Exide PAR20 manual Equalizing Charge, Effects of Float Voltage, Table D, Ohmic Measurements

13.5 Effects of Float Voltage
Float voltage has a direct effect on the service
life of your battery and can be the cause of
thermal instability.
A float voltage above the recommended values reduces
service life. Table D shows the effects of float voltage
(temperature corrected) on battery life.
	TABLE D
FLOAT VOLTAGE EFFECTS ON LIFE
Temperature corrected 25°C (77°F)		Percent
Float voltage per cell		Reduction
Minimum	Maximum	in Battery Life
2.23	2.25	0%
2.28	2.30	50%
2.33	2.35	75%

Voltage records must be maintained by the user in accor- dance with the maintenance schedule published in this manual. To obtain the optimum service life from the bat- tery, it is important to make sure the batteryʼs float voltage is within the recommended range.

13.6 Float Current and Thermal Management Increased float current can portend a condition known as thermal runaway, where the battery produces more heat than it can dissipate. VRLA batteries are more prone to thermal runaway because the recombination reaction that occurs at the negative plate, and reduces water loss, also produces heat. High room temperature, improper applica- tions, improper voltage settings, and incorrect installation practices can increase the chances of thermal runaway. As with good record-keeping practices, monitoring float current can prevent a minor excursion from becoming a major issue.

13.7 AC Ripple

AC ripple is noise or leftover AC waveform riding on the DC charge current to the battery that the rectifier did not remove. It is usually more pronounced in UPS than tele- com systems. Proper maintenance of the UPS capacitors will reduce the amount of ripple going into the battery.

Establishment of absolute limits for AC ripple has always been problematic because the degree of damage it caus- es depends on the wave shape, peak-to-peak magnitude and frequency. Accurate characterization of AC ripple requires an oscilloscope and even then, only represents a picture of the ripple at that moment in time.

Whatever its exact characteristics, AC ripple is always harm- ful to batteries. Depending on its particular properties, ripple can result in overcharge, undercharge and micro-cycling that can prematurely age the battery. The most common and damaging result of AC ripple is battery heating which can lead to thermal runaway. AC ripple will decrease battery life and should be reduced as much as possible.

13.8 Ohmic Measurements

Impedance, resistance and conductance testing is collectively known in the industry as ohmic measurements. Each mea- surement is derived using a manufacturer-specific and propri- etary algorithm and / or frequency. This means that one type of measurement cannot be converted or related easily to another. “Reference” ohmic values are of dubious value because so many factors can affect the way the readings are made and displayed by the devices. Connector configuration and AC ripple as well as differences between readings of temperature and probe placement will prevent the ohmic devices from generating consistent and meaningful data. The meters work better with monoblocs and small capac- ity VRLA products and less well with large (>800-Ah) VRLA and flooded battery designs. Users should be par- ticularly skeptical of data taken on series-parallel VRLA battery configurations as the feedback signal to the device may follow unforeseen paths that can overwhelm it.

It is best for users to establish their own baseline values for their battery as specifically configured. Do not rely on reference values.

If users wish to enhance normal maintenance and record- keeping with ohmic measurements, GNB recommends the trending of this data over time. Use a first set of read- ings taken 6 months after initial charge and installation as the baseline data. Because cell positioning within the string (connector configuration to a particular cell) can affect the reading, always compare each cell at baseline to itself in the new data. Standalone ohmic data is not suf- ficient to justify warranty cell replacement.

Responsible ohmic device manufacturers acknowledge that there is no direct relationship between percent ohmic change from baseline and battery capacity. A change from baseline of 25% or less is in the normal noise or variability range. Changes between 25% and 50% may call for addi- tional scrutiny of the system. An IEEE compliant discharge test is usually warranted on systems exhibiting more than a 50% change from baseline. Consult a GNB representa- tive for specific questions about ohmic data.

SECTION 14: EQUALIZING CHARGE

14.0 General

Under normal operating conditions an equalizing charge is not required. An equalizing charge is a special charge given a battery when non-uniformity in voltage has devel- oped between cells. It is given to restore all cells to a fully charged condition. Use a charging voltage higher than the normal float voltage and for a specified number of hours, as determined by the voltage used.

Non-uniformity of cells may result from low float voltage due to improper adjustment of the charger or a panel voltmeter

20 which reads an incorrect (higher) output voltage. Also, vari-