Dry Cell Batteries In Ignition Use

Schematic for building a simple device

Figure 'A'

Content Tools

PO Box 832, Prentiss, MS 39474, all rights reserved

The modern dry cell battery is a marvel of technology combining power and durability into a relatively compact, inexpensive package. Unfortunately, its ability to provide reasonable service in ignition applications is sometimes misunderstood. Misunderstandings can promote misuse and lead to poor performance; the end result is the battery often receives an underserved reputation for general unsuitability. In defense, a battery (like any other device) cannot be expected to give optimum service beyond the limits of its design. However, ignition operation within these capabilities is possible. Hopefully this article will enable the restorer to recognize the capabilities of dry batteries and correct problems that can be encountered in their use.

To illustrate some of the variables involved in battery design let's investigate the NED A (National Electrical Distributors Association) 918 series: a 6 volt 'lantern' style widely available from several manufacturers, and often used for hit & miss engine operation. Specifications for three variations of the NED A 918 offered by 'Eveready' are given in Table 'A'. Note that the current ratings for the various configurations range from to 1 amps. Also be aware of how temperature affects these batteries. High temperature raises output but speeds up deterioration due to increased cellular chemical activity. Conversely, cooler temperatures improve storage life but decrease output. Carbon-zinc types suffer the most from temperature extremes; zinc chloride is better; with alkaline models being the least effective of the lot.

Batteries under consideration for ignition use must possess the ability to adequately handle the drain imposed by the intended system. Fresh dry cells are capable of brief outputs well in excess of their rated current levels and will recover from such excursions if allowed to 'rest'. However, repeated use at elevated drains will eventually lead to failure. How soon failure occurs is hard to predict. Chemical makeup, age, storage/operating temperatures, duration of high-level drain, and 'rest' interval, all enter into the picture. Abuse of any, or a combination of these factors can lead to a disappointingly short service life. Knowing the operating parameters for these batteries is helpful, but more needs to be known about the power requirements of the ignition device itself before a determination of suitability can be reached.

High current drain caused by ignition equipment with inherently low resistance is probably the chief factor in many cases of early battery demise, Automotive ignition coils were designed to operate in conjunction with power supplies capable of furnishing very high current. But even under such circumstances, some type of internal or external resistance was utilized to limit current flow. Used without a resistor, an automotive coil will wreak havoc on a dry cell. 'Buzz' type coils are also current hogs, and a significant amount of power is required merely to operate their magnetic vibrators. Even ones designed for small engine service were often originally intended to use four or five 1 volt high-current 'ignition' cells in series. Restorers desiring to use a 6V lantern battery with these types of coils may wish to experiment with a current limiting resistor of between 2 to 5 ohms, rated at 20 watts minimum. Insert the resistor in series between the coil and battery the objective being to raise system resistance and thus reduce current flow to a level within the battery's capability. Low-tension coils can also pose a problem: most requiring fairly high current, again due to low internal resistance. Select a coil specifically designed for low current drain (6 to 10 ohms internal resistance) as external limiting resistors are not recommended with low-tension systems.

Best battery life is seen on very slow turning engines (particularly those with some type of ignition cutout governor), since ignition 'on' cycles can be as few as 5 to 10 per minute. Service can be further enhanced by insuring that contact dwell during these 'on' cycles is of no longer duration than necessary to provide adequate ignition. Remember that dwell, though measured in degrees of rotation, is really an expression of time. A coil requires a given amount of time to respond no matter what the engine speed, so manufacturers originally specified enough dwell to cover ignition needs at the highest anticipate a RPM. When operating at low RPM, longer dwell is just 'icing on the cake', and serves only to decrease battery service life.

Dwell is particularly important on low-tension igniters. Some systems were designed to have the points in a closed position between firing cycles; while others allowed the points to remain open until just before ignition. A few manufacturers provided a means of selecting either mode of off-cycle point disposition depending upon whether a battery or magneto was used. One example is the Associated Manufacturers Company which gave the following instructions on igniter setup: (The igniter points should separate about 1/6'. When operating engine on batteries the igniter points should be held apart by a small coil spring attached to the igniter hammer stop and the left hand post on igniter body. Either 'open' or 'closed' position of igniter points can be used for operating engine on magneto.) In 1913 directions for their engines, the International Harvester Company recommended igniter points be set to dwell closed for about 60 degrees of crankshaft rotation prior to opening, and cautioned that less dwell might not allow adequate time to properly energize the coil. International's figures were based on an operational speed of 400 RPM experimenting with slower speeds and different coils might well yield satisfactory operation at less dwell on your particular engine. If the system is not adjustable, you may wish to fashion some type of cutoff mechanism to prevent battery drain during off-cycle operation. A sketch and description of such a device appears in the December 1985 issue of Gas Engine Magazine on pages 20 and 21.

TABLE 'A'

'Eveready'

731

1231

521

number/name

Lantern Battery

Super Heavy Duty

Energizer

type

carbon-zinc

zinc chloride

alkaline

voltage

6

6

6

max. suggested

1/2 amp

3/4 amp

1? amps

prolonged high

fair

good

very good

output ability

   

cost

low

moderate

more expensive

Characteristics of three NEDA No. 918 series 'Eveready' batteries.

Courtesy: Battery Products Division of Union Carbide Corporation.

Though a declining battery is often capable of furnishing extended service at very light current drains, the inability to provide output at or near rated levels is an indication of general weakness. A battery in this condition may give erratic or unsatisfactory performance. Weakness (brought on by age, high temperature, or high current drain) is a result of chemical changes within the battery. Chemical deterioration causes a rise in internal resistance; eventually to a point that restricts voltage output, particularly at high current drains. Oddly, there is very little difference in voltage readings between fresh and depleted cells on open circuit. So accurately judging the present condition of dry cells requires that measurements be taken under load. Equipment designed for such testing is available, but beware of inexpensive models as they may not expose the battery to enough load to be of use. For example: 'Eveready' recommends that a 6V lantern battery be tested with a load of 5 ohms, but a multi-battery tester sold by Radio Shack gives about 600 ohms on the six volt setting. Since higher resistance means lighter load, the 5 ohm test will give a much better indication of a battery's suitability to high-drain ignition use. Obviously a questionable battery could receive satisfactory rating on a 600 ohm test, yet fail at 5 ohms.

Figure 'A' shows the schematic for building a simple device to correctly load test lantern batteries. The tester is for use on 6V dry cells only (though it can be used for comparative purposes on 6V storage batteries) do not use it to test units rated at over 6 volts. R1 is a 5 ohm power type resistor with a minimum rating of 10 watts. If such a resistor is not available, wire two 10 ohm, 10 watt resistors (Radio Shack no. 271-132; priced at two for $.89) in parallel to give 5 ohms, 20 watts. If you already own a multimeter, all that is required is to solderR1 between the test leads (you may want to add alligator clips to a separate set just for battery testing). Connect the leads so as to place the load (R1) across the battery terminals, and read voltage output. Be sure to observe proper meter polarity and keep test periods brief since the load is rather severe. More accurate readings will be taken with the meter set to indicate at half scale or higher. To evaluate a battery, compare reading with the following percentages of rated voltage:

0.0-61% (0.0V-3.66V)replace

61%-72% (3.66V-4.32V)weak

72%-105% (4.32V-6.3V)good

Bear in mind that these readings reflect general battery condition and do not necessarily indicate it to be fit or unfit for service. A system with inherently low current needs can often function on comparatively weak batteries, while a less efficient device is rendered inoperable.

A more versatile arrangement would be to mount a 0-8 DC voltmeter and R1 in some type of enclosure. Locating a momentary push button switch at position 'X' in the schematic will allow for taking readings with or without a load. This can be useful in determining the compatibility of the battery with a particular coil. For instance, suppose you have determined a battery is good by testing it under load with the switch closed; you can then substitute a coil for the load resistor by temporarily connecting it across the battery and observing the meter reading with the switch open. If the reading falls in the weak or replace range, then the coil will likely overdraw the battery. Following these basics will enable you to get all of the inexpensive, trouble free service that a dry cell battery can provide. Remember:

Buy a battery known to be fresh.
Store it in a cool place.
Operate it in warm surroundings.
Keep drains within specifications.

Good luck!

Those desiring more information on electricity, ignition and batteries, may wish to read a book by the author entitled: Make and Break Ignition Manual available from LoneOak Distributing Company, PO Box 832, Prentiss, MS 39474 for $9.95 ppd.