Dry Cell Batteries In Ignition Use


| March/April 1986



Schematic for building a simple device

Figure 'A'

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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.