Deep Cycle Batteries

By RIVER RAT - May 1, 2002
Deep Cycle Battery Every fisherman has from one to four deep cycle batteries in their boat. Some fisherman remove them from their boat at the end of the fishing season, some leave them in the boat. Some trickle charge them all winter, some charge them once in a while. Is there a right way or a wrong way to maintain your deep cycle battery ? I hope to answer this question, and give you some information that you will find very useful. Unless you are quite wealthy, you will want to take care of your batteries, and they will take care of you for a long time.. Average battery life has become shorter as energy requirements increase. Life span depends on usage; 6 months to 48 months, yet only 30% of all batteries actually reach the predicted life span . Proper maintence, and understanding how your battery works is necessary to achieve this. First lets see just how a Deep cycle battery works. The Lead acid battery is made up of plates, lead and lead oxide (various other elements are used to change density, hardness, porosity, etc.) with a 35% sulfuric acid and 65% water solution. This solution is called electrolyte which causes a chemical reaction that produce electrons. When you test a battery with a hydrometer you are measuring the amount of sulfuric acid in the electrolyte. If your reading is low, that means the chemistry that makes electrons is lacking. So where did the sulfur go? It is stuck to the battery plates and when you recharge the battery the sulfur returns to the electrolyte. Deep cycle batteries have thicker plate design and can survive over a hundred discharge cycles.

There are six simple steps in testing a deep cycle battery: inspect, recharge, remove surface charge, measure the state-of-charge, load test, and recharge. If you have a non-sealed battery, it is highly recommended that you use a good quality temperature compensated hydrometer; these can be purchased at an auto parts store for between $5 and $20. A hydrometer is a float type device used to determine the state-of-charge by measuring the specific gravity of the electrolyte in each cell. It is a very accurate way of determining a battery's state-of-charge and its weak or dead cells. To troubleshoot charging or electrical systems or if you have a sealed battery, you will need a digital voltmeter with 0.5% or better accuracy. A digital voltmeter can be purchased at an electronics store like Radio Shack for between $20 and $50. Analog voltmeters are not accurate enough to measure the millivolt differences of a battery's state-of-charge, or the output of the charging system. The purchase of a battery load tester is optional, these run about $ 70 at auto parts stores. If you use your electric trolling motor every day, as I do buy one. A more accurate way of testing the capacity of a lead acid battery is by using a conductance tester.

Ok, so lets visually inspect the battery for obvious problems. Check the electrolyte levels. Is the level below the top of the plates, are the cables corroded , corroded terminal clamps, dirty or wet battery top, loose hold-down clamps, loose cable terminals, or a leaking battery case? If the electrolyte levels are low in non-sealed batteries, allow the battery to cool and add distilled water to the level indicated by the battery manufacturer. If this is not indicated, use ¼ inch below the bottom of the plastic filler tube vent wells. The plates need to be covered at all times. Do not overfill the cells, especially in hot weather, because heat will cause the electrolyte to expand and overflow. Owners and operators of all types of equipment know how important it is to be able to obtain maximum efficiency. The strength and performance of the equipment's battery play a key role in determining the duration of its operating time. The ideal battery would be one that produces and maintains the high power levels needed to deliver peak performance. The two most common types of batteries used today are the maintenance-free lead calcium and the low maintenance lead antimony battery. Both batteries are ideal in certain applications, but neither battery alone can fulfill the rugged demands placed on them by equipment used today. The calcium battery uses less water than the lead antimony battery, however, it cannot be used successfully in deep discharge applications where it is prone to capacity loss. Breakdown of the grid-paste interface occurs when a calcium battery is discharged repeatedly, resulting in battery capacity losses of 40% or more. Additional maintenance expenses are incurred as a result of the battery's premature failure. The calcium battery uses less water than the lead antimony battery, however, it cannot be used successfully in deep discharge applications where it is prone to capacity loss.

Breakdown of the grid-paste interface occurs when a calcium battery is discharged repeatedly, resulting in battery capacity losses of 40% or more. Additional maintenance expenses are incurred as a result of the battery's premature failure. The lead antimony battery will withstand repeated charge/discharge cycles and will generally accept charge more readily than a calcium battery. However, the higher charge acceptance of the antimony battery causes increased water consumption and the resultant external corrosion problems associated with the sulfuric acid fumes being carried out for the battery in the evaporation process. If the lead antimony battery is not properly maintained, battery trays and cables will require regular replacement due to corrosion and boil over problems. To achieve longer battery life, the lead antimony battery requires frequent water additions to maintain proper electrolyte levels and the corrosion must be regularly removed from posts, cables, hold-downs, and battery trays. All types of equipment require the proper battery to minimize downtime and optimize profitable operating time. Battery problems cost the operator valuable time and money.

Thanks to the latest in modern technology, a product called THERMOIL has been developed to reduce, if not completely solve today's battery problems. Most folks don't know that just the gases from the battery condensing on metal parts cause most corrosion. As I stated above, Thermoil® is a unique and patented inexpensive battery additive that when added to your new or existing battery will increase battery life, increase shelf life, eliminate corrosion, greatly reduces water consumption, explosion, toxic fumes and will help to keep your battery working under any condition from -50º to 400º Fahrenheit and is Guaranteed 100%. This product is a must for all lawn mower, motorcycle, car, truck, boat, RV, bus, tractor, utility, scrubber, sweeper, golf cart, solar or any other new or used non-sealed lead acid battery. I use Thermoil in every battery I have, and believe me, it works exactly as the manufacture said it would. If you go to their web site you can order it from there, and you will be happy that you did. No more wet tops on your batteries, no more corrosion on the battery posts, and cables, this is a great product, and I know several in the RV industry that recommend it to all there customers because it really works. Next recharge the battery to 100% state-of-charge. If the battery has a difference of .03 specific gravity reading between the lowest and highest cell, then you should equalize it.

There are up to four phases of battery charging: bulk, absorption, equalization and float. The bulk stage is where the charger current is constant and the battery voltage increases. You can give the battery whatever current it will accept not to exceed 20% of the ampere-hour rating and this will not cause overheating. The absorption phase is where the charger voltage is constant and current decreases until the battery is fully charged. This normally occurs when the charging current drops off to 1% or less of the ampere-hour capacity of the battery. For example, the ending current for a 100 ampere-hour battery is 1.0 amp or less.

The optional equalizing phase is a controlled 5% overcharge, which equalizes and balances the voltage and specific gravity in each cell, the effect of increasing the charge voltage. Equalizing reverses the build-up of chemical effects like stratification, where acid concentration is greater in the bottom of the battery. It also helps remove sulfate crystals that might have built up on the plates. The frequency recommendation varies by manufacturer from once a month to once a year, from 10 to 100 deep cycles, or when a specific gravity difference between cells reaches .03 (or 30 "points"). To equalize, fully recharge the battery; next, increase the charging voltage to the manufacturer's recommendations (if you cannot find one, add 5%). Heavy gassing should start occurring (be very careful about safety precautions). Take specific gravity readings in each cell once per hour. Equalization has occurred once the specific gravity values no longer rise during the gassing stage. The optional float phase is where the charge voltage is reduced, held constant and used indefinitely to maintain a fully charged battery.

Surface charge is the uneven mixture of sulfuric acid and water within the surface of the plates as a result of charging or discharging. It will make a weak battery appear good or a good battery appear bad. You need to eliminate the surface charge by one of the following methods:

  • Allow the battery to sit for four to twelve hours to allow for the surface charge to dissipate.
  • Apply a load that is 33% of the ampere-hour capacity for five minutes and wait five to ten minutes.
  • With a battery load tester, apply a load of at least one half the battery's CCA rating for 15 seconds and wait five to ten minutes.
If the battery's electrolyte is above 110° F (43.3° C), allow it to cool. To determine the battery's state-of-charge with the battery's electrolyte temperature at 80° F (26.7° C), use the following table. The table assumes that a 1.265 specific gravity reading is a fully charged, wet, lead acid battery. For other electrolyte temperatures, use the Temperature Compensation table below to adjust the Open Circuit Voltage or Specific Gravity readings. The Open Circuit Voltage will vary for gel cell and AGM type batteries, so check the manufacturer's specifications.

Digital Voltmeter Open Circuit Voltage Approximate State-of-Charge Hydrometer Average Cell Specific Gravity Electrolyte Freeze Point
12.65 100% 1.265 -75° F
(-59.4° C)
12.45 75% 1.225 -55° F
(-48.3° C)
12.24 50% 1.190 -34° F
(-36.7° C)
12.06 25% 1.155 -16° F
(-26.7° C)
11.89 Discharged 1.120 -10° F
(-23.3° C)

[Source: BCI]

Electrolyte Temperature Fahrenheit Electrolyte Temperature Celsius Add or Subtract to Hydrometer's SG Reading Add or Subtract to Digital Voltmeter's Reading
160° 71.1° +.032 +.192
150° 65.6° +.028 +.168
140° 60.0° +.024 +.144
130° 54.4° +.020 +.120
120° 48.9° +.016 +.096
110° 43.3° +.012 +.072
100° 37.8° +.008 +.048
90° 32.2° +.004 +.024
80° 26.7° 0 0
70° 21.1° -.004 -.024
60° 15.6° -.008 -.048
50° 10° -.012 -.072
40° 4.4° -.016 -.096
30° -1.1° -.020 -.120
20° -6.7° -.024 -.144
10° -12.2° -.028 -.168
-17.8° -.032 -.192

Electrolyte temperature compensation, depending on the battery manufacturer's recommendations, will vary. If you are using a non-temperature compensated HYDROMETER, make the adjustments indicated in the table above. For example, at 30° F (-1.1° C), the specific gravity reading would be 1.245 for a 100% State-of-Charge. At 100° F (37.8° C), the specific gravity would be 1.273 for 100% State-of- Charge. This is why using a temperature compensated hydrometer is highly recommended and more accurate than other means. If you are using a DIGITAL VOLTMETER, make the adjustments indicated in the table above. For example, at 30° F (-1.1° C), the voltage reading would be 12.53 for a 100% State-of-Charge. At 100° F (37.8° C), the voltage would be 12.698 for 100% State-of-Charge.

For non-sealed batteries, check the specific gravity in each cell with a hydrometer and average the readings. For sealed batteries, measure the Open Circuit Voltage across the battery terminals with an accurate digital voltmeter. This 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 its six cells. If the built-in indicator is clear or light yellow, then the battery has a low electrolyte level and should be refilled and recharged before proceeding. If sealed, the battery is toast and 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. You should replace the battery, if one or more of the following conditions occur:

If there is a .05 (sometimes expressed as 50 "points") or more difference in the specific gravity reading between the highest and lowest cell, you have a weak or dead cell(s). 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). If you know that a battery has spilled or "bubbled over" and the electrolyte has been replaced with water, you can replace the old electrolyte with new electrolyte and go back to Step 3.2 above. Battery electrolyte is a mixture of 25% sulfuric acid and distilled water. It is cheaper to replace the electrolyte than to buy a new battery. If digital voltmeter indicates 0 volts, you have an open cell. . If the digital voltmeter indicates 10.45 to 10.65 volts, you probably have a shorted cell or a severely discharged battery. A shorted cell is caused by plates touching, sediment ("mud") build-up or " treeing" between the plates.

If the battery is fully charged or has a "good" built-in hydrometer indication, then you can test the capacity of the battery by applying a known load and measuring the time it take to discharge the battery until 20% capacity is remaining. Normally a discharge rate that will discharge a battery in 20 hours can be used. For example, if you have an 80-ampere-hour rated battery, then a load of four amps would discharge the battery in approximately 20 hours (or 16 hours down to the 20% level). New batteries can take up to 50 charge/discharge cycles before they reach their rated capacity. Depending on your application, batteries with 80% or less of their original capacity are considered to be bad.

For non-sealed batteries, check the electrolyte level. Make sure it is covering the plates, and it is not frozen before starting to recharge. Do not add distilled water if the electrolyte is covering the top of the plates because during the recharging process, it will warm and expand. After recharging has been completed, recheck the level. Reinstall the vent caps BEFORE recharging, recharge ONLY in well-ventilated areas, and wear protective eye ware. Do NOT smoke or cause sparks or flames while the battery is being recharged because batteries give off explosive gasses. If your battery is an AGM or a sealed flooded type, do NOT recharge with current ABOVE 12% of the battery's RC rating (or 20% of the ampere-hour rating). Gel cells should be charged over a 20-hour period and never over the manufacturer's recommended level or over 14.1 VDC.

Follow the battery and charger manufacturer's procedures for connecting and disconnecting cables and other steps to minimize the possibility of an explosion or incorrectly charging the battery. You should turn the charger OFF before connecting or disconnecting cables to a battery. Do not wiggle the cable clamps while the battery is recharging, because a spark might occur, and this could cause an explosion. Good ventilation or a fan is recommended to disperse the gasses created by the recharging process. If a battery becomes hot, over 110° F (43.3° C), or violent gassing or spewing of electrolyte occurs, turn the charger off temporarily or reduce the charging rate. This will also prevent "thermal runaway" that can occur with VRLA batteries.

If you are recharging gel cell batteries, a manufacturer's charging voltages can be very critical. Sometimes, you might need special recharging equipment. In most cases, standard deep cycle chargers used to recharge wet batteries cannot be used to recharge gel cell and AGM batteries because of their charging profiles; using them will shorten battery life or cause "thermal runaway". Match the charger (or charger's setting) for the battery type you are recharging or floating.

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. Timers that will cut-off the charger will help prevent overcharging the battery. For discharged batteries, the following table lists the recommended battery charging rates and times:

Reserve Capacity (RC) Rating Slow Charge (RECOMMENDED) Fast Charge
80 Minutes or less [32 ampere hours or less] 15 Hours @ 3 amps 5 Hours @ 10 amps
80 to 125 Minutes [32 to 50 ampere hours] 21 Hours @ 4 amps 7.5 Hours @ 10 amps
125 to 170 Minutes [50 to 68 ampere hours] 22 Hours @ 5 amps 10 Hours @ 10 amps
170 to 250 Minutes [68 to 100 ampere hours] 23 Hours @ 6 amps 7.5 Hours @ 20 amps
Above 250 Minutes [over 100 ampere hours] 24 Hours @ 10 amps 6 Hours @ 40 amps
[Source: BCI]

The best method is to slowly recharge the battery at 70° F (21.1° C) over a 10 to 20 hour period (C/10 to C/20) using an external constant voltage (or tapered current charger) because the acid has more time to penetrate the plates and there is less mechanical stress on the plates. C-rate is a measurement of the charge or discharge of battery overtime. It is expressed as the Capacity of the battery divided by the number of hours to recharge or discharge the battery. For example, assume that the ampere-hour capacity of the battery is 220, then it would take 11 hours to recharge or discharge the battery using a C/20 rate. A constant voltage or "automatic" charger applies regulated voltage at approximately 13.8 to 16 volts, based on the manufacturer's recommendations and temperature. A 10 amp constant voltage charger will cost between $30 and $60 at an auto parts store is suitable for most simple recharging or charging applications.

More expensive three stage microprocessor controlled chargers are available that will automatically provide bulk, absorption and float charging. A four-stage charger will provide an equalizing charge in addition to the bulk, absorption and float charging. An excellent automatic constant voltage battery charger is a 15-volt regulated power supply adjusted to the manufacturer's recommendations or, if not available, to voltage ranges below with the electrolyte at 70° F (21.1° C):

Battery Type Charging Voltage Float Voltage Equalizing Voltage
Wet Low Maintenance 14.4 13.2 15.1
Wet Maintenance Free 14.8 13.4 15.5
Sealed &VRLA 14.4 13.2 15.1
AGM 14.4 13.6 15.5
Gel Cell 14.1 13.2 N/A
Wet Deep Cycle 14.5 13.2 15.8

To compensate for electrolyte temperature, which has a negative temperature compensation coefficient, adjust the charging voltage .0028 (2.8 millivolts) to .0033 (3.3 millivolts) volts/cell/degree F. For example, if the temperature is 30° F (-1.1° C), then increase the charging voltage to 15.19 volts for a wet low maintenance battery. If 100° F (43.3° C), then decrease the charging voltage to 13.81 volts. If left unattended, cheap, unregulated trickle or manual battery chargers can overcharge your battery because they can "decompose" the water out of the electrolyte. Avoid using fast, high rate, or boost chargers on any battery that is sulfated or deeply discharged. The electrolyte should never bubble violently while recharging because high currents only create heat and excess explosive gasses.


Starting (Used as a Deep Cycle) 0 to 12 months
Marine/RV to 6 years
Golf Cart to 6 years
Gelled Deep Cycle to 8 years
AGM to 10 years
Ni-Cad to 10 years
Telecommunications (Float) to 10 years
Fork Lift to 10 years
Industrial (Traction) to 20 years
Industrial (Stationary) to 20 years
Ni-Fe to 20 years

Recharging slowly and keeping your battery well maintained are the best ways to extend the life of your battery. Recharge a deep cycle battery as soon as possible after each use to prevent sulfating. In warmer climates and during the summer, "watering" is required more often. Check the electrolyte levels and add distilled water, if required. Never add electrolyte to a battery that is not fully charged' just add distilled water and do not overfill. The plates must be covered at all times. High ambient temperatures (above 80%deg; F [26.7° C]) will shorten battery life because it increases positive grid corrosion and growth.

Shallower the average depth-of-discharge (DoD), increases the battery life. For example, a battery with an average of 50% DoD will last twice as long or more as an 80% DoD; a 20% DoD battery will last five times longer than a 50% DoD. For example, golf cart batteries will average 225 cycles at 80% DoD and increase to 750 cycles at 50% DoD. Try to avoid DoD that is less than 10% or greater than 80%. Industrial traction and stationary deep cycle batteries are designed for 80% DoD and most Marine/RV deep cycle batteries are designed for 50% DoD.

Deep Cycle Battery
Depth-of-Discharge (DoD)
[Source: Concorde]

When in storage, recharge when the state-of-charge drops to 80% to prevent lead sulfating.

Maintaining the correct state-of-charge while in storage, electrolyte levels, tightening loose hold-down clamps and terminals, and removing corrosion is normally the only preventive maintenance required for a deep cycle battery. Avoid "opportunity charging." Size the battery so that there is a minimum of one cycle per day. Never discharge below 10.5 volts.

Loss of electrolyte due to heat or overcharging. Undercharging. Old age (positive plate shedding) or "Sludging". Excessive vibration. Freezing or high temperatures. Using tap water which causes calcium sulfating. Positive grid corrosion or growth due to high temperatures. Fast recharging at rates greater than C/4.

Batteries naturally self-discharge 1% to 15% per month while in storage, and lead sulfating will start occurring when the state-of-charge drops below 80%. If left in a vehicle, disconnecting the negative cable will reduce the level of discharge by eliminating the parasitic load. Cold will slow the self-discharge process down and heat will speed it up. Use the following six simple steps to store your batteries:

  1. Physically inspect for damaged cases
  2. Remove any corrosion
  3. Clean and dry the battery tops.
  4. Fully recharge the batteries.
  5. Check the electrolyte levels and add distilled water as required, but avoid overfilling.
  6. Store in a cold dry place, but not below 32° F (0° C)
Depending on the ambient temperature and self-discharge rate, periodically test the state-of-charge using the procedure in Section 4. When the state-of-charge drops below 80%, recharge the batteries using the procedures in Section 6. An alternative would be to connect an automatic voltage regulated, solar panel or "smart trickle" charger to "float" batteries. Based on the manufacturer's recommendations, use an automatic or smart charger that has been manufactured for this purpose and battery type. You may also use a setting of 13.02 to 13.8 volts for wet batteries and 13.2 to 14.1 volts for VRLA batteries, compensated for temperature, and the correct automatic or smart charger that has been designed not to overcharge the batteries.

The following graph from Concorde demonstrates the effect of temperature on float voltage requirements.

Deep Cycle Battery
[Source: Concorde]


Storing a battery on a concrete floor will discharge them.
A hundred years ago when battery cases were made of porous materials, such as wood, storing batteries on concrete floors would accelerate their discharge. Modern battery cases, made of polypropylene or hard rubber, which are better sealed, so external leakage, causing discharge, is no longer a problem. However, the top of the battery must be clean and dry. Temperature stratification within large batteries could accelerate the internal " leakage" or self-discharge if the battery is sitting on a cold floor in a warm room or is installed in a submarine.

Driving or running an engine will fully recharge a battery.
Some of factors affecting a charging system's ability to charge a battery are: how much current from the alternator is diverted to the battery to charge it, how long the current is available and the temperature.

A battery will not explode.
Recharging a wet lead-acid battery normally produces hydrogen and oxygen gasses. While spark retarding vent caps help prevent battery explosions, they occur when jumping, connecting or disconnecting charger or battery cables, and starting the engine. While not fatal, battery explosions cause thousands of eye and burn injuries each year.

When battery explosions occur when starting an engine, here is the usual sequence of events: One or more cells had a high concentration of hydrogen gas (above 4.1%) because the vent cap was clogged or a defective valve did not release the gas. The electrolyte levels fell below the top of the plates due to high under hood or engine compartment temperatures, overcharging, or poor maintenance. A low resistive bridge or "treeing" formed between the top of the plates such that when the current started to flow, it caused an arc or spark in one of the cells. That combination of events ignites the gas, blows the battery case cover off and spatters electrolyte all over the engine compartment. The largest number of battery explosions while starting an engine occurs in hot climates.

A battery will not lose its charge sitting in storage.
Depending on the type of battery, it has natural self-discharge or internal electrochemical "leakage" at a 1% to 20% rate per month that will cause it to become sulfated and fully discharged over time. Higher temperatures accelerate this process. Batteries stored at 95° F (35° C) will self discharge twice as fast than one at 75° F (23.9° C).

Maintenance free batteries never require maintenance.
In hot climates, water in the electrolyte is "decomposed" due to the high temperatures and normal charging of a wet maintenance free battery. Water can also be lost due to excessive charging voltage or charging currents. Non-sealed batteries are recommended in hot climates so they can be refilled with distilled water when this occurs.

Pulse chargers, aspirins or additives will revive sulfated batteries.
Using pulse chargers or additives is a very controversial subject. Most battery experts agree that there is no conclusive proof that more expensive pulse charges work any better than constant voltage chargers to remove sulfating. They also agree that there is no evidence that additives or even aspirins provide any long-term benefits.

On really cold days turn your headlights on to "warm up" the battery up before starting your engine.
While there is no doubt that turning on your headlights will increase the current flow in a car battery; it also consumes valuable capacity that could be used to start the engine. Therefore, this is not recommended. For extremely cold temperatures, externally powered battery warmers, battery blankets, or engine block heaters are highly recommended. AGM and Ni-Cad batteries perform better in extremely cold temperatures than wet cell batteries.

Batteries last longer in hot climates than in cold ones.
Batteries last approximately two thirds as long in hot climates as cold ones. Heat kills batteries, especially sealed wet lead acid batteries.

Deep cycle batteries have a memory.
Lead acid deep cycle batteries do not have the so called "memory effect" that first generation Ni-Cad batteries have.

Discharging, like charging, depends on a number of factors such as: the initial state-of-charge, depth-of-discharge, age, capacity of the battery, load and temperature. For a fully charged battery at 70° F (21.1° C), the ampere-hour rating divided by the load in amps will provide the estimated life of that cycle. For example, a new, 72-ampere-hour battery with a 10-amp load should last approximately 7.2 hours. As the battery ages, the capacity is reduced. Well, now you know as much as I know about Deep Cycle Batteries. It seems every one has a different way of charging their batteries. Some do it correctly, and many DON'T, and I hope they read this article, as it will help them do it correctly to avoid spending hard earned money needlessly.

Like always, if any of you have any questions on this article, please feel free to contact me. or I'd like also to thank my friends at Crestliner Boats, Magic products,,,, Hummingbird locators, St.Croix Rods, Harriet's Kitchen Nook, in Wis.Rapids. Heckels marine, Amherst Marine. Comprop Prop, ISG Jigs, Reeds Sporting Goods, Big Fish Tackle Co.,, and others who have helped me along the way. This article may not be reproduced without my written permission. This article may not be reproduced without my written permission.

River Rat has been fishing the Petenwell Flowage for over 40 years and owns Gone Fishing Guide Service and enjoys primarily fishing for walleyes but is well educated on many other species. He is also a Field Editor for as well as other sites and is very knowledgeable on the history and fishing tactics of the Petenwell Flowage and Wisconsin River.