CHARGING GUIDELINES FOR GLOBAL/DAVIDSON TUBULAR PLATE BATTERIES

Your Global/Davidson battery set has been specially formulated for long cycle life and has unique voltage and specific gravity characteristics. The battery charging set points specified in this guide are for these batteries only, and should not be assumed appropriate for other batteries.

It is far easier to discharge a battery than to recharge it. Both the charge and discharge of batteries involve significant inefficiencies. As a rule of thumb for each amp hour of power taken out of a battery 1.5 amp hours must be replaced. In order to complete the chemical process that recharges a battery the electrolyte must reach 104° F. The method used to "heat up" the electrolyte is the application of charging current (amps) to a discharged battery. If insufficient charge is applied to bring the electrolyte to this temperature the batteries will never fully recharge, and over time, their capacity will be reduced.

The minimum amount of initial charging current, which must be applied to recharge your battery, is 10% of the rated capacity of the battery. For example an 850 amp/hour battery (85T-17 @ 20 hour rate¹) will require an initial start up rate of 85 amps to be supplied by the selected battery charger. A combination of charging sources such as generator/PV array can be used to achieve the initial charge rate but it is imperative that sufficient current is available to fully recharge the battery.

In addition to minimum charge rates there are charging voltage settings that are specific to your Global/Davidson battery set.

Bulk Charge Voltage is the voltage at which the batteries will receive most of the charging. When the battery has reached its bulk voltage level the charge current will slowly decrease as batteries become more charged. As a battery becomes more charged it presents more resistance to the flow of current than when discharged, hence the rate of amps declines during a charging cycle. This part of the charging cycle is called the "absorption" stage. We recommend 30 minutes of absorption time per 100 amp hours of battery bank rated capacity (72 hour rate).

All batteries will self-discharge over time, even if they are not used. In stand-by applications where utility power is available batteries are often finish charged at float voltage. This is the voltage level that can be applied to the batteries to maintain their charged state and, at the same time, minimize loss of electrolyte. In generator cycle charging applications it is rare to "float" the batteries as it represents a costly and inefficient use of fuel.

Regular "equalization" charging is required as part of a battery maintenance routine. An equalization charge is an intentional overcharge that causes rapid bubbling of the electrolyte, which in turn helps, dislodge sulfation that occurs naturally during normal charge/discharge cycling. Equalization charge settings involve both voltage and time specifications.

Table 1 shows the appropriate settings for Bulk, Absorption, Float and Equalization stages of charging. Many programmable chargers and inverter/chargers have provisions for custom voltage, current, and time settings. Your Global/Davidson batteries will perform best if charged at the settings shown in Table 1.²

Table 1: Recommended Voltage and Current settings for recharging Global/Davidson Batteries

The specific gravity of your Global/Davidson battery is somewhat different from other brands of battery. They are actually fully charged at a lower specific gravity than most batteries and as a result they will actually be charged before most hydrometers will indicate full charge. The most accurate way to use a hydrometer to measure state of charge is to take note of when the specific gravity stops rising. Take readings every 1/2 hour during the last part of Absorption Stage, when the hydrometer does not rise from your last reading the batteries are fully charged.

The correct fully charged specific gravity of your Global/Davidson battery is:
1.265 - 1.275 (will vary dependent on battery age, condition and ambient temperature)

Another result of the special formulation and design of these long lasting deep cycle batteries is that they tend to operate at lower voltages than most higher specific gravity batteries, thus the safe operating voltage range of Global/Davidson batteries is lower than most other batteries.

It is quite normal and safe to operate your batteries as low as 2 Vdc per cell⁵. They should always be recharged when they reach a standing voltage of less than 1.975 per cell⁶. Table 2 shows the conversion of Volts per cell settings to 12, 24 and 48 Vdc values.

Table 2: Converting Volts per cell to battery bank voltage.

¹The "@ X hour" rate refers to the length of time it would take to discharge the battery. Most batteries are rated at the 6, 20 and 72 hour rates. Back to source.

²Charger should be temperature compensated and adjust all voltage values accordingly. Back to source.

³This is a rule of thumb estimate of time, confirm with hydrometer (see recommended specific gravity). Back to source.

⁴Float charging is usually only done when utility power is available. Please note that if batteries are continuously held at a float charge they will loose capacity, regular charge/discharge and equalization cycles are recommended for Global/Davidson batteries. Back to source.

⁵Battery voltage will drop significantly below this point during application of large loads but should recover quickly if batteries are in good condition. Back to source.

⁶In this reference standing voltage is defined as the voltage of a battery after it has neither been charged nor discharged for an eight-hour period. The most practical time to assess battery state of charge is early morning as nighttime current draw is usually minimal. Back to source.