Battery voltage will drop for many reasons, the main one being corrosion. In an auto the battery is usually a lead acid type and is therefore subject to corrosion from acid seepage around the terminals. The corrosion is resistant to the flow of electrical current causing the voltage beyond the terminal to drop or in some cases be absent.
High amperage loads such as the engine starter can cause so much voltage drop that the circuit no longer has power.
Check the battery condition with a battery load tester. Battery load testers are available from most auto parts retailers at minimal cost and is a good addition for your tool kit. This will test the battery under load and see if it can perform as intended. Follow the manufacturer’s instructions for the proper use of a battery load tester. If the battery voltage drops below 8 Volts when placed under a high amp load, then it will need replaced.
Another early test to conduct is the alternator charging voltage. This test is performed by using the 20 V DC range on your VOM to test voltage at the battery terminals as the engine is running. The voltage should be in the 13.5 -15 V range. If it is not in this range, you will need to do further testing to find the problem and correct it.
Tools Used to Check Voltage Drop
A good VOM (volt ohm meter) is very useful for checking electrical problems and is invaluable for checking voltage drop.
Read and understand the instructions that come with the VOM or look up a couple of videos explaining how to use a VOM. They are really simple to use and can be purchased relatively inexpensively.
A 12V test light will assist you in testing also as it can indicate a no voltage or voltage present. This author uses both in electrical diagnostic work.
Testing for Voltage Drop
A voltmeter is used to test for voltage drop. The meter is set for DC 20 volt range and connected to the battery to get a reference voltage. (12.5V) After getting a reference voltage the meter is then connected between battery ground terminal and the load being tested.
Do not disconnect any wires at this time. The load is then applied and a new meter reading is observed. Any reading less than 12V will indicate a voltage drop unless the load is a high amperage draw such as a starter. Under most circumstances a starter load can be acceptable if the voltage is 8V or above when starter is engaged.
Repairing the Voltage Drop Problem
If your test indicated a low voltage, disconnect the circuit in question and set your VOM for Ohms. Test the suspect wire by placing one lead of the VOM on one end of the wire and the other VOM lead on the other end of the wire.
A low number setting is good as there should be little resistance in the wiring itself. A reading somewhere around 1 Ohm or less will indicate a good wire. Anything over 1 Ohm should be investigated further for extra resistance.
Any problems observed should be corrected such as replacing the end terminal of the wire being tested or cleaning all corrosion from the terminal using a wire brush, sandpaper, or a sharp scraper (such as a knife). You need to expose shiny metal to be able to make a minimal resistance connection.
A good tool to have is a battery post and terminal wire brush cleaner. It is designed to clean corrosion from the battery post and the cable terminal. It is not very expensive.
The aim is to get clean shiny metal showing again. A good procedure is to start with the battery cable ends and clean them thoroughly. Reattach them and then do a new voltage test. An improvement in voltage reading will show you are on the right track for solving your problem.
Clean each suspect circuit and check them for voltage when under load both before and after cleaning. Carefully inspect any crimped terminal ends for corrosion and if you are not able to clean them replace the terminals with new ones.
After cleaning the terminals you should see an improvement in the voltage drop for the cleaned wire.
Disconnect your vehicle battery to remove the possibility of getting a power surge into your VOM when you are testing Ohms resistance. Voltage applied during this test can damage your VOM.
It would be good to check resistance of the wire in question. Read the instructions for using your VOM and follow the suggestions to obtain readings that you can use to troubleshoot voltage drop problems. Set the VOM for Ohms and attach the probes to each end of the wire. Note the reading. It should be very low (less than .4 Ohms).
A good habit to get into when cleaning wiring terminals is to use dielectric grease on the terminals and plugs when reassembling as this will slow any future corrosion.
Vehicles operated in high humidity climates and where salt is used on roadways in winter are extremely susceptible to electrical wiring corrosion.
Places to Check
One of the common points for corrosion to cause problems is the grounding studs and bolts used throughout the vehicle. The ground (black) cable from battery to a terminal on the engine needs cleaned.
Any other grounding points such as the vehicle frame, the vehicle body, and any lighting will need the grounding point cleaned. Many vehicles have multiple grounding bolts on the engine to ground various electrical components. Locate these and clean them thoroughly for good service.
Most vehicles have a smaller ground wire built in with the battery ground cable and this wire connects directly to the vehicle body metal such as inner fender or radiator support.
This author has found many voltage drop problems in auto service to be problems in the grounding part of the circuit. The grounds definitely need checked.
The positive (red) battery cable should be checked where it connects to the starter relay or solenoid. This is not a real common corrosion place, but it can corrode.
Sometimes a relay will have internal corrosion on its contact points causing a voltage drop. Use your VOM (volt ohm meter) set to resistance to measure and compare the suspect relay resistance reading with a known good relay. The best remedy here is installing a replacement relay as relays are not easily serviced.
Other Causes of Voltage Drop
Extra long runs with undersize wiring can cause voltage drop. Wire should be sized to match the expected load plus the resistance of the length of run and then oversized to the next larger wire size.
All wiring terminals should be soldered, not just crimped. A good solid mechanical and electrical connection is necessary for proper operation of electrical components.
Modern vehicles are extremely dependent on electrically operated components, from computers to power steering. With this in mind there should be no scrimping on any of the electrical wiring. For the DIY person doing their own auto maintenance paying close attention to battery cables and other wiring components will be very worthwhile.
A thorough cleaning of battery posts and battery cable ends is a good place to start your maintenance process. Move on to the ground bolts and studs with their respective wires and terminals. If you can maintain the electrical connections in a clean state, you should not have voltage drop problems.
Close attention should be given to any wiring connections related to the computer sensors and controls. The vehicle performance is totally dependent on input information from these sensors. Corrosion of these wires can severely affect the driving performance of your auto.
Modern transmissions no longer have a shift linkage, but have an electrical connection through a wire to select the desired gear range and travel direction.
Speed sensors communicate with the auto computer along with all the information inputs from the engine to control most of the events occurring with the auto as you travel. The correct gear for the transmission is selected using all the inputs that travel along wiring to the computer.
These sensors work with small voltage, usually 5V or less. Clean terminals and intact wiring is extremely important for the continued good performance of your auto. Any voltage drop here can have bad repercussions with the drivability of your auto. Since the voltage is low coming from the computer, any loss due to voltage drop can have adverse effects on the vehicle.
Knowing how to conduct voltage drop testing will help you as you do your auto service and maintenance. The small investment in tools for testing and repair of voltage drops can be repaid multiple times.
A service and repair manual for your particular vehicle will come in very helpful when making voltage drop tests. It will give you the correct voltage readings expected at the various sensors related to the computer.
Voltage drops will always be a problem, but armed with information and testing tools, you can stay ahead of them and keep you vehicle operating as designed.
Some Problems From Voltage Drop
Batteries usually mount under the hood near front of vehicle. They are used to supply voltage to operate the fuel pump near the rear of the vehicle. This makes for a long wire and DC electricity does not like long runs.
The voltage will drop and if it is too much of a drop it will cause your fuel pump to under-perform and heat up. This will shorten its life by several percentage points.
It will be in your best interests to keep this wiring clean and as free of voltage drop as you can make it.
This problem can become bad enough to keep your fuel pump from making the correct pressure to correctly operate your vehicle as it was designed for.
Some of the modern cars use an electric power steering unit and it can suffer from voltage drop. When you steer for a turn the power steering motor requires a lot of current and full voltage.
If you have a problem that causes voltage drop, this motor can be starved for electricity and not perform as it should causing steering to be difficult or impossible. For your safety when driving your auto, you should be aware of voltage drop and how to remedy it.
Headlights can become dimmed by voltage drop. It is imperative you keep your headlight wiring corrosion free to maintain the brightness of your headlights. Be sure to clean the grounding wire of the headlights. It is usually a black colored wire.
Voltage drop in the electric feed wire for your auto’s computer needs to supply full voltage without any drop to maintain the ability of the computer to control all of the auto functions it is responsible for.
A very common problem found in vehicles is corroded battery posts and the corresponding cable terminal. This corrosion can become so bad that the vehicle alternator is unable to maintain the needed charge in the battery. This causes a low voltage situation that is made worse by any wire terminal plug corrosion.
This can cause you to be stranded without enough battery power to restart your vehicle. This relates back to an earlier caution about keeping battery posts and cable terminals cleaned on a regular schedule.
A Word About EVs
If you are owner of an electric vehicle, you have a completely different set of problems. Engineers have tried to eliminate voltage drop as the best they can, but it sometimes creeps in. If you notice drop in vehicle performance of your EV, then take it to a service center for checking. This is best left to trained professionals as the voltage of many EV’s battery is enough to be lethal.
This is one time to enlist the services of a professional technician. Your personal safety is much more important than saving a few dollars trying to do it