The Model "A" Battery & Generating System - Part 2

Battery Performance, State of Charge, and Charging Requirements

Part 1 of this series presented the general performance requirements and inter-relations of the DC generator, Reverse-Current Cut-Out, and Storage Battery. This article continues with discussion of storage battery performance, measurement of state of charge, and charging requirements.

During normal starting (ignition "on," lights "off"), the average battery discharge current measured was approximately 225 amperes. The "cold" total starting time was between 5 to 15 seconds for an average of 10 seconds. So the battery discharge was approximately 10/60 x 225 or some 37 ampere-minutes. This must be replenished by the charging system for further engine restarts.

The lead acid battery has been in use since the early 1900's. The electro-chemical characteristics have been well researched and published¹ but are not well known to the using public. It is reliable and performs with excellent repeatability when properly recharged after any discharge.

It fails "gracefully" with time and proper use but fails catastrophically (and always at the worst time) when misused! The misuse in auto applications normally occurs during "recharge" and "deep-discharge" cycles-running the battery to total discharge followed by long time standing in the discharged state.

State of Charge - Almost all battery and charging/ system instructions begin "Start with a fully charged battery." How is that condition known? It is seldom stated how it is determined. But, there are simple measurements to determine the state of charge.

• Single cell open circuit (no load) voltage
• Single cell voltage while charging at a specified charging current
• Specific gravity of the electrolyte in each cell of the battery

Specific Gravity - The most reliable and simplest measurement is the specific gravity value in each cell. Each cell test is necessary since they are connected in series and weak cells have the greatest influence on total battery performance.

Figure 1 shows the relation between state of charge and electrolyte specific gravity.

Note that the plot is referenced to 80 degree F electrolyte temperature. Other temperatures require correction of the measurement (Table 1).

SPECIFIC GRAVITY CORRECTION vs. TEMPERATURE

° F Add Correction 140 + 0.0240 120 + 0.0160 100 + 0.0080 80    0 60 - 0.0080 40 - 0.0160 20 - 0.0240

Table I - Specific Gravity Correction vs. Temperature

Specific gravity hydrometers are easy to use and available in many price ranges at almost all auto supply stores.

Cell Voltage - If one has an accurate voltmeter, the open circuit (no load) voltage is also a state of charge indicator. Table II, lists the relation for 3 cell (6 volt) and 6 cell (12 volt) batteries.

Cell/ battery voltage rises at a non-uniform rate during charging and reaches a limiting value when charge is complete. The limit is primarily dependent upon the charging current, the temperature, and internal resistance of the cells. So no change of cell/ battery voltage (at a low charging current) is also a measure of the "state of charge."

OPEN CIRCUIT EMF/CELL/BATTERY vs. SPECIFIC GRAVITY

Specific Gravity Cell EMF 3 Cell EMF 6 Cell EMF 77° F Volts Volts Volts 1100 1.962 5.886 11.772 1200 2.050 6.150 12.300 1250 2.098 6.294 12.588 1280 2.128 6.384 12.768

Table II - Open Circuit EMF/Cell/Battery varies with Specific Gravity of electrolyte.

Ampere-Hour Capacity - Batteries were/ are rated at some ampere-hour capacity. It meant that the voltage per cell did not fall below a specific final value at a specified current discharge for some discharge time. It was states as:

Amp-hours Capacity

8 or 20 Hours Amperes Current Discharge

So an 80 A-HR battery could be discharged at a current of 80/8 or 10 amperes for 8 hours to be within its rating. Note that 10 amperes is the nominal current drain of the "A" headlights (8 amperes) and approximately 2 amperes running ignition value.

Charging Requirements - Batteries is normal/ partial discharge state can absorb energy rapidly without overheating and consequent gassing. The critical temperature is 125 °F. Higher temperatures cause unstable conditions within the cell. So the charging limitation is temperature rise and excessive gassing.

By tests², "the charging rate in amperes should be kept below the ampere-hours then out of the battery"..."the condition of gassing and temperature will then also be met."

This statement is known as the AMPERE-HOUR LAW. Fig. 2 shows the relationship for a 100 A-HR discharged battery.

The figure clearly shows-as the battery is charged, the charging current should be continuously reduced, finally approaching essentially zero or a small value. The mathematical relation is

I=(A)/(2.72t)

Where I = Amperes t = Time in Hours A = Number of A-HR out of battery when charging begins, i.e., t = 0

Charging-Cell/Battery Voltage - On charge, the point of gassing is determined by the voltage. The quantity of gas depends upon the portion of current not absorbed by the battery. That not absorbed current goes to heat the electrolyte.

Gassing begins when the cell terminal voltage reaches about 2.3 volts/ cell (6.9 volts, 3 cell battery; 13.8 volts, 6 cell battery). Table III shows the relations.

GASSING ON CHARGING

Cell Voltage 3 Cell Voltage Gassing Gas Composition by Volume Hyd Oxy 2.20 6.6 None - - 2.30 6.9 Slight 52% 47% 2.40 7.2 Normal 60% 38% 2.50 7.5 Hard 67% 33%

Table III - Cell gassing during battery charging.

Constant Voltage Charging - Considering the foregoing conditions, a constant-voltage charging generator or source is desired. The fixed or constant voltage source should not exceed about 2.4 volts per cell. The nominal value for the 3 cell Model "A" battery is 3 x 2.4 or 7.2 volts. Modern automotive regulated charging systems are set to the nominal 2.4 volt/ cell value. A report of such regulated charging systems is contained in reference³.

Always be aware that hydrogen gas is present in the air space of the cells. DO NOT use a lighted match to observe the electrolyte level or cause a spark to occur near the cell vents on top of the battery - else your insurance policy may be suddenly actuated!

Overcharging - To attain the state-of-charge of a lead-acid battery at its maximum level, it is necessary to overcharge by at least 5 percent, but no more than 40 percent. Repeated excessive overcharge is harmful in several ways:

• the positive grid severely corrodes, causing a loss in electrical conductivity.
• hydrogen and oxygen gas evolved during overcharge tends to dislodge material from the plates.
• decomposition of water, and the resulting high acid concentration, is harmful to the plates and separators, and requires frequent water addition.
• prolonged overcharging heats the battery, accelerating corrosion and separator deterioration.

Charging Rules - The important rules for charging lead-acid batteries are:

• The temperature of the electrolyte should not exceed 125°F.
• Care should be exercised not to over-charge a battery beyond the limits specified, with this provision: the amount of overcharge a battery can safely accommodate is dependent on its history. A new battery can sustain a longer overcharge than one that has had long use.
• A battery that has been stored for long periods develops a coarse grained sulfate on both the positive and negative plates. Such batteries should be charged only at rates no higher than the 20-hour rate: A-HR/20 amperes current.

Cycle Life - The unregulated 3rd brush DC generator charging system of the Model "A" does not properly control the charging current of the battery. The following cycle life statement thus does not hold.

The normal cycle life of an automotive, thin plate construction, lead-acid battery is between 150 and 250 cycles, when the depth of discharge is assumed to be 50 percent or greater, and the temperature 80°F. The cycle life would be substantially greater than the range above, if, for example, the depth of discharge were less. Under the type of service for which automotive batteries are specifically designed, the life ranges from 2 to 5 years. It should be noted that the ordinary duty cycles encountered in modern passenger automobile service are high-rate, shallow discharges, and that charging is constant voltage-regulated and temperature-compensated.

Low Temperature - The "Cranking Performance at 0°F" of a battery provides an indication of a battery's high-rate capability. By definition, "Cranking Performance at 0°F" is the number of amperes a battery can sustain for 30 seconds at 0°F, at a voltage not lower than 1.2 volts per cell. This rating has been established by the SAE (Society of Automative Engineers) and the BCI (Battery Council International) formerly AABM (American Association of Battery Manufacturers).

As a general rule, the Cranking Performance in amperes is on a one-to-one relationship with the CID (cubic inch displacement) of an automobile engine; that is, if the CID of an internal combustion engine is 312, the battery selected for starting that engine should have a Cranking Performance of no less than 312 amperes. For the Model "A" engine of 200.5 CID, the value is 200 amperes.

Reserve Capacity - An additional battery test rating that has been established is the "Reserve Capacity Test." This test is conducted as follows: A fully charged battery is discharged at 25 amperes to a cutoff voltage of 1.75 volts per cell. The number of minutes the battery operates under those conditions constitutes the Reserve Capacity. The Reserve Capacity provides a measure of the length of time an automobile can operate in the event of a failure of the charging system, with its head and tail lights and its electric windshield wipers turned on.

Self-Discharge - The self-discharge rate of a fully-charged lead-acid battery is temperature dependent. At room temperature, the self-discharge rate for a freshly manufactured battery ranges from 5 to 11 percent loss per month (Fig. 3).

Modern Automotive Batteries - The general features for present day automotive batteries are shown in Table IV⁴.

Test results of the standard "A" reverse-current cutout, a solid state diode replacement, unregulated DC generator, and the "regulated" modification pictured in Part I, will be the subject of a following article.

Table IV

Automotive Battery

General Features

Cap. Avail. at 20-Hr. Rate (Ah) 33-340* Open Circuit Voltage (V) 2.05-2.10 Open Circuit Batt. Voltage (V) 6, 8, 12 Nom. Operating Voltage at 20-Hr. Rate (V) 1.98 Nom. End-of-Chg. Voltage at 20-Hr. Rate (V) 2.53 Recommended Charging Rate C/20 or higher - do not exceed electrolyte temp. of 125°F or overcharge recommendation. Recom. Overchg. at 20-Hr. Rate (%) 5 - 40 Recom. Dischg. Temp. (°F) 70 - 90 Functional Dischg. Temp. (°F) -40 - 140 Optimum Chg. Temp. (°F) 50 - 115 Permis. Chg. Temp. (°F) -40 - 125 Recom. Trickle Chg./Float/Finishing Rate Trickle Chg. C/100 - continuous overcharging not recommended. Recom. Storage Temp., Wet (°F) -40 - 115 Permis. Storage Temp., Wet (°F) -40 - 120 Recom. Storage Temp., Dry (°F) -4- - 115 Self-Dischg. Rate/Mo. at R.T., Wet (%) 5 - 11 Self-Dischg. Rate/Mo. at R.T., Dry (%) 1 - 5 Watt Hr./Lb 12.7 - 21.8 Watt Hr./Cu. In. 0.79 - 1.6 Impedance (ohms) 0.0010 - 0.0024 Cycle Life (cycles) 150 - 250 Electrolyte Sulfuric acid solution - sp gr 1.265 - 1.300 Electrochemical Equation Pb + PbO2 + 2H2SO4® 2PbSO4 + 2H2O Typical Applications Automotive starting, lighting, and ignition (SLI); Diesels, buses, and trucks; Farm equipment; Snowmobile, garden tractors, lawn mowers, and small engines; Marine *20-hr. ratings and all calculations based thereon are for reference only. The 20-hr. rating is no longer recognized as a minimum performance requirement by either the Battery Council International (BCI, formerly AABM) or the Society of Automotive Engineers (SAE).

¹G. W. Vinal Sc. D., Storage Batteries, 4th. Ed.
²J. L. Woodbridge, "Storage Battery Charging," Trans. Am. Inst. Elec. Eng. 54, 516 (1935).
³B. M. Leece, "Generation of Current for Automotive Apparatus", Journal of Soc. of Automotive Engineers, June 1928, pg 658.
⁴ Gould Battery Handbook, 1985.

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