Constant Voltage Charger Selection

For optimum system cost and performance the battery and charger should be specified as a system. Not only does the charger capability determine the recharge time required but it also has a significant impact on the service life of the battery.

Typically, other than cost, the time required to recharge a battery is the major concern of the battery system user. The recharge time required will be a direct function of the output voltage and current capability of the charger, depth of discharge of the battery and the battery temperature. Utilizing a higher charging voltage or greater charging current capability can be correctly assumed to reduce the recharge time required to attain the desired state of charge. However, higher recharging voltage and current availability may also have a negative impact on battery service life. Other important considerations in the charger selection relate to the battery's tolerance for AC ripple voltage present on the charger DC output and the limitations of any critical load connected in parallel.

Therefore, the selection of the charger must consider the characteristics and limitations of the battery and any parallel connected load as well as the desired recharge time and charger cost.

A profile of battery current acceptance and charger output voltage during a typical recharge is illustrated in Figure 1. This profile will vary depending on the depth of the preceding discharge and the charger output characteristics. In general however, that period is known as the "bulk phase" takes the battery to approximately an 85% to 90% state of charge (SOC) while the so called "absorption phase", with declining current acceptance, takes the battery to a 95% SOC. The last 5% of capacity is restored during what is called the "float phase". Due to the very low current acceptance during this "float phase" an extensive period of time is required to attain the necessary ampere hours of recharge to a 100% SOC.

The VRLA battery has internal resistance and other characteristics such that it is not 100% efficient during discharge or recharge. As a result, to attain a 100% state of charge (SOC) it requires that the battery be recharged with about 110 Ampere hours for every 100 Ampere hours removed during the discharge. Due to the relatively low charging current acceptance of the battery once it has attained approximately a 90% state of charge, the time to attain a 95% or 100% state of charge is greatly extended. Note in Figure 2 that it requires 4 times as long to reach 100% SOC as it does to reach 90% SOC.

Attainment of 100% SOC requires an inordinate amount of time, as shown in Figure 2. It is usually most practical size the charger to provide for 90% to 95% SOC and then oversize the battery by 5% to 10% to assure it can deliver the required autonomy following a reasonable recharge period.

Selection of the appropriate charging system for the battery will typically be a function of the cost and bulk of the charger and battery performance in terms of recharge time and service life. The charger selected for a cyclic application is typically lower capacity, less sophisticated and less costly than the charger / rectifier utilized in a float service application. In the case of float service applications such as for UPS and telecommunications systems, the requirements of the critical load will also be a major factor in the charger / rectifier selection because the critical load may have more stringent voltage regulation and purity requirements than the battery itself.

There are several characteristics that the VRLA battery charger should posses regardless of the application and these would include:

The voltage at which the battery is charged is critical to recharge time, performance and service life. The minimum charging voltage required to fully recharge a lead acid battery is a direct function of the electrolyte specific gravity (SG) which determines the fully charged open circuit voltage (OCV) of the cell. The Dynasty VRLA batteries have an electrolyte specific gravity from 1.280 to 1.300 and the recommended charging voltages are as noted in Table 1.

CCB Series	Electrolyte Specific Gravity	Recommended Float Voltage/Cell	Recommended Equalization Voltage/Cell
GEL	1.280 (gel)	2.25 to 2.30 v/c	2.38 to 2.42 v/c
CD	1.300 (AGM)	2.26 to 2.30 v/c	2.40 to 2.45 v/c
DD	1.300 (AGM)	2.26 to 2.30 v/c	2.40 to 2.45 v/c
HD(Hi-Rate)	1.300 (AGM)	2.26 to 2.30 v/c	2.40 to 2.45 v/c
MD	1.300 (AGM)	2.26 to 2.30 v/c	2.40 to 2.45 v/c
TD	1.300 (AGM)	2.26 to 2.30 v/c	2.40 to 2.45 v/c
ND	1.300 (AGM)	2.23 to 2.25 v	2.29 to 2.31 v
GND(OPZV)	1.280 (gel)	2.23 to 2.25 v	2.34 to 2.36 v
TND(OPZS)	1.240 (ASP)	2.23 to 2.25 v	2.35 to 2.40 v

Exceeding the recommended charging voltages will result in excessive electrolysis of the water in the electrolyte, gassing, dryout and premature wear out of the battery plates.

Using less than the recommended float voltage will result in the inability to bring the battery to a full state of charge, following discharge, resulting in residual lead sulfate remaining in the plates and declining capacity with successive cycles. The impact of improper float charging voltage on service life is noted in Figure 2.

The required output current capability of the charger is typically determined per the equation;

The battery has an internal resistance (R_i) and as charging current (I) passes through the battery, heat will be generated as a function of the square of the current multiplied by the internal resistance of the battery (watts = I²R_i). There is additional heat generated as a result of the exothermic oxygen recombination cycle, which occurs at the negative plate of the VRLA battery .With high current availability, deep depth of discharge (DOD) and higher charging voltages the duration of the high charging current may be such as to cause the battery to become excessively warm. To maintain battery heating below 10°C (18°F) during recharge, the charging current availability should be limited to the recommendations noted in Table 2. These recommendations reflect both the rate and depth of discharge of the battery being recharged.

Max. Rate Discharge Pd. to End Point Voltage	Approx. % Depth of Discharge Relative to 20 Hour Rated Capacity	Recommended Charging Current Limit Relative to Battery 20 Hour Rated Capacity
15 Minute	45%	C/1 (100 amps per 100 AH)
30 Minute	55%	C/2 (50 amps per 100 AH)
1 Hour	60%	C/3 (33 amps per 100 AH)
3 Hour	75%	C/4 (25 amps per 100 AH)
8 Hour	90%	C/5 (20 amps per 100 AH)
20 Hour	100%	C/6 (17 amps per 100 AH)

Table 2- Recommended Maximum Charging Current (I_C) VS. Depth of Discharge (DOD)

When the DC charging voltage contains an AC ripple voltage (V_rms) component an AC ripple current (I_rms) will flow through the battery in addition to the normal DC charging current. Due to the low internal resistance (R_I) of the battery the resulting AC ripple current can be very large, up to 100 times as large as the DC float current, and result in significant heating (I_rms²R_i) of the battery in float service applications. Since the battery service life is reduced by 50% for every 18°F ( 10°C) above its rated operating temperature of 77°F (25°C) any long term increase in it's temperature should be avoided. The recommended charger output maximum allowable AC ripple voltage (V_rms) and current (I_rms) are noted in Table 3.

Application	Max. AC (rms) Ripple Voltage	Max. AC (rms) Ripple Current
Cycle Service Charger	1.5% of DC Charging Voltage	0.15C Amperes rms (15 Amps rms per 100 Ah Capacity)
Float Service Charger	0.5% of DC Float Charging Voltage	.05C Amperes rms (5 Amperes rms per 100 Ah of Capacity)