Modern day vehicles are increasingly seeing changes in the specifications of vehicle charging systems from make to make, and from model to model. And so unlike years gone by, we now see sweeping variations in battery size and alternator output. And with the introduction of smart alternators, we also see automated restrictions on how much charge output those systems are permitted to generate. This bulletin relates solely to 12v electrical systems which are used to power vehicle accessories and does not relate to a hybrid or electric vehicle motor and its charging system.
Smaller or aging batteries might still attain a full charge level of 12.7 volts, but when subjected to high current loads from vehicle accessories like lights, air compressors, and winches, they can fail to meet a high enough discharge rate, or they can pre-maturely discharge due to having too small a charge capacity (Ahr rating). Smaller alternators are limited in output amps and therefore can rely too heavily on batteries to supply the remainder of the current draw required.
Smart alternators are programmed to intentionally limit alternator output and keep batteries in a minimum acceptable state, all in the name of extremely small gains in fuel efficiency. But smart alternators can mean that an otherwise acceptable battery and alternator combination can be forced to operate at levels short of what is needed to support more powerful accessories or longer run time durations.
Air compressors are an accessory that can be used for long periods of reasonable amp draw. A charging
system that cannot keep up can show a drop in the supply voltage resulting in stalls on start-up causing blown fuses (brushed) or throttling and/or protection modes (brushless).
For this reason, ARB highly recommends that you consider measuring vehicle charging systems in order to assure that the customer’s system is well matched to their accessories and is in good working order.
Battery load testing has been around for some time, and the testers still used today have not changed much in design. They are essentially a variable resistor made from carbon pile that is fitted with both an ammeter gauge (to measure amps) and a voltmeter gauge (to measure volts). The basic idea is to subject your charging system to a particular load in amps (A), and to then observe how many volts (V) your charging system is pulled down to while supplying that load. Volts observed above a certain number would denote a pass.
For our purposes the load is going to depend upon the size of the accessory that you intend to power for any longer than a few minutes. Please use the chart below to determine your test load value and then follow the test procedure after the chart to complete the test.
To carry out the required test, ensure you have access to a Carbon Pile Load Tester with a minimum of 500A such as the Projecta BLT200.
1. With the vehicle off, turn off all lights and accessories in or on the vehicle.
2. Connect the load tester to the vehicle’s battery using the alligator clamps. Red to positive first, then Black to negative second.
3. Turn the load dial anticlockwise to remove all load. The ammeter gauge should then read zero amps.
4. Read the battery volts from the voltage gauge. This is the battery’s resting state voltage. If the battery
reads less than 12.5 volts then the battery either needs to be recharged or the battery is damaged and
should be replaced. Discontinue the test until the battery reads over 12.5 volts in its resting state.
5. Start the vehicle’s engine. Note: fast idle can be used if necessary to pass the test
6. Read the battery volts again. This is the vehicle’s charge voltage being supplied by the alternator. If the
charge voltage now reads less than 13 volts then the alternator is not providing sufficient charge to the
battery.
7. Start turning the load dial clockwise. The ammeter should start to show a load value in amps. Keep turning the dial until the amps match the test load value from the table above.
NOTE: This step should be done quickly as the internals of this type of load tester will heat up quickly under high amps and long-term running will cause damage. Many testers are fitted with a timer to prevent long testing times.
8. Immediately read the voltage gauge while the ammeter gauge is showing a load equivalent to the test load value. If the voltage is less than 12.5 volts then the vehicle’s charging system is NOT keeping up with
supplying the applied load. If the voltage is greater than 12.5 volts BUT the voltage value is visibly dropping, then the vehicle’s charging system is NOT keeping up with supplying the applied load. If the voltage is greater than 12.5 volts and it is holding steady (or increasing) then the vehicle is capable of supplying the load required to run the product from the table.
NOTE: The test loads from the table above do not account for any vehicle lighting or other accessories that may be used at the same time as the air compressor. If it is intended to run the air compressor together with additional accessories, then the load from these accessories should be added to the test load value above.
NOTE: The results of this test should be considered something that may degrade over time as the charging system ages, and therefore a vehicle that only marginally passes may, in the future, require charging system maintenance to continue to pass.
Comments
0 comments
Please sign in to leave a comment.