbed Glass Mat (AGM). 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) but arguably can have an overall lower total cost of ownership due to a longer lifetime and no "watering" labor costs, only IF they are properly maintained and recharged.


[Source: Hawker]

4.3. 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 in models of the same brand or from brand to brand. You can also find BCI size information online at . 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 correct 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 automotive applications, the SAE Post is the most popular, followed by GM Side and then the combination "dual" SAE Post and GM Side. "L" terminal is used on some European cars, motorcycles, lawn and garden devices, snowmobiles, and other light duty vehicles. Stud terminals are used on heavy duty and deep cycle batteries. The positive SAE Post terminal is slightly larger (by 1/16") than the negative one. Terminal locations and polarity will vary.

[Source: BCI]

Battery manufacturers or distributors will often "private label" their batteries for large chain stores. An alphabetical list in order of the largest battery manufacturers/distributors in North America and some of their brand names, trademarks and private labels maybe found at or contact Bill Darden at Ownership, branding, Web addresses and telephone numbers are subject to change. For example, on September 29, 2000, Exide purchased GNB and Johnson Controls purchased Gylling Optima. Exide is the largest battery manufacturer in the world, and Johnson Controls is the largest manufacturer in the Americas.

4.4. Freshness

Determining the "freshness" of a battery is sometimes difficult. NEVER buy a wet lead acid battery that is MORE than THREE months old because by then it has started to sulfate unless it has periodically been recharged (this is not the usual practice of many retailers) or it is "dry charged". The exceptions to this recommendation are AGM and Gel Cell batteries, which can be stored up to 12 months before the state-of-charge drops 80% or below. Please see Section 12 for more information on sulfation. Dealers will often place their older batteries in storage racks so they will sell first. The new batteries can often be found in the rear of the rack or in a storage room. The date of manufacture is stamped on the case or printed on a sticker.

Some of the manufacturer's date coding techniques are as follows:

4.4.1 Delphi (AC Delco and some Sears DieHard)

Dates are stamped on the cover near one post. The first number is the year. The second character is the month A-M skipping I. The last two digits are codes about geographic areas. Example 0BN3=2000 Feb.

[Source: Interstate Batteries]

4.4.2. Douglas

Douglas used the letters of their name to indicate the year of manufacture and the digits 1-12 for the month. D=1994 O=1995 U=1996 G=1997 L=1998 A=1999 S=2000 Example S02=2000 Feb.

4.4.3. East Penn, GNB (Champion), and Johnson Controls Inc. (Interstate and some Sears DieHard)

Usually on a sticker or hot-stamped on the side of the case. A=January, B=February, and the letter I is skipped. The number next to the letter is the year of SHIPMENT. Example B0=Feb 2000

[Source: Interstate Batteries]

4.4.4. Exide (some Sears non-Gold DieHard)

The fourth or fifth character is the month. The following numeric character is the year. A-M skipping I. Example RO8B0B=Feb. 2000.

[Source: Interstate Batteries]

4.4.5. Trojan

Stamp on post, 2 Months AFTER manufacture date.

If you cannot determine the date code then ask the dealer or contact the manufacturer. Like bread, fresher is definitely better and does matter.

4.5. Warranty

As with tire warranties, battery warranties are NOT necessarily indicative of the quality or cost over the life of the car. Most manufacturers will prorate 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 prorated warranty. The exception to this is the FREE replacement warranty period. The quality of the warranty represents the risks that the manufacturer is willing to assume. A longer free replacement warranty period is better.

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