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Charging Deep Cycle Batteries

Charging, Equalizing & Storing Deep Cycle Batteries

Proper battery charging and equalizing will help ensure the longest life possible from your deep cycle batteries. The simple steps outlined below will make charging your deep cycle batteries a breeze

  • Initial Charging

    Due to the potential for self-discharge during shipping and storage, batteries should be given a full charge prior to first use. See the next section for details on proper charging.

  • Normal Charging

  • Proper charging is imperative to maximize battery performance. Both under- or over-charging batteries can significantly reduce the life of the battery. Most chargers are automatic and pre-programmed, while others are manual and allow the user to set the voltage and current values.

    • Charge only in well-ventilated areas.
    • Batteries should be fully charged after each use. “Use” is defined as 30 minutes or more of runtime.
    • Before charging, make sure the charger is set to the appropriate program for deep cycle flooded/wet, AGM, or Gel batteries.
    • Charging time will vary depending on battery size, charger output, and depth of discharge.
    • Lead-acid batteries donot have a memory effect and should not be fully discharged prior to charging.
    • Temperature compensated charging is desirable for all batteries, but should always be used with AGM and Gel batteries. Temperature compensation coefficients raise the voltage/cell for temperatures below 77°F (25°C), and lower the voltage/cell for temperatures above 77°F (25°C).
    • Never charge a frozen battery.
    • Avoid charging at temperatures above 122°F (50°C).
  • Charging Flooded Batteries

    • Prior to charging, check electrolyte levels to verify that the plates are covered with electrolyte. If plates are exposed, add enough water (distilled or deionized) to just cover the plates prior to charging.
    • Make sure that vent caps are in place and are properly secured.
    • Trojan recommends a 3-phase I-V-I profile for charging its flooded batteries
      • Phase 1: Constant current bulk charge
        A constant current equal to 10-13% of C20 is applied as the voltage slowly increases. The bulk phase ends when the voltage rises to the absorption voltage.
      • Phase 2: Constant voltage absorption charge
        A constant voltage equal to 2.35-2.45 V/cell is applied as the current slowly declines. The absorption phase ends when the current falls to the finish current.
      • Phase 3: Constant current finish charge
        A constant current equal to 1-3% of C20 is applied as the voltage increases. The finish phase ends when the battery is fully charged. Flooded batteries will gas (bubble) toward the end of the finish phase to ensure proper mixing of electrolyte.
      • Refer to Table 5 and Figure 6 for system charging parameters and typical voltage and current profiles.
      • In applications where batteries are infrequently used, it is desirable to compensate for self-discharge by keeping a low charging voltage on the batteries. This is called the float voltage. Avoid using continuous float charging on flooded batteries that are charged more than once/week. When float charging is used, it is started immediately after the end of the finish stage.
    • Following charge completion, add water if required as directed in the section "Watering" on Deep Cycle Battery Maintenance
    Flooded/Wet Batteries - Normal Charging Parameters @ 25°C (77°F) - Table 5
    Trojan Battery Line Maximum 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 Signature 13% 2.45 4 1-3% 2.7 2.25 Fahrenheit:
    -2.8 mV x
    (Tbattery-77)
    Celsius:
    -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

  • Charging Valve-Regulated Lead Acid (VRLA) Batteries (known as AGM and Gel)

  • There are various ways to charge VRLA batteries. For optimum performance and life, Trojan Battery recommends the following:

    • Use a 2-phase I-V profile
      • Phase 1: Constant current bulk charge
        A constant current equal to a percentage of C20 is applied as the voltage slowly increases.
        The recommended currents are as follows:
        • AGM: 20% of C20
        • Gel: 10-13% of C20
        The bulk phase ends when the voltage rises to the absorption voltage.
      • Phase 2: Constant voltage absorption charge
        A constant voltage equal to 2.35-2.45 V/cell is applied as the current slowly declines. The absorption phase ends when the current stabilizes at a low value of approximately 0.005 x C20.
    • Refer to Table 6 as well as Figure 7 and Figure 8 for system charging parameters and typical voltage and current profiles.
    • In applications where batteries are infrequently used, it is desirable to compensate for self-discharge by keeping a low charging voltage on the batteries. This is called the float voltage. Avoid using continuous float charging on VRLA batteries that are charged more than once/month. When float charging is used, it is started immediately after the end of the finish stage.
    • VRLA charging voltages should always be temperature compensated per Table 6.
    Valve Regulated Batteries - Normal Charging Parameters @ 25°C (77°F) - Table 6
    Trojan Battery Line Maximum 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)
    AGM 20% 2.4 2 - N/A 2.25 Fahrenheit:
    -2.8 mV x
    (Tbattery-77)
    Celsius:
    -5 mV x
    (Tbattery-25)
    GEL 13%

    *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

  • Equalizing (flooded/wet batteries only)

  • When batteries are used in a battery bank, over time some of the batteries can drift to a lower state of charge than others. This charge imbalance can lead to sulfation and premature battery failure. In order to ensure that these charge imbalances are corrected, a process called “ equalization” is used. Equalizing is an overcharge performed after fully charging deep-cycle flooded/wet batteries. An equalizing charge prevents electrolyte stratification and reduces sulfation, which are leading causes of battery failure. Trojan recommends equalizing for 2-4 hours in the following situations:

    • Periodically (every 30 days)
    • When batteries have a low specific gravity after charging (<1.235), or
    • When the range of specific gravities between cells is > 0.030 points
    • Reference Section 9.3 for instructions on specific gravity measurement.

    WARNING! Deep-cycle AGM or Gel batteries should NEVER be equalized.

    Equalization can be performed either automatically (as programmed on the charger) or by following the procedure below:

    • Check the battery’s electrolyte level in each cell to make sure the plates are covered before charging.
    • Check that all vent caps are secured properly on the battery before charging.
    • Set charger to equalizing mode.
    • The batteries will gas (bubble) during the equalization process.
    • Measure the specific gravity every hour.
    • Discontinue the equalization charge when the specific gravity no longer rises.
  • Storage of Deep Cycle Batteries

  • The following tips will help ensure that your batteries emerge from storage in good condition:

    • Charge batteries before placing them in storage.
    • Store in a cool and dry location, protected from the elements.
    • Disconnect from equipment to eliminate potential parasitic loads that may discharge the battery.
    • Batteries gradually self-discharge during transit and storage, so monitor the specific gravity or open-circuit voltage of flooded batteries every 4 - 6 weeks. Monitor the open circuit voltage for AGM or Gel batteries every 2 - 3 months.
    • Batteries in storage should be charged when they decline to the following state of charge (SOC):
      • Flooded deep cycle batteries: 70% SOC
      • AGM/Gel deep cycle batteries: 75% SOC
    • Refer to Table 7 for the relationship between SOC, specific gravity (flooded only) and open-circuit voltage. If charging is needed, follow the normal charging procedure outlined in Section 5.2.
    • When batteries are taken out of storage they should be given an initial charge as outlined in Section 5 prior to use.
    • Storing Deep Cycle Batteries in Hot Environments

    • Storage in hot environments (greater than 90°F or 32°C) can negatively impact batteries. Avoid direct exposure to heat sources, if possible, during storage. Batteries self-discharge faster at high temperatures. If batteries are stored during hot summer months, monitor State-of-Charge on a regular basis as follows:

      • Flooded batteries: check specific gravity or voltage every 2 - 4 weeks.
      • AGM or Gel batteries: check voltage every 1 - 2 months.
    • Storing Deep Cycle Batteries in Cold Environments

    • If possible, avoid locations where freezing temperatures are expected during storage. Batteries can freeze in cold temperatures (less than 32°F or 0°C) if they are not fully charged. If batteries are stored during cold winter months, it is critical that they be kept at a high state of charge as outlined above.

    Freezing Point of Electrolyte
    Specific Gravity Temperature
    °C °F
    1.28 -68.9 -92
    1.265 -57.4 -71.3
    1.25 -52.2 -62
    1.2 -26.7 -16
    1.15 -15 -5
    1.1 -7.2 19

    State of Charge (SOC) as a Function of Specific Gravity & Open-circuit Voltage - Tabel 7
    FLOODED/WET STATE OF CHARGE AS A FUNCTION OF SPECIFIC GRAVITY AND OPEN-CIRCUIT VOLTAGE
    STATE OF CHARGE (%) SPECIFIC GRAVITY CELL 6 VOLT 8 VOLT 12 VOLT
    100 1.277 2.122 6.37 8.49 12.73
    90 1.258 2.103 6.31 8.41 12.62
    80 1.238 2.083 6.25 8.33 12.5
    70 1.217 2.062 6.19 8.25 12.37
    60 1.195 2.04 6.12 8.16 12.24
    50 1.172 2.017 6.05 8.07 12.1
    40 1.148 1.993 5.98 7.97 11.96
    30 1.124 1.969 5.91 7.88 11.81
    20 1.098 1.943 5.83 7.77 11.66
    10 1.073 1.918 5.75 7.67 11.51
    AGM STATE OF CHARGE AS A FUNCTION OF OPEN-CIRCUIT VOLTAGE
    STATE OF CHARGE (%) SPECIFIC GRAVITY* CELL 6 VOLT 8 VOLT 12 VOLT
    100 1.295 2.14 6.42 8.56 12.84
    75 1.245 2.09 6.27 8.36 12.54
    50 1.195 2.04 6.12 8.16 12.24
    25 1.145 1.99 5.97 7.96 11.94
    0 1.095 1.94 5.82 7.76 11.64
    GEL STATE OF CHARGE AS A FUNCTION OF OPEN-CIRCUIT VOLTAGE
    PERCENTAGE CHARGE SPECIFIC GRAVITY* CELL 6 VOLT 8 VOLT 12 VOLT
    100 1.295 2.14 6.42 8.56 12.84
    75 1.265 2.11 6.33 8.44 12.66
    50 1.215 2.06 6.18 8.24 12.36
    25 1.155 2 6 8 12
    0 1.125 1.97 5.91 7.88 11.82

    * Although the specific gravity cannot be measured in a VRLA battery, an approximate value can be useful in determining the freezing point of electrolyte.

Disclaimer:
Information was derived from Trojan Battery and Trojan Battery User's Guides.