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Equipment
Truck Checks: Total connected load


June 1, 2009
By Don Henry

Put up your hand if you’ve had battery or starting problems on your fire truck. The  problem could be the total connected load ( TCL), one of the most misunderstood factors affecting apparatus operation. The relationship between TCL and alternator output is very important and is often overlooked during construction of new apparatus and in the diagnoses of problems during repair of older apparatus. By definition, TCL is the amount of electrical power needed to operate the vehicle while on scene or enroute.

Put up your hand if you’ve had battery or starting problems on your fire truck. The  problem could be the total connected load ( TCL), one of the most misunderstood factors affecting apparatus operation. The relationship between TCL and alternator output is very important and is often overlooked during construction of new apparatus and in the diagnoses of problems during repair of older apparatus. By definition, TCL is the amount of electrical power needed to operate the vehicle while on scene or enroute.

TC1 

A set of three group 31 batteries 

 
TC2 

It is not possible to test these batteries until the posts are cleaned. This is an 8D battery; this type of battery clamp can cause unwanted voltage drops. 

 
TC3 

Figure 1: A typical output curve. Note the difference between cold and hot output.

 
TC4 

A large output alternator but with the wrong belt for the pulley. The belt needs to match the alternator requirements. Note adjustable voltage
regulator upper left corner. 


 

When determining how much electrical power is available we first need to examine the batteries. The batteries must, by NFPA definition, be high-cycle batteries, not to be confused with deep-cycle batteries.

Deep-cycle batteries work well with electrical loads that discharge, such as golf carts or electrical fishing motors, and they work well when a small amount of energy is needed for a long time. Deep-cycle batteries can be recharged many hundreds of times but they cannot be used to deliver the large amounts of energy needed to start a diesel-engine. Neither are batteries that work well for delivering high cold-cranking amps (CCA) suitable for fire service work, as fire trucks are usually started in warm fire halls.

A high-cycle battery can be discharged for a long period of time and also provides the energy needed to restart a large diesel-engine. Fire-truck batteries also need to be able to deliver current over long periods of time after an alternator failure at the fire scene, and this is why the high-cycle batteries are ideal.

For many years D batteries were the batteries of choice, either 4D, 6D or 8D, with 8D being the most popular. But these batteries are not high-cycle. The high-cycle battery most often specified now is the group 31 battery. It is a high-cycle battery that can supply both the high-amp discharge needed for starting and the long, continuous current needed in the event of low alternator output or alternator failure.

The first step after determining that the battery is high-cycle and is fully charged is to test the brand of alternator. The remainder of the test will provide the necessary data to project the on-scene survival time for any vehicle when the charging system fails. (Note: the following tests will be invalid if any of the batteries are defective.)

1. Test to determine total connected load 
 a. Brand of alternator ____________
At the time of delivery the manufacturer should have given the user a nameplate displaying the rating of the alternator and the output of the alternator at both engine idle and at a 200-F (93ºC) engine-compartment temperature. This information is useful to determine if the alternator is the original or a smaller, cheaper replacement. 
 
 b. Rated charge voltage (Vc) ____________
This number should be given by the manufacturer as a cold setting or a hot setting. (A cold alternator, say at -30 C, will have a voltage maximum setting of close to 14.4 to 14.6 volts but as the alternator warms up because of high under-hood temperatures that are found on fire apparatus, the voltage will be lower, often in the range of 13.8 to 14 volts at 70 C. This is automatically accomplished by the solid state voltage regulator.) Be careful, however, as a typical cold setting may be 14.4 volts DC and 14 volts DC at operating temperature. Most alternators have some method to reduce charge voltage when at operating temperature. Use a digital voltmeter to take this measurement.
 
 c. Maximum output rating
Cold, 77 F (25 C) Ac ____________
Hot, 200 F (93 C) Ah ____________
Note that amperage cold and amperage hot may differ by as much as 20 per cent on the low side for some brands of alternators.

Get a chart of the alternator performance from either the truck-builder or the alternator-manufacturer. To measure the  amperage you will need an amperage gauge and a method of applying a load to the alternator. You can use a carbon pile or turn on the electrical loads such as lights, sirens and primers, as long as you can apply enough loads. Most alternators must spin the rotor shaft at least 5000 rpm to reach full output. This means the engine needs to turn at least 1200 rpm to allow full alternator output.

For example: A typical setup– alternator pulley is 2.5 inches (6.35mm) in diameter; engine drive pulley is 9 inches (22.8mm) in diameter; engine full-speed maximum governed rpm is  2100 rpm. The relationship of the drive to driven pulley is 3.6 times – for every one revolution of the engine, the alternator turns 3.6 rpm. For the alternator to spin at 5000 rpm the engine must run at 1388 rpm.

You will have to do this math for every truck you look at and calculate the pulley relationships. Sometimes people put the wrong pulley on the alternator. It might seem that the solution to the problem of low alternator output at idle is simply to put a smaller pulley on the alternator, which would make it turn faster at a lower engine rpm. However, it would also turn faster at a higher engine rpm and at the engine maximum governed speed of 2100 rpm the alternator rotor would be turning at 7560 rpm. Most manufacturers recommend that the rotor bearing not turn at more than 8000 rpm for long bearing life. If a smaller pulley were used, the rotor shaft and its bearing could turn at far too many rpm, which would result in a shortened life cycle.

  
 d. Amperage output at engine idle ____________
Take this measurement with the engine at its normal idle  speed and with an engine compartment temperature of at least 200 F (93 C).
  
 e. Amperage output at 1000 engine rpm
Take this with the engine as close to 1000 rpm as possible, with an engine compartment temperature of at least 200 F (93 C).
  
2. Vehicle engine type 
 Engine manufacturer ____________
Model ____________
Cubic inch displacement ____________
14-volt system ____________ or ____________
28-volt system ____________
  
3. Total connected load
To determine the total connected load you can either:
 
 a. Assign a value to all the electrical accessories that might be on at any given time – this information should be available from the manufacturer of each particular component.  
 b. Perform a test of all the possible loads that could be in use. This could be measured with an amperage meter connected at the batteries when the engine is off so that all power must come only from the battery packs. As some of this load will be applied only while en route and some only when at scene, you will have to take two measurements with the appropriate loads switched on or off. Consult your department’s SOPs to find out what loads and lights are used in which situations.   
 Total component load, component’s amperage draw  
  a. Dedicated loads (loads you can switch off)
1. Engine fuel management ____________
2. Transmission computer ____________
3. Other loads ____________
4. Amount necessary to float charge the battery pack ________
If the engine computer senses that the voltage is too low it will shut down the engine. This shut down cannot be disabled like the high-temperature or low oil-pressure shutdowns in fire-service vehicles. A typical low voltage shut down is 10.2 volts.A typical load for the engine could range from 10 to 35 amps, depending on the engine type, and about 10 amps for the transmission. To maintain the batteries at peak performance a single 8D battery will need 15 amps, while a group 31 will need approximately five amps per battery.
Total amps dedicated  ________________________
  
  b. Switchable loads (worse case condition)
1. Calculated amp value ____________ at rated test voltage (VT) ____________ or
2. Actual amp value ________ at battery test voltage _________
This test voltage will most likely be about 12.0 to 12.1 volts depending on the condition of the batteries and the size of the loads, and is measured at the battery. Be careful and take safety precautions around the batteries. To avoid any sparks, use a clamp-on type amperage meter. Here are some typical load draws – you will need to fill in your own numbers.
Lights running ____________
Lights at scene ____________
Sirens ____________
Driveline retarder (electrical type) ____________
Air conditioning (fans and compressor clutch) _______
Radios and cellular phones ______
Primer pump ____________
Total amperage load, switchable ____________
Total connected load (3A plus 3B) ____________
  
4. Fill in the blanks to find the Voltage Factor (Vf)  
  (Vc)___ – (Vt)____+  (Vd)___
= _______ +1 + (Vf)______
(Vt)______
  

■ Voltage factor (Vf)
It is not enough to just put back in to the electrical system what you took out to run the loads. This is because the electrical power you used was at 12.2 volts and the power you must put back in is most likely going to be in the range of 14 to 14.4 volts. This voltage factor is used as a multiplier to find the total connected load. This adjusts the TCL for any given charge voltage value when the test voltages in B-1 and B-2 are known. This larger number is the true value of the TCL when the system is operating at charge voltage and represents an increase in amps anywhere from 15 per cent to 25 per cent.

The voltage factor is determined by first obtaining the voltage difference (Vd) between the system’s charge voltage (Vc) and its voltage (Vt) during the load test or, if calculated, the operating voltage at which the manufacturer rated the component lamps or devices. This voltage difference (Vd) is then divided by the test voltage (Vt). The resulting answer is the voltage factor (Vf). By multiplying the voltage factor by the TCL you will know the true amount of electrical power needed for the system.

5. True value of total connected load at charge voltage
 a. Total connected load from
section 3B ____________
 b. Rated charge voltage (Vc) ____________
This number should be given by the manufacturer as a cold setting or a hot setting. (A cold alternator, say at -30 C, will have a voltage maximum setting of close to 14.4 to 14.6 volts but as the alternator warms up because of high under-hood temperatures that are found on fire apparatus, the voltage will be lower, often in the range of 13.8 to 14 volts at 70 C. This is automatically accomplished by the solid state voltage regulator.) Be careful, however, as a typical cold setting may be 14.4 volts DC and 14 volts DC at operating temperature. Most alternators have some method to reduce charge voltage when at operating temperature. Use a digital voltmeter to take this measurement.
6. Output of alternator hot (Ah, from section 1) ___________
 Adjusted TCL from section 5 _____
Is the alternator’s output when hot greater than the adjusted TCL?
If yes the alternator is properly sized. If no, then a larger alternator is needed or you will have electrical problems.
7. CID (cubic inch displacement) of engine _____ x 2 = minimum CCAs (cold cranking amps) required. The metric calculation is 125 CCAs per liter of engine displacement. Battery manufacturers will give you the CCA rating of the battery and this measurement is always taken at 0 F ( -18 C).
Check with the manufacturer – many recommend three times the cubic inch displacement but this is usually calculated for diesel engines that have to start outside in cold weather and therefore may not apply to your fire truck.
8. Number of batteries needed to meet the reserve capacity (Rc) requirements if the alternator is off-line (failure of alternator or engine stall).
 a. TCL (adjusted from line 5) _______
b. 25 (Rc load test) A divided by B = number of batteries ___________
 

■ Explanation of reserve capacity test
Rc is the amount of time in minutes that a battery can deliver 25 amps and maintain a voltage above 1.75 volts per cell. As a 12-volt battery will have six cells, at the battery terminals the voltage should be not less than 10.5 volts. This is very important for the newer electronically controlled engines and transmissions, as a low voltage will cause these engines and transmissions to shut down. Simply put, how long, in minutes, can the truck operate with no alternator output?

9. Total CCAs available.
 a. CCA rating of battery _______
b. Number of batteries in the pack (item 8) ____________
c. (A x B) = ________________ (This value must be larger than the total from line seven)
 

■ Summary
This chart was not made up by someone at the taxation office! This information was originally presented by a true genius and supporter of the fire service named Dennis Litchenstine – we truly stand on the shoulders of giants like Dennis. An EVT who is aware of the special electrical demands placed on a vehicle in the fire service and the NFPA standards in place to ensure those demands are met is well placed to offer expert advice to the decision makers who purchase fire apparatus. This expert knowledge of electrical systems and electrical demands also ensures the EVT is capable of carrying out appropriate preventive maintenance, testing and record keeping. Correct maintenance of the most appropriate electrical components will ensure that firefighters are not left in the dark and incommunicado at the emergency scene. If the TCL is not correct then you will need to install a load-management device, load-shedding device or a larger alternator. It is also waste of time and money to install a larger output alternator if you do not also install larger gauge wires to and from the alternator.

An excellent source for larger alternators is www.ceniehoff.com It also has excellent wire charts to determine the size and length of wire for a given amperage. If your alternator is undersized it may be necessary to use load-management devices and load-shedding devices – they will be the subject for a future article.•


Don Henry teaches in the Automotive Services Technician and Heavy Equipment Technician programs at Lakeland College in Vermilion, Alta. He can be reached at don.henry@lakelandcollege.ca


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