Battery Maintenance

The key to achieving optimum performance and long battery life is to follow a regular care and maintenance program.

While reviewing our battery maintenance tips, please keep in mind that all battery systems are unique. Battery type, charger technology, equipment loads, cable size, climate, and other factors can all vary. Slight or significant, these differences will require battery maintenance to be adjusted accordingly. These are only guidelines to follow for proper battery care. Each particular system will always require a degree of customized attention.

Before Getting Started

  • Make sure you know your system voltage, battery compartment size (length, width and height) and your energy needs.

  • Determine whether you want to use a deep cycle flooded or AGM battery.

Step 1: Determine Your Battery Voltage And How Many Batteries To Use

1-1 Based on your system voltage, you must first decide which battery is needed and how many to use in order to meet your requirements. For example, you may connect a series of eight 6V batteries, six 8V batteries or four 12V batteries for a 48-volt system. The size of your battery compartment, your performance requirements and costs may limit your options.

1-2 Make sure there is enough space between batteries to allow for minor battery expansion that occurs during use and to allow proper airflow to keep battery temperature down in hot environments.

Series Connect-Parallel Connect-Series:Parallel Connect
Step 2: Choose Your Best Battery Model

2-1 When choosing your battery model, first consider your battery compartment space, as this may limit your options. Within your size restrictions you may have several battery options to choose from. For example, you can use a T-605, T-105 or T-125 in the same space, as they are the exact same physical size. The difference between these batteries is the amount of energy they offer.

2-2 Next consider your energy needs. If replacing an existing battery, use it as a reference point. If your old battery provided enough energy, it can be replaced with a similar capacity battery. If you need more energy you can size up, or if you need less energy you can size down.


If you do not know what battery to use, contact your equipment manufacturer for their recommended battery specification. Trojan Battery also offers outstanding technical support provided by full-time applications engineers to help you select your ideal batteries.

Step 3: Select Your Best Terminal

3-1 Finally determine which terminal option best meets your needs based on the type of cable connections you plan to use. Look for the terminal(s) available for the battery you have selected.


Make sure you use the proper cable size when connecting your batteries so the connections do not overheat. For information regarding correct wire sizes, you can refer to the National Electric Code, Trojan Battery User’s Guide, or contact Trojan’s technical support team in the US and Canada at 800-423-6569, extension 3045, or internationally at +1-562-236-3045.

Battery Type

Lead acid batteries are generally classified by application (what they are used for) and by construction (how they are made). Deep cycle batteries are used for various types of applications specific such as RV, golf cars, renewable energy, and marine.

There are two popular construction types: flooded batteries (wet or wet cell) and VRLA batteries (Valve Regulated Lead Acid). In flooded lead acid batteries, the electrolyte is a solution of sulfuric acid and water that can spill out if the battery is tipped over. In VRLA batteries, the electrolyte is suspended in a fiberglass-mat (AGM and AES AGM technology), allowing these batteries to be mounted in a variety of positions.

Before getting started, be sure to identify the type of battery involved. The information on this page addresses charging and maintenance for both deep cycle flooded and VRLA batteries.


There are many tools that may help in properly caring for and maintaining batteries. Below is a list of basic items that Trojan recommends for this task:

Recommended Equipment
  • Baking Soda

  • Distilled Water

  • Goggles & Gloves

  • Hydrometer

  • Post Cleaner

  • Vaseline

  • Voltmeter

  • Wrench

CAUTION: Always wear protective clothing, gloves and goggles, when handling batteries, electrolyte, and charging your battery.

Batteries should be carefully inspected on a regular basis in order to detect and correct potential problems before they can do harm. It is a great idea to start this routine when you first receive the batteries are first received.

Inspection Guidelines

1. Examine the outside appearance of the battery.

  • Look for cracks in the container.

  • The top of the battery, posts, and connections should be clean, free of dirt, fluids, and corrosion. If batteries are dirty, refer to the Cleaning section for the proper cleaning procedure.

  • Repair or replace any damaged batteries.

2. Any fluids on or around the battery may be an indication that electrolyte is spilling, leaching, or leaking out.

  • Leaking batteries must be repaired or replaced.

3. Check all battery cables and their connections.

  • Look closely for loose or damaged parts.

  • Battery cables should be intact; broken or frayed cables can be extremely hazardous.

  • Replace any cable that looks suspicious.

4. Tighten all wiring connections to the proper specification (see below). Make certain there is good contact with the terminals.

Terminal TypeDry Torque (in-lb)Dry Torque (Nm)
DWNT, ELPT, EUT, LT, UT, WNT95-10511-12
EAPT, AP50-706-8
DT, EHPT50-70 (AP Connector) 95-105 (Stud Portion)6-8 (AP Connector) 11-12 (Stud Portion)
AP (Adapter) Reliant50-70 (AP Connector) 95-105 (Stud Portion)6-7 (AP Connector) 11-12 (Stud Portion)
DT50-70 (AP Connector) 95-105 (Stud Portion)6-8 (AP Connector) 11-12 (Stud Portion)
LT (Adapter)55-70 (Nut) 85-95 (Bolt)6-8 (Nut) 10-11 (Bolt)

WARNING: Do not overtighten terminals. Doing so can result in post breakage, post meltdown, or fire.


Visual inspection alone is not sufficient to determine the overall health of the battery.

Both open-circuit voltage and specific gravity readings can give a good indication of the battery’s charge level, age, and health. Routine voltage and gravity checks will not only show the state of charge but also help spot signs of improper care, such as undercharging and over-watering, and possibly even locate a bad or weak battery. The following steps outline how to properly perform routine voltage and specific gravity testing on batteries.

I. Specific Gravity Test (Flooded batteries only)
  1. Do not add water at this time.

  2. Fill and drain the hydrometer 2 to 4 times before pulling out a sample.

  3. There should be enough sample electrolyte in the hydrometer to completely support the float.

  4. Take a reading, record it, and return the electrolyte back to the cell.

  5. To check another cell, repeat the 3 steps above.

  6. Check all cells in the battery.

  7. Replace the vent caps and wipe off any electrolyte that might have been spilled.

  8. Correct the readings to 80º F (26.6º C): • Add 0.004 to readings for every 10º F (5.6º C) above 80º F (26.6º C) • Subtract 0.004 for every 10º (5.6º C) below 80º F (26.6º C)

  9. Compare the readings.

  10. Check the state of charge using Table 1 below.

The readings should be at or above the factory specification of 1.277 +/- 0.007. If any specific gravity readings register low, then follow the steps below.

  1. Check and record voltage level(s).

  2. Put battery(s) on a complete charge.

  3. Take specific gravity readings again.

If any specific gravity readings still register low then follow the steps below.

  1. Check voltage level(s).

  2. Perform equalization charge. Refer to the Equalizing section for the proper procedure.

  3. Take specific gravity readings again.

If any specific gravity reading still registers lower than the factory specification of 1.277+/- 0.007 then one or more of the following conditions may exist:

  1. The battery is old and approaching the end of its life.

  2. The battery was left in a state of discharge too long.

  3. Electrolyte was lost due to spillage or overflow.

  4. A weak or bad cell is developing.

  5. Battery was watered excessively previous to testing.

Batteries in conditions 1 – 4 should be taken to a specialist for further evaluation or retired from service.


II. Open-Circuit Voltage Test

For accurate voltage readings, batteries must remain idle (no charging, no discharging) for at least 6 hrs, preferably 24 hrs.

  1. Disconnect all loads from the batteries.

  2. Measure the voltage using a DC voltmeter.

  3. Check the state of charge with Table 1 below.

  4. Charge the battery if it registers 0% to 70% charged.

If battery registers below the Table 1 values, the following conditions may exist:

  1. The battery was left in a state of discharge too long.

  2. The battery has a bad cell.

Batteries in these conditions should be taken to a specialist for further evaluation or retired from service.

Percentage of ChargeSpecific Gravity Corrected toOpen Circuit Voltage 6VOpen Circuit Voltage 8VOpen Circuit Voltage 12VOpen Circuit Voltage 24VOpen Circuit Voltage 36VOpen Circuit Voltage 48V



Flooded batteries need water. More importantly, watering must be done at the right time and in the right amount or the battery’s performance and longevity suffers.

Water should always be added after fully charging the battery. Prior to charging, there should be enough water to cover the plates. If the battery has been discharged (partially or fully), the water level should also be above the plates. Keeping the water at the correct level after a full charge will prevent having to worry about the water level at a different state of charge.

Depending on the local climate, charging methods, application, etc., Trojan recommends that batteries be checked once a month until you get a feel for how often your batteries are need watering.

 General Watering Instructions
  • Add water, never acid, to cells. (Distilled water recommended)


  • For fully charged standard deep cycle batteries, add water to the level of 1/8 below bottom of vent well (see diagram A below).

  • For fully charged Plus Series batteries, add water to the maximum water level indicator (see diagram B below).

  • If the batteries are discharged, only add water if the plates are exposed. Add just enough water to cover the plates, then charge the batteries. Once fully charged, add water to the proper level indicated above.

  • After watering, secure the vent caps on the batteries.

General Watering Instructions
Step-By-Step Watering Procedure
  1. Open the vent caps and look inside the fill wells.

  2. Check electrolyte level; the minimum level is at the top of the plates.

  3. If necessary add just enough water to cover the plates at this time.

  4. Put batteries on a complete charge before adding any additional water (refer to the Charging section).

  5. Once charging is completed, open the vent caps and look inside the fill wells.

  6. Add water until the electrolyte level is 1/8″ below the bottom of the fill well.

  7. A piece of rubber can be used safely as a dipstick to help determine this level.

  8. Clean, replace, and tighten all vent caps.

Important Things to Remember
  1. Do not let the plates get exposed to air. This will damage (corrode) the plates.

  2. Do not fill the water level in the filling well to the cap. This most likely will cause the battery to overflow acid, consequently losing capacity and causing a corrosive mess.

  3. Do not use water with a high mineral content. Use distilled or deionized water only.

CAUTION: The electrolyte is a solution of acid and water so skin contact should be avoided.

WARNING: Never add acid to a battery.


Batteries can attract dust, dirt, and grime. Keeping them clean will help you spot signs of trouble if they appear, and avoid problems associated with grime.

  1. Check that all vent caps are tightly in place.

  2. Clean the battery top with a cloth or brush and a solution of baking soda and water. When cleaning, do not allow any cleaning solution or other foreign matter to get inside the battery.

  3. Rinse with water and dry with a clean cloth.

  4. Clean battery terminals and the inside of cable clamps using a post and clamp cleaner. Clean terminals will have a bright metallic shine.

  5. Reconnect the clamps to the terminals and thinly coat them with an anti-corrosive spray or silicon gel.

  6. Keep the area around batteries clean and dry.


Periods of inactivity can be extremely harmful to lead acid batteries. When placing a battery into storage, follow the recommendations below to ensure that the battery remains healthy and ready for use.

NOTE: Storing, charging or operating batteries on concrete is perfectly OK.

The Most Important Things to Avoid
  1. Freezing. Avoid locations where freezing temperature are expected. Keeping a battery at a high state of charge will also prevent freezing. Freezing results in irreparable damage to a battery’s plates and container.

  2. Heat. Avoid direct exposure to heat sources, such as radiators or space heaters. Temperatures above 80° F (26.6º C) accelerate the battery’s self-discharge characteristics.

Step-By-Step Storage Procedure
  1. Completely charge the battery before storing.

  2. Store the battery in a cool, dry location, protected from the elements.

  3. During storage, monitor the specific gravity (flooded) or voltage. Batteries in storage should be given a boost charge when they show a 70% charge or less. See Table 1 in the Testing Section of the Trojan Battery User's Guide.

  4. Completely charge the battery before re-activating.

  5. For optimum performance, equalize the batteries (flooded) before putting them back into service. Refer to the Equalizing section for this procedure.


Charging batteries properly requires administering the right amount of current at the right voltage. Most charging equipment automatically regulates these values. Some chargers allow the user to set these values. Both automatic and manual equipment can present difficulties in charging. Tables 2 & 3 list most of the necessary voltage settings one might need to program a charger. In either case the original instructions for your charging equipment should also be referenced for proper charging. Here is list of helpful items to remember when charging.

  1. Become familiar with and follow the instructions issued by the charger manufacturer.

  2. Batteries should be charged after each period of use.

  3. Lead acid batteries do not develop a memory and do need not be fully discharged before recharging.

  4. Charge only in well-ventilated areas. Keep sparks or flames away from a charging battery.

  5. Verify charger voltage settings are correct (Table 2).

  6. Correct the charging voltage to compensate for temperatures above or below 77°F (25°C).  Subtract 0.0028 volt per cell for every 1°F (0.005 volt per cell for every 1°C) above 77°F (25°C) or add 0.0028 volt per cell for every 1°F (0.005 volt per cell for every 1°C) below 77°F (25°C).

  7. Check water level (see the Watering section).

  8. Tighten all vent caps before charging.

  9. Prevent overcharging the batteries. Overcharging causes excessive gassing (water breakdown), heat buildup, and battery aging.

  10. Prevent undercharging the batteries. Undercharging causes stratification which can lead to premature battery failure.

  11. Do not charge a frozen battery.

  12. Avoid charging at temperatures above 120° F (48.8° C).

Trojan Battery LineMaximum Charge Current* (% of C20)Absorption Voltage** (V/cell)Maximum Absorption Phase Time (hours)Finish Current (% of C20)Equalization Voltage (V/cell)Float Voltage (V/cell)Temperature Compensation (V/cell)
Solar Premium and Signature13%2.4541-3%2.702.25Fahrenheit: -2.8 mV x (Tbattery -77); Celsium: -5 mV x (Tbattery -25)

*If charging time is limited, contact Trojan Technical Support for assistance. **In cases where the charger has a bulk voltage setting rather than a current, use the above absorption settings.

Charger Voltage Setting12V24V36V48V
Absorption / Bulk Charge14.428.843.257.6
Float Charge13.527.040.554.0
Additional VRLA Charging Instructions
  1. Become familiar with and follow the instructions issued by the charger manufacturer.

  2. Verify charger has necessary VRLA setting.

  3. Set charger to VRLA voltage settings (Table 3).

  4. Do not overcharge VRLA batteries. Overcharging will dry out the electrolyte and damage battery.

Trojan Battery LineAGM
Maximum Charge Current* (% of C20)20%
Absorption Voltage** (V/cell)2.40
Maximum Absorption Phase Time (hours)2
Finish Current (% of C20)-
Equalization Voltage (V/cell)N/A
Float Voltage (V/cell)2.25
Temperature Compensation (V/cell)Fahrenheit: -2.8mV x (Tbattery -77) | Celsius: -5mV x (Tbattery -25)
Charger Voltage Setting12V24V36V48V
Absorption / Bulk Charge14.428.843.257.6
Float Charge13.527.040.554.0

*If charging time is limited, contact Trojan Technical support for assistance. **In cases where the charger has a bulk voltage setting rather than a current, use the above absorption settings.

Charger Selection

Most deep cycle applications have some sort of charging system already installed for battery charging (e.g. solar panels, inverter, golf car charger, alternator, etc.). However, there are still systems with deep cycle batteries where an individual charger must be selected. The following will help in making a proper selection.

There are many types of chargers available today. They are usually rated by their start rate, the rate in amperes that the charger will supply at the beginning of the charge cycle. When selecting a charger, the charge rate should be between 10% and 13% of the battery’s 20-hour AH capacity. For example, a battery with a 20-hour capacity rating of 225 AH will use a charger rated between approximately 23 and 30 amps (for multiple battery charging use the AH rating of the entire bank). Chargers with lower ratings can be used but the charging time will be increased.

Trojan recommends using a 3-stage charger. Also called “automatic”, “smart” or “IEI” chargers, which prolong battery life with their programmed charging profile. These chargers usually have three distinct charging stages: bulk, acceptance, and float.



Equalizing is an overcharge performed on flooded lead acid batteries after they have been fully charged.

It reverses the buildup of negative chemical effects like stratification, a condition where acid concentration is greater at the bottom of the battery than at the top. Equalizing also helps to remove sulfate crystals that might have built up on the plates. If left unchecked, this condition, called sulfation, will reduce the overall capacity of the battery.

Many experts recommend that batteries be equalized periodically, ranging anywhere from once a month to once or twice per year. However, Trojan only recommends equalizing when low or wide ranging specific gravity (>0.030) are detected after fully charging a battery.

Step-By-Step Equalizing
  1. Verify the battery(s) are flooded type.

  2. Remove all loads from the batteries.

  3. Connect battery charger.

  4. Set charger for the equalizing voltage

    (See Table 2 in the Charging section). 

    If your charger doesn’t have an equalization mode, you can unplug the charger and re-plug it back in. This also will conduct the equalization charge.

  5. Start charging batteries.

  6. Batteries will begin gassing and bubbling vigorously.

  7. Take specific gravity readings every hour.

  8. Equalization is complete when specific gravity values no longer rise during the gassing stage.


Discharging batteries is entirely a function of your particular application.

However, below is list of helpful items:

  1. Shallow discharges will result in a longer battery life.

  2. 50% (or less) discharges are recommended.

  3. 80% discharge is the maximum safe discharge.

  4. Do not fully discharge flooded batteries (80% or more). This will damage (or kill) the battery.

  5. Many experts recommend operating batteries only between the 50% to 85% of full charge range. A periodic equalization charge is a must when using this practice.

  6. Do not leave batteries deeply discharged for any length of time.

  7. Lead acid batteries do not develop a memory and do not need to be fully discharged before recharging.

  8. Batteries should be charged after each period of use.

  9. Batteries that charge up but cannot support a load are most likely bad and should be tested. Refer to the Testing section for proper procedure.

Tips For Maximizing Your Battery Life


Store and operate your batteries in a cool, dry place.

For every 18° F (10° C) rise above room temperature (77° F or 25° C), battery life decreases by 50%.

Charge your batteries fully after each period of use.

Allowing your batteries to sit in a low state of charge for extended periods will decrease their capacity and life.

If you store your batteries for an extended period of time, be sure to charge them fully every 3 to 6 months. Lead acid batteries will self-discharge 5% to 15% per month, depending on the temperature of the storage conditions.

Monitor battery voltage and specific gravity of the electrolyte regularly to verify full recharging. As a general rule of thumb, the total amps from your PV panels should be sized between 10% and 20% of the total amp-hours (Ah) of the battery pack.

Many charge controllers have equalization settings that you can set to help ensure the health of your batteries. Equalize your batteries at least once per month for 2 to 4 hours, longer if your batteries have been consistently undercharged.

Voltage Settings6V12V24V36V48V
Daily Charge7.414.829.644.559.3
Float Charge6.713.52740.554
Equalize Charge8.116.232.448.664.8

Water your batteries regularly. Flooded, or wet cell batteries require watering periodically. Check your batteries once a month after installation to determine the proper watering schedule. Add water after fully charging the battery and use distilled water.

Battery Maintenance FAQs

What is the shelf life of my battery?

The limiting factor of battery’s shelf life is the rate of self-discharge, which itself is temperature dependent. Valve-regulated lead acid (VRLA) batteries like AGM batteries self-discharge less than 3% per month at 77ºF (25ºC). Flooded batteries self-discharge up to 15% per month at 77ºF (25ºC). VRLA batteries should not be stored for more than 6 months at 77º F (25ºC) without being recharged. The specific gravity (SG) or voltage of flooded batteries should be monitored every 4-6 weeks and batteries should be given a boost charge when they are at 70% stat of charge (SoC). When batteries are taken out of long storage, recharging is recommended before use.

Can I use specific gravity to troubleshoot my battery?

A battery's specific gravity is an excellent tool to determine its state of charge and ability to accept a charge. Unfortunately, using SG can also lead to an erroneous conclusion if we do not specify the conditions under which the measurements were taken.

The two major sources of error in measuring SG are lack of temperature compensation and water addition. Since the specific gravities of batteries are affected by temperature, the measurements must be compensated for temperature. The best way to compensate for cold temperatures is to subtract 4 points from your reading for every 10º F (5.5ºC) degrees below 80º F (27ºC). To compensate for hot temperatures add 4 points to your reading for every 10º F (5.5ºC) degrees above 80ºF (27ºC).

Also, adding water should only be done once the batteries are fully charged as the electrolyte levels vary as a function of state of charge and are highest once the batteries are fully charged. Use distilled water only and note that adding water will lower the SG of your batteries. The SG cannot be used as a reliable source unless we account for the two sources of error mentioned above.

Is it normal for my flooded batteries to lose water?

It is completely normal for flooded batteries to lose water. They lose water because of the gassing that they go through with every charge. Gassing is an important part of the recharge process. Gassing allows the electrolyte to mix well and helps the battery get back to a fully charged state at the end of the charge process.

Unfortunately, gassing also causes water to be lost. Water should only be added once batteries are fully charged as the electrolyte levels vary as a function of state of charge and are highest once the batteries are fully charged. The correct electrolyte level is about 1/8th of an inch below the bottom of the vent well. use Only use distilled water, and when adding water to the L16RE-2V battery, ensure you add water to all three caps.

When should I water my flooded batteries?

Water is lost when flooded batteries are charged. The best time to water your batteries is always at the end of the charge cycle; however, if the plates are exposed to air, add just enough water to cover the plates before starting the charge cycle. When the charging process is complete, re-check the electrolyte and top off as needed.

How often should I water my flooded batteries?

It is best to check your new batteries regularly as this will give you a good sense of how often your application will require battery watering. WARNING: A brand new battery may have a low electrolyte level. Charge the battery first and then add water if needed. Adding water to a battery before charging may result in overflow of the electrolyte.

Can I add de-ionized water to my flooded batteries instead of distilled water?

Yes, de-ionized water can be used instead of distilled water. However, distilled water is preferred because although de-ionized water has all free ions removed, it may still contain some minerals that may be harmful to the battery. In addition, distilled water is generally more available than de-ionized water.

What is the proper electrolyte level?

Liquid levels should be 1/8 inch below the bottom of the vent well (the plastic tube that extends into the battery). The electrolyte level should not drop below the top of the plates.

What is the proper torque value for my battery connections?

See the chart below for the proper torque value.

WARNING: Do not over-tighten terminals. Doing so can result in post breakage, post meltdown, and fire.

Terminal TypeTerminal Type Torque (in/lbs)Torque (N*m)
ELPT, EHPT, EUT, LT, WNT, DWNT, UT95-10511-12
EAPT, AP50-706-8
Can a flooded battery freeze?

The only way a battery can freeze is if it is left in a state of partial or complete discharge. As the state of charge in a battery decreases, the electrolyte becomes more like water and the freezing temperature increases. The freezing temperature of the electrolyte in a fully charged battery is -92ºF (-69ºC). At a 40% state of charge, electrolyte will freeze if the temperature reaches approximately 16º F (-9ºC).

What is the life expectancy of my battery?

The life expectancy of batteries is very dynamic and depends on a number of application specific variables such as the proper sizing of the battery bank, depth of discharge, type of loads, battery maintenance regime, ambient temperature, and charging algorithm.

Can I do a partial replacement of my flooded batteries?

We do not recommend partial battery replacement of flooded battery banks. The behavior of batteries during discharging and charging varies throughout their lifespan; if all the batteries are the same age, they all will have similar responses.

The danger with replacing only one battery is that the older batteries will tend to require more charge than the newer ones, and since the new batteries are in the same circuit, they will also be overcharged. There is also the chance that the older batteries will be undercharged since the overall voltage response of all the batteries (old and new) will not be a good representation of either group. The charging system might erroneously think that ALL the batteries have reached the desired voltage and may stop the charging of the batteries prematurely.