Knowing your batteries - Final Part 6, Some Procedures

by Larry Janke on 3 Jan 2010 This is the sixth and final part of a series by http://www.semarine.com/store/home.php!Larry_Janke from Southeast Marine Services. This week he talks about some procedures - determining appropriate absorption time settings for charging flooded lead acid batteries and for Equalisation

The most common causes of battery undercharging are:

1. Improper charging voltages

2. Low temperature

3. Inadequate absorption voltage duration







The following tools are required:
1. An accurate float hydrometer, one that reads in numbers, not floating balls or a swinging needle,
or
2. A refractometer and a glass thermometer which can be immersed in the electrolyte with a scale sufficient to cover expected temperature ranges.

The following formula will allow calculation of approximate absorption time.

H = ((C x 0.25 x 1.15) / R) x 2

R= Available charge rate.
C = battery capacity in Amp Hours at 20 Hour rate.
H = Hours at absorption voltage to full charge.
For example: If R =110 amps, C= 820 Amp Hours
H= ((820x0.25x1.5)/110) x 2 H=4.29 hours
(Courtesy Fred King P.E.)

To further refine absorption voltage duration for a given battery bank some experimentation is necessary. Assuming the batteries are new or healthy and specific gravity is 1.265 to 1.275 at 25C. (78 F).

Proceed as follows:

1. Discharge the batteries to specific gravity 1.200 at 78 F. as corrected for the temperature of the electrolyte in each cell.

For every 10 degrees below 78 subtract 0.004 from the specific gravity value on the hydrometer or refractometer. For every 10 degrees above 78 add 0.004.
For example if the temperature is 98 degrees F. we would add 0.008 to the gravity reading If the indicated specific gravity is 1.250 the corrected specific gravity would be 1.258 1.250 + 0.008 = 1.258

To convert between Celsius and Fahrenheit temperatures, use the following formulae. C= (F-32)/1.8, F= (Cx1.8) + 32

Typical Charging Voltages

2. Record the specific gravities and temperatures.

3. Set the battery charger to an absorption output voltage of 2.4 volts per cell (VPC) 14.4 VDC for a 12 volt system. 28.8 VDC for a 24 volt system and 57.6VDC for a 48 volt system.

4. Begin charging and check the specific gravity and temperature of each cell every hour till specific gravity reaches about 1.250. Thereafter, check the specific gravity and temperature each half hour. Do not allow electrolyte temperature to exceed 48 C. or 118 F. If, electrolyte temperature reaches these values, before specific gravity reaches 1.265 as corrected for temperature, discontinue charging and allow the system to cool for 12 hours before resuming see paragraph 6 below.

5. If electrolyte temperatures remain below 48C., or 118 F. continue charging until specific gravity reaches 1.265 as corrected for temperature, continue for 1additiona hour. The total time elapsed from the beginning of the charge to this point is the appropriate time setting for the absorption stage of the charging system. This may take far longer than you would expect depending upon the size of the battery bank, the magnitude of any loads on the system during the charge cycle, the available, ambient temperature and charging current. If charging current is below 15% of battery bank capacity in amp hours at the 20 hour rate, longer absorption time will be required. Optimal battery charging rate is between 16-24% of capacity.

6. After cooling the batteries, resume charging at a lower voltage, 12 volts 0.2, 24 volts 0.4 and 48 volts 0.8. Repeat steps 4 and 5. If cell electrolyte temperatures again exceed allowable values, stop charging and call your battery manufacturers technical support for further assistance.

7. If, during steps 4 and 5, specific gravities reach a plateau and will not rise above that point, it may be necessary to perform an equalizing charge as set forth below. This may occur on all cells or only some of them.

8. Do not try to depend on the charger’s temperature sensor, it only measures one point in the system and will not provide the required information.

If ambient temperatures exceed 78F by more than 20 degrees, absorption voltage should be reduced by the amounts set forth in paragraph 6. If ambient temperature is more than 20 degrees below 78F. add the same to the charge rate. Adjustment for temperature will require some experimentation to achieve a proper balance between charge rate and temperature. Batteries below 0 C. or 32 F. will charge with great difficulty if at all.

Float Voltages:
Once the appropriate absorption time has been established, float voltages should be about 2.2 VPC at 78F. 13.2 volts for a 12 volt system, 26.4 volts for 24 volt system and 52.8 volts for a 48 volt system, at 78F. If water consumption exceeds 2-3 oz per cell every 30 days reduce float voltage in.01 volt increments till water usage stabilizes. If ambient temperature is below 50 F. (10C.) increase float voltage by .01 for each 20 degrees F below 78.

Equalization:
Equalization is the process whereby individual cells are forced to similar specific gravity values and should be performed when individual cell specific gravities after charging, are consistently below 1.265, differ by 0.015; the batteries seem sluggish or non-responsive or have been on a float charge for 90 or more days. If batteries are moderately cycled with less than 30% of capacity removed, and regularly and completely recharged to specific gravity 1.265, equalization will seldom be necessary.

Equalization is usually thought of as a periodic process where batteries are subjected to a higher than normal voltage usually about 2.58 volts per cell, 15.5 volts for a 12 volt system, 31 volts for a 24 volt system and 62 volts for a 48 volt system at a limited current for a predetermined length of time. Electrolyte temperature must be kept below 48 C. (118 F.).

This procedure will raise cell specific gravity in the least amount of time but is also somewhat injurious to the battery as a whole, but not as injurious as allowing accumulated harden lead sulphate to continue to crystallize. Some cells will be overcharged to allow those lagging behind to catch up. Also, the batteries should be isolated from all DC loads during the process as the higher voltage can damage or destroy lighting and electronic appliances. Sealed lead acid batteries, absorbed glass mat batteries (AGM) and gel batteries should never be equalized. Serious consequences such as explosion, fire and serious injury or death may result.

The tools necessary to accomplish equalization are a charger of sufficient output to accomplish the task within a reasonable period of time, 20% of battery bank capacity is a good rule a lesser charge rate will also work but more slowly. In any event charge rate should not be less than 12% of battery bank capacity. E.g. for a 400 amp hour bank charger capacity should be between 80-90 amps and so on. Also required are an accurate hydrometer or refractometer and a thermometer which reads in the 100-125F. range (45-55C.)

Discharge the battery bank to SG 1.200, record each cell specific gravity and temperature. Disconnect the negative battery cable. Allow the batteries to sit for approximately 3 hours and measure the voltage with an accurate digital meter (two digits after the decimal point). It should be about 2.03 volts per cell or 12.2 volts for a 12 volt system, 24.4 volts for a 24 volt system and 48.8 volts for a 48 volt system. Reconnect the negative cable and begin charging at 2.58 volts per cell, 15.5 volts for a 12 volt system, 31 volts for a 24 volt system and 62 volts for a 48 volt system. Some chargers and inverter chargers have an equalization option and most allow voltage adjustment.

If the charger has an equalization time setting, set it to one hour. Voltage will begin