Truck Checks: Batteries for the bewildered
Batteries for the bewildered
December 7, 2007 By Don Henry
Of all the devices on fire apparatus the batteries and their systems have the most amount of myth surrounding them then any other system on the truck. Batteries have two tasks, to provide a very large amount of current to start the engine and to act as an electrical sponge to stabilize and absorb voltage spikes. In fact, their correct name is a lead-acid storage battery.
APPARATUS BATTERY MYTHS
Myth number one: Batteries make electricity.
False. Because of a chemical reaction they cause electrons to flow. The easiest way is to look at a battery is like your bank account. Your bank account generally does not make money; it only stores it (at least mine does). If you don't put money in your account you can't get it out. Batteries are very similar; if the alternator does not put the energy in, you don't get it out when you go to start the engine.
Myth number two: Batteries die on cement flours.
False. Batteries do self discharge. If a battery is kept cool and dry then the rate of self discharge is less. In firehalls we tend to keep the batteries in fire apparatus warm and wet. Monthly washing of the batteries should be discouraged. Batteries that have water on the outside case have a small current drain from the positive (+) post to the negative (-) post; this is called a parasitic drain. As technicians we often conduct battery service and charging by placing the battery when out of the vehicle on a piece of plywood rather than directly on a cement floor. Of course we all know wood is an insulator and that's what keep the battery from discharging on the cement floor, right? Wrong. The real reason is batteries often leak acid and the acid eats the cement floor. It's easier to discard a piece of plywood then repair the cement floor. The batteries electrons don't run off to mother earth.
Myth number three: Maintenance batteries are better then maintenance-free batteries because I can add water to a maintenance battery.
False. Maintenance batteries have fill holes on the top of the batteries cells that allows you to replace the water that has been lost due to the normal cycling of charging and discharging of the battery. There is one advantage with a maintenance-type battery. It is possible to detect with a hydrometer which cell of the six cells on a 12-volt battery is defective. This is very similar to a dog chasing a car; okay dog, now you have caught the car what are you going to do with it? Even if you know which cell is bad it is not feasible to repair only one cell, you simply replace the whole battery.
With a truck in frontline service with a maintenance-type battery and a onboard battery charger it would not be uncommon to have to top up the battery with water every month. This is another reason not to use maintenance-type batteries.
Myth number four. The higher the battery's CCA output (cold cranking amps) the better; buy the biggest battery you can.
This is false for two reasons. The only way you can put more CCA capacity in a battery of the same size is to make more plates. The only way you can make more plates in the same sized cell is to make the plates thinner. Over the life of the battery the plates will deteriorate, either from normal use or vibrations. Thinner plates will not last as long as thicker plates. In a previous article we talked about the group 8-D battery and the group 31 battery. Group 31 batteries have very solid plates and are made for the unique work encountered by the fire service. Group 31 batteries are the batteries that NFPA 1901 specifies for fire service use. Thank goodness they only come in a maintenance-free style. A common industry standard for the sizing of the battery pack is three CCAs per cubic inch of engine displacement. For example, an inline 6-71 Detroit diesel would be 6 x 71 (cubic inches per cylinder) is 426 times three, equals 1278 CCAs. Therefore two 650 CCA batteries would work. If you want the redundancy of a dual battery system then you would need to double this with four batteries.
Myth number 5: Car batteries last longer than fire service batteries.
Not really. On a car battery in Canada we expect at least five years of service, yet many departments change their truck batteries after only two years. Why the difference? As I said at the beginning you should keep your batteries dry and cool. For most of the year in this country, from October to April, your battery in your personal car will be quite cool. This cool period causes batteries to self discharge at a lower rate then if they were warm. Because we keep our fire apparatus in this country in warm firehalls we do not get the five-year life that we expect from our personal car batteries.
Other activities like deep discharges, parasitic drains, over- and under-charging also shorten a battery's life, but the biggest cause in my opinion is not having the batteries properly secured. When I see a battery held down with a rubber tarp strap I know the vibration and road shock will drastically shorten the battery life (see photo 2).
Myth number 6: I can run my truck off only the batteries; I do not need an alternator.
Wrong again. Most modern electronic engines and transmissions will not function below a voltage of less then 10.2 volts. On one of our teaching engines in our fire apparatus maintenance program I am able to start the engine and then reduce the battery voltage as the engine runs. At 10.3 volts the engine begins to surge like it is running out of fuel and at 10.2 volts the engine simply does not run. NFPA 1910 requires the use of a low voltage warning buzzer to activate if the voltage drops below 11.8 volts for more than 120 seconds. Batteries with the bigger reserve capacity will keep the truck running longer but when the voltage gets too low the truck simply quits.
Reserve capacity is the ability of a battery to sustain a minimum electrical load in the event of a charging system failure or a prolonged charging system deficit. In my opinion, this reserve capacity is what really sets the group 31 batteries apart from other batteries.
A Few loose ends
Many older apparatus used battery disconnect devices (see photo 3 on previous page). The reasons for these devices were twofold; one was to isolate the batteries from any electrical drain, so as not to have a parasitic drain and kill the batteries when the truck was in not in service, i.e. in the firehall. The other reason was to have a redundant battery system. You would start the engine on, say, the left or number one battery pack for a week, then the next week switch to the right hand or number two battery. It would not make much sense to always start the engine with both sets of batteries every day as you would never know if one set of batteries was defective until the engine could not start. On trucks that use electronic engines, transmissions and other controls, this switch has been largely discontinued. You will have a master on-off switch, usually a rocker type on the dash. This switch will disconnect the loads like the lights inside the compartments but it does not disconnect the electronic engine or transmission. These have memories inside their computers that must be keep powered, and this can require about 100 milliamps. This parasitic drain could kill a set of batteries in as short a time as three months. Even shorter if the batteries are wet.
Print this page