Deep Cycle Battery FAQ

A word of caution. Batteries contain a sulfuric acid electrolyte which is a highly corrosive poison, that will produce gasses when recharged and explode if ignited. This will hurt you--BAD! When working with batteries, you need to have plenty of ventilation, remove jewelry, wear protective clothing and eye wear (safety glasses), and exercise caution. Whenever possible, please follow the manufacturer's instructions for testing, jumping, installing and charging. This FAQ assumes a six cell battery commonly used for 12 volt negatively grounded system in most recreational applications. For six volt batteries, divide the voltage by two.

The technical stuff is in [brackets].

11. Isolators

A. Remove the surface charge before testing and check specific gravity in each cell, (See Section 3)
B. Recharge as soon as possible after discharge. (See Section 6)
C. Size charger so that it will recharge over a 10 to 12 hour period at 13.8 to 14.6 volts, (See Section 6)
D. Buy the freshest and largest Reserve Capacity (or Ampere Hour) battery that will fit your requirements, (See Section 4)
E. Perform preventative maintenance, especially during warm weather, and (See Section 7)
F. Shallower the average discharge, the longer the battery life. (See Section 7).

Because only the rich can afford cheap batteries.....

A good quality deep cycle battery will cost between $50 and $100 and, if properly maintained, will give you at least 200 deep discharge cycles of service. The purpose of a deep cycle battery is to provide power for trolling motors, lighting and other accessories for motor home and marine applications. Dead batteries almost always occur at the most inopportune times, for example, across the lake or during bad weather.

Normally a battery "ages" as the active plate material sheds (or flakes off) due to the expansion and contraction that occurs during the discharge and recharge cycles. Heat and vibration accelerate this "aging" process. Eventually, the sediment builds up and can short the cell out. Another major cause of faulty batteries is sulfation. When batteries are stored discharged or for over six months, lead sulfate makes the plates very hard and dense and the battery less capable or unable to be recharged. When the active material in the plates can no longer sustain a discharge current and the battery "dies".

Most of the "defective" batteries returned to the manufacturer are good. This suggests that most SELLERS of new batteries do not know how or take the time to properly test or recharge batteries.

A. Visually inspect for obvious problems, for example, damaged case, corroded terminals or cables, loose hold-down clamps or cable terminals, or low electrolyte.
B. If you have just recharged your battery, then eliminate any surface charge by one of the following methods; otherwise, go to the next step:
   1. Allow the battery to sit for two to three hours,
   2. Apply a 25 amp load for three minutes and wait five minutes, or
   3. With a battery load tester, apply a 150 amp load for 10-15 seconds.
C. Use the following table, determine the battery's state-of-charge. The best way to measure the state-of-charge is to check the specific gravity in each cell with a hydrometer. A temperature compensating hydrometer will cost approximately five dollars at an auto parts store. If the battery is sealed, then the correct procedure to test it is to measure the battery's voltage with a good quality digital DC voltmeter with an accuracy of .5% or better.

Open Circuit
Battery Voltage
Average Cell
Specific Gravity
12.65+ 100% 1.265+
12.45 75% 1.225
12.24 50% 1.190
12.06 25% 1.155
11.89 0% 1.120

[If the temperature of the electrolyte is below 70 degrees F (21.1 degrees C), then add .012 volts (12 millivolts) per degree below 70 degrees F to the reading. A 100% state-of-charge for a AGM (absorbent glass mat) battery will be approximately 12.80 VDC and 12.90 VDC for a gel cell.]

Check both the specific gravity in each cell with a external hydrometer AND the battery terminal voltage with a digital voltmeter without the engine running. For sealed batteries, measuring the battery's voltage without the engine running with a digital voltmeter is the only way you can determine the state-of-charge. Some batteries have a built-in hydrometer which only measures the state-of-charge in ONE of it's six cells. If the indicator is clear or light yellow, then the battery has a low electrolyte level and should be refilled before proceeding, or if sealed, the battery should be replaced.

If the state-of-charge is BELOW 75% using either the specific gravity or voltage test or the built-in hydrometer indicates "bad" (usually dark), then the battery needs to be recharged BEFORE proceeding. Replace the battery, if one or more of the following conditions occur:

1. If there is a .050 or more difference in the specific gravity reading between the highest and lowest cell, you have a weak or dead cell(s),
2. If the battery will not recharge to a 75% or more state-of-charge level or if the built-in hydrometer still does not indicate "good" (usually green, which is 65% state-of-charge or better),
3. If digital voltmeter indicates 0 volts, you have an open cell, or
4. If the digital voltmeter indicates 10.45 to 10.65 volts, you have a shorted cell. [A shorted cell is caused by plates touching, sediment build-up or "treeing" between plates.


A. Reserve Capacity or Ampere Hour Rating

The most important consideration in buying a deep cycle battery is the Reserve Capacity (RC) or Ampere Hour (AH) rating that will meet or exceed your requirements. RC is the number of minutes a fully charged battery at 80 degrees F is discharged at 25 amps before the voltage falls below 10.5 volts. Some deep cycle batteries are rated in Ampere Hours. To convert Reserve Capacity to Ampere Hours, multiple RC by .6. For example, a battery with 120 minute RC will have approximately 72 Ampere Hours. This means that the battery should produce one amp for 72 hours of continuous use. Since shallower the average discharge increases the battery life, more RC is better in every case.

[If more RC is required, two six volt batteries can be connected in series or two (or more) 12 volt batteries can be connected in parallel. Within a BCI group size, generally the battery with larger RC will weigh more because it contains more lead.]

B. Type

Car batteries are especially designed for high initial cranking amps (usually for five seconds) to start a car and no deep cycle discharges. Deep cycle (and marine) batteries are designed for prolonged discharges at lower amperage. [The plates in car battery are more porous and thinner than in a deep cycle battery.] A deep cycle battery will typically outlast two to four car batteries used in deep cycle applications. A "dual marine" battery is a compromise between a car and deep cycle battery and is used to start small engines and to provide deep cycle discharge capability.

Using two battery setups through a diode isolator is popular in recreational vehicle (RV) applications. A car battery is used to start the engine and deep cycle battery is to supply power to the accessories. When purchasing a isolator, be sure that it matches your alternator or charging system. [The batteries are connected to a diode isolator and both are automatically recharged by the RV's charging system when engine is running. For additional information on multi-battery applications, call (800) 845-6269 or (503) 692-5360 and request a free copy of "Introduction to Batteries and Charging Systems" by Ralph Scheidler.]

The two most common types of deep cycle batteries are flooded ( a.k.a. wet or liquid electrolyte) cell and valve regulated (VR).

1. Flooded cell
Flooded cell deep cycle batteries are divided, like their car battery counterparts, into low maintenance (the most common) and maintenance free based on their plate formulation. [Low maintenance batteries have lead-antimony/calcium (dual alloy or hybrid) plates; whereas, the maintenance free batteries use lead-calcium/calcium.] The advantages of maintenance free batteries are less preventative maintenance, longer life, faster recharging, greater overcharge resistance, reduced terminal corrosion and longer shelf life, but are more prone to deep discharge (dead battery) failures due to increased shedding of active plate material and more expensive.

2. Valve Regulated
Valve Regulated (VR) batteries are divided into two groups, gel cell and Absorbent Glass Mat (AGM). VR batteries are spill proof, so they can be used in closed 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 could have an overall lower cost due to a longer lifetime.

C. Size

An internationally adopted Battery Council International (BCI) group number (24, 27, 31, etc.) is based on the physical case size, terminal placement and terminal polarity. Within a group, the RC ratings, warranty and battery type will vary in models of the same brand or from brand to brand. Generally, batteries are sold by model, so some of the group numbers will vary for the same price. This means that for the same price you can potentially buy a physically larger battery with more RC than the battery you are replacing. Be sure that the replacement battery will fit, the cables will correct to the correct terminal, and that the terminals will not touch anything else.

The battery manufacturers publish application guides that will contain the BCI group number replacement recommendations, and battery size, and RC specifications. Manufacturers might not build or the store might not carry all the group numbers.

Battery manufacturers or distributors will often "private label" their batteries for large chain stores. A list of the largest domestic battery manufacturers/distributors in North America and my understanding of some of their brand names, trademarks and private labels can be found in Car Battery FAQ

D. Freshness

Determining the "freshness" of a battery is sometimes difficult. Never buy a flooded battery that is more than six months old. Stamped on the case or printed on a sticker is the date of manufacture. It is usually a combination of alpha and numeric characters with letters for the months starting with "A" for January (generally skipping the letter "I") and digit for the year, for example, "F5" for June, 1995. Like bread, fresher is definitely better.

E. Warranty As with tire warranties, battery warranties are not necessarily indicative of the quality or cost over the life of the application. Manufacturers will prorate warranties based on the list price, so if a battery failed half way or more through its warranty period, buying a new replacement might cost you less. The exception is the free replacement warranty period. This represents the risk that the manufacturer is willing to assume. A longer free replacement warranty period is better.


A. Thoroughly wash and clean the old battery, battery terminals and case or tray with water to minimize problems from acid or corrosion. Heavy corrosion can be neutralized with a mixture of baking soda and water. Also, mark the cables so you do not forget which one it is which when you reconnect.

B. Remove the NEGATIVE cable first because this will minimize the possibility of shorting the battery when you remove the other cable. Next remove the POSITIVE cable and then the hold-down bracket or clamp. If the hold down bracket is severely corroded, replace it. Dispose the old battery by exchanging it when you buy your new one or by taking it to a recycling center. Please remember that batteries contain large amounts of harmful lead and acid.

C. After removing the old battery, be sure that the battery tray and cable terminals or connectors are clean. Auto parts stores sell a cheap wire brush that will allow you to clean the inside of a terminal clamps and the terminals. If the terminals, cables or hold down brackets are severely corroded, replace them. Corroded terminals or cables will significantly reduce starting capability.

D. Thinly coat the terminal and terminal clamps with a high temperature grease or petroleum jelly (Vaseline) to prevent corrosion.

E. Place the replacement battery so that the NEGATIVE cable will connect to the NEGATIVE terminal. Reversing the polarity of the electrical system will severely damage or DESTROY it.

F. After replacing the hold-down bracket, reconnect the cables in reverse order, i.e., attach the POSITIVE cable first and then the NEGATIVE cable last.

G. Before using the battery, check the electrolyte levels and state-of-charge. Refill or recharge as required.


In addition to the earlier cautions, some more words of caution:

A. NEVER, NEVER disconnect a battery cable from vehicle with the engine running because the battery acts like a filter for the electrical system. Unfiltered [pulsating DC] electricity can damage expensive electrical components, e.g., radio, charging system, etc.

B. Check the electrolyte level and be sure it is not frozen BEFORE recharging.

C. Do NOT add water if the electrolyte is covering the top of the plates because during the recharging process, it will warm up and expand. After recharging has been completed, RECHECK the level.

D. Reinstall the vent caps BEFORE recharging and recharge ONLY in well ventilated areas. NO smoking, sparks or open flames because while the battery is being recharged because they give off explosive gasses.

E. If your battery is a VR type or a sealed flooded type, do NOT recharge with current ABOVE 12% of the battery's RC rating (or the 20% of the amp hour rating).

F. Follow the charger manufacturer's procedures for connecting and disconnecting cables and operation to minimize the possibility of an explosion, but generally you should turn the charger OFF before connecting or disconnecting cables to a battery.

G. If a battery becomes hot, or if violent gassing or spewing of electrolyte occurs, turn the charger off temporarily or reduce the charging rate.

H. Insure that in car charging with an external charger will not damage the electrical system with high voltages. If this is even a remote possibility, then disconnect the car's negative battery cable from the battery BEFORE connecting the charger.

I. If you are recharging VR (AMG or gel cell) batteries, manufacturer's charging voltages can be very critical and you might need special recharging equipment. In most cases standard car or flooded deep cycle battery chargers can not be used.

Use an external constant current charger which is set not to deliver more than 12% of the RC rating of the battery and monitor the state-of-charge. For fully discharged batteries, the following table, published by BCI, lists the recommended battery charging rates and times:

Reserve Capacity
(RC) Rating
Slow Charge Fast Charge
80 Minutes or less 15 Hours @ 3 amps 5 Hours @ 10 amps
80 to 125 Minutes 21 Hours @ 4 amps 7.5 Hours @ 10 amps
125 to 170 Minutes 22 Hours @ 5 amps 10 Hours @ 10 amps
170 to 250 Minutes 23 Hours @ 6 amps 7.5 Hours @ 20 amps
Above 250 Minutes 24 Hours @ 10 amps 6 Hours @ 40 amps

The BEST method is to SLOWLY recharge it using an external constant voltage (or tapered current charger) because the electrolyte has more time to penetrate the plates. A constant voltage "automatic" charger applies regulated voltage at approximately 14.4 volts. A 10 amp automatic charger will cost between $30 and $60 U.S. at an auto parts store.

[An excellent automatic constant voltage battery charger is a 15 volt regulated power supply adjusted to 14.4 volts at 80 degrees F (26.7 degrees C). If 32 degrees F (0 degrees C), then increase the charging voltage to 15.3 volts. When charging a maintenance free battery, add .2 volts. ]

If left unattended, a cheap, unregulated trickle battery chargers can overcharge your battery because they can "boil off" the electrolyte. Do NOT use fast, high rate, or boost chargers on any battery that is sulfated or deeply discharged. This condition requires a constant current from one to two amps for 60 to 120 hours. The electrolyte should NEVER bubble violently while recharging because high currents only create heat and excess explosive gasses. 7. CAN I INCREASE THE LIFE OF MY BATTERY?

Recharging slowly and keeping your battery well maintained are the best ways to extend the life of your battery. For cold climates, keeping the battery fully charged will help. In the warmer climates and during the summer, check the electrolyte levels more frequently and add distilled water, if required. Never add acid--just distilled water and do not overfill. The shallower the average discharge, the longer the battery life.

Recharge a deep cycle battery as soon as possible after each use. Maintaining the correct electrolyte levels, tightening loose hold-down clamps and terminals, and removing corrosion is normally the only preventative maintenance required for a battery. 8. WHAT ARE THE MOST COMMON CAUSES OF BATTERY FAILURES?

A. Loss of electrolyte due to heat or overcharging,
B. Sulfation in storage,
C. Undercharging with voltages less than 13.8 volts,
D. Old age,
E. Vibration,
F. Freezing,
G. Using tap water,
H. Corrosion.


A. Storing a battery on a concrete floor will discharge them.
Modern lead acid battery cases are better sealed, so external leakage causing discharge is no longer a problem. [Temperature stratification within large batteries can accelerate the internal "leakage" or self discharge if the battery is sitting on an extremely cold floor in a warm room or installed in a submarine.]

B. Driving a vehicle will fully recharge a battery.
There are a number of factors affecting alternator's ability to charge a battery. The greatest factors are how much current from the alternator is diverted to the battery to charge it, how long the current is available and temperature. Generally, running the engine at idle, short "stop-and-go trips", or during bad weather at night will not recharge the battery.

C. A battery will not explode.
While recharging, a battery produces hydrogen and oxygen gasses. If a spark occurs, an explosion can occur. Remember the "Hindenburg"?

D. A battery will not lose its charge sitting in storage.
A battery has internal electrical leakage that will cause it to become fully discharged and sulfated over time. Prior to storing a battery, it should be fully charged and recharged when it reaches 80% state-of-charge or six months, whichever occurs first.

E. How long will a deep cycle battery last?
Discharging, like charging, depends on a number of factors. The most important ones are the initial state-of-charge, capacity of the battery, load and temperature. For a fully charged battery at 80 degrees F, the ampere hour rating divided by the load in amps will provide the estimated life of that cycle. For example, a 72 ampere hour battery with a 10 amp load should last approximately 7.2 hours.


Additional information sources about deep cycle batteries can be found in the links page.

Copies of the Deep Cycle Battery FAQ
are also available by requesting them by e-mail from Bill Darden