BATTERIES IN PARALLEL FAIL TO FUNCTION PROPERLY
One Takes Over-Charge While Other is Undercharged
SPECIFICATION for motor fire apparatus, recently issued by a fire prevention organization recommends two batteries for each piece of motor fire apparatus—one battery to serve the siren where an electric siren is to be used and the other to supply current for the electric systems of the fire apparatus. With such an arrangement, the batteries would have to be charged in parallel from the generator.
As a matter of fact the battery companies, Willard. U.S.L., and Exide, have all advised that they will remove their guarantee from any battery if an attempt is made to charge in parallel. No two batteries have exactly the same internal resistance and as the battery is continued in use, the resistance changes. Any attempt to charge in parallel would mean that one battery would get practically all the charge and would probably burn up. They mean by this the charge rate in that particular battery would be extremely high and excessive heat would develop, causing a considerable shortening of life of the battery. The primary purpose would not be achieved as both batteries would not receive full charge, one being badly over-charged and the other undercharged. The only remedy they can offer is adequate capacity in one battery or the selection of two batteries in series, as for example two six-volt batteries to be used on a twelve-volt system.
Siren Is a Heavy Battery Drain
A good siren requires from seventyfive to 150 amperes to get it rolling and fifty to seventy-five amperes to keep it rolling.
And while this draught on the batteries may be considered high, there is no serious problem in providing necessary battery and generator capacities to handle it, or even greater demands.
While it would be impracticable to employ a generator big enough to meet the current demand of a very large and inefficient electric siren, a combination of proper battery capacity with suitable generator will solve the problem. The solution consists of providing large capacity batteries so that during the period of pumping, the output of the generator will be sufficient to replace the current consumed in using the siren en route to the fire.
At least one company installs two generators on apparatus like city service trucks, service aerials, and large aerials, where no pumping is involved. One of the these generators cuts in to charge at about 350 rpm of the engine and thus at an idle speed the battery is actually charging. The second generator cuts in at about 500 engine rpm and so at speeds above 500 engine rpm, both generators are charging. This method provides sufficient charge capacity to keep the batteries in proper condition. Generators of one company have a charge capacity of nearly 600 watts; batteries have a cranking capacity to crank the engine continuously for a period of nineteen minutes with an 190 HP engine and twenty-four minutes with an 240 hp engine, this at a temperature of 65° F.
As a matter of fact, it is felt that a false sense of security is created. The operator will feel that he has an auxiliary battery and therefore he will pay little attention to either battery, and particularly in volunteer fire departments which, by the way, constitute the vast majority of fire departments in the United States. The apparatus generally does not receive any too good attention and with infrequent runs, the Fire Company is more apt to have battery trouble under the present set-up than where a single set of batteries is provided.
In an investigation of troubles with ignition system a few years ago by one manufacturer, in an effort to improve the reliability of apparatus, it found the percentage of failures in service of the magneto far exceeded failures of the battery system. Another factor was that practically all trucks today are equipped only with battery system and thus adequate magneto test facilities are no longer available in many cities. In one case, the closest magneto testing station to one large city is located at Buffalo, N.Y., 150 miles away. This holds true of all the cities in this territory, and would include places like Binghamton, Ithaca, Corning, etc.
As one prominent chief reported, if it had not been for the battery system there would have been many times when they would have failed to respond to a fire as magneto failures have occurred several times enroute to a fire and they found it necessary to switch to the battery and yet they had not a single battery ignition failure to report. Some chiefs still want magnetos on apparatus.
When the duplex jobs were built for Los Angeles, three different sets of magnetos were used before the apparatus left the factory, and it was necessary to make one replacement of magnetos before the acceptance test was finished at Los Angeles. These were Scintilla magnetos and are supposed to be just about the best available.