D.C. (direct current) Electrical

Electrical :

The following scenarios are from accumulated experience as a mechanic. First of all direct current electrical systems are associated with an electrical system that has the capabilities of a storage system and mostly associated with a common lead acid battery.

There are many types of batteries made out of different materials however for this basic explanation we will stay focused on a lead acid or paste battery having an optimum 100% voltage @13.2Vdc (Volts direct current). This first set of scenarios will be basic problems and how to solve them, as we go into this we will explain the Digital Volt Ohm Meter.

Scenario #1 - No action when start switch is turned on. If you have a system with a 12Vdc battery there are 6 individual cells that optimally have a potential voltage of 2.2 Vdc per cell. These cells are positioned in a series configuration whereas one cell starts the chain with negative and has a positive, that positive is connected to the next cells negative and then that cells positive is connected to the following cells negative post. As these cells in series go on they accumulate the voltage of each and are added together (2.2 + 2.2 + 2.2 + 2.2 + 2.2 + 2.2 = 13.2Vdc). This is a perfect brand new battery and after the battery has been used it should have at least 12.58 to 12.68 Vdc. This needs to be measured with a meter (DVOM) digital volt ohm meter.


For example when one cell is out or damaged you will read approximately 10.2 Vdc when checking across the - & + battery posts. The post is the lead pieces coming out of the battery, a terminal is attached to the cable so it's important to touch the meter wires to the battery posts and not the terminal ends when doing a primary voltage test. Primary voltage is checked directly at the supply posts of the electrical tank (battery).


The primary voltage is the critical point here and on a 12Vdc mobile system which is considered conventional in todays standards. A DVOM meter is the optimum way to get down to the actual cause of an electrical problem. In todays electronic products the old fashioned method of a light bulb tester or just jumper wires and a flash is not the way to go. This method of testing can damage an electronic circuit without blowing it out immediately, it will cause a failure later down the road.

The DVOM meter is inexpensive and can be purchased at any hardware store for around 20 dollars. If you have one or have purchased one recently and would like to get a brief instruction on using one then go to our DVOM page on this site.

The DVOM above from Jameco Electronics cost is around 12 dollars, to get it here costs 18 bucks with shipping. You can purchase one of these from Lowes or ACE hardware for around $25.00. This meter is essential to easily find your problem. All problems with engine no start can be found in these three steps and in this order, Fuel, Fire, Compression.


We are concerned now with fire, the electrical spark. Without the spark on a dc system the fuel will not explode and the engine will not start. If it is a problem with the primary source (battery) nothing will operate. This is where we begin.

Look at the round piece almost centrally located on the face, this is the dial that can be rotated in either direction according to what you are checking for. Right now we are testing for primary voltage in the dc circuit. A circuit is a complete loop of electrical conduit called wire. The complete circuit is when something is working by the flow of electrons which is essentially electrical volume, these electrons are stored in a battery which is directly associated with the size and weight of the battery. The bigger and heavier the battery is the most likely it will hold a larger volume of accumulated electrons. As these electrons accumulate in the battery the voltage rises slowly until the maximum saturation of 2.2 Vdc per cell is achieved. As these electrons flow within a circuit they have a direction (hence dc) these are negatively charged energy and are attracted to the more positive charges. What this means is that the electrons flow from the metal frame of the mobile piece of equipment and usually termed the ground into the ground cable (conduit) and through the negative (-) terminal of the battery ground cable.

If your ground cable is not firmly attached the flow of electrons are slowed down from resistance and that drops the voltage which you are needing to test for. In this first and primary voltage test you can leave everything attached to the battery but make sure to press the meter leads (wires/points) lightly into the battery post and make sure your not on the terminals at this test. Below we will show the meter set up for dc testing only at this point.

In picture one above you will see that the center dial has been rotated to the left (counter-clockwise) by three clicks to 20. This is the scale setting for a 12vdc test. This scale will allow you to test up to 20Vdc . The V is indicating volts and the solid line and dashed line underneath it is the symbol for dc electrical. Picture 2 shows the meter leads plugged into the correct receptacles on the meter face. The common (COM) lead is generally black and is in the bottom receptacle on the lower right of this meter. The red lead of the meter is plugged into the V receptacle. At this time don't worry about the 10A receptacle you need to first understand this primary voltage test. Below are pictures of actual battery tests performed on vehicles that will exhibit what you should be seeing if your battery is not the problem. 

Picture 1 above shows 13.01Vdc in this small motorcycle battery, the battery has been in service for a month or so and the driver just pulled into the shop. This battery has a surface charge meaning that it has just been charged without discharging the surface voltage. To discharge a surface charge state in the battery I usually hook up a light or turn on the lights for 15 seconds and then read the voltage on the battery after the surface charge has been taken off. This battery voltage reading is excellent and shows the charging system is working fine. 

Picture 2 shows a used lawn mower battery that had just been charged up with a battery charger. The Vdc is 13.07 without the surface charge taken off. The battery is slightly larger than the motorcycle battery and for reference the amp hour rating on batteries was standard for determining the actual capacity of the battery being tested. In an amp hour rating for example a 10 amp hour battery can hold one half the volume of a 20 amp hour battery and, voltage is not in the  rating. In an amp test on a battery the load is applied until the 12Vdc battery voltage is dropped to 9Vdc for 15 seconds and the amp meter is read. This reading for example with this battery here could show possibly 65 amps. A cold crank rating has been accepted in todays battery ratings for example; a 500 cold crank number would be that battery will produce 500 amps for 15 seconds of continued cranking of the engine. This would surely toast a starter, the engine should start within 3 seconds of cranking.

Picture 3 shows a slightly larger battery in a John Deere tractor. You will notice the Vdc is 12.73 and again the surface charge has not been taken off, the operator just stopped the tractor and you can see that the voltage shows only 3 tenths of a volt difference from the previous battery #1 & #2. This is huge for a Vdc no load test, a no load means that there is no current being drawn out of the battery at the time of the test. After primary voltage is established and noted down then a disconnection of the battery terminals from the battery posts is necessary. You could assume as a mechanic that this charging system is working properly also. For the no load test you can just remove the negative (ground) terminal from the battery post. This will be explained below after we understand more clearly the primary voltage test on the battery.

Picture 4 is most likely what you will find when testing a battery and dc system that is good. The surface charge is off this battery and it is a truck with a diesel engine. The 12.59 Vdc is common to almost any good 12Vdc battery with no load and after the surface charge has been removed. The newer trucks and other mobile vehicles that have computer controlled fuel systems draw current all the time as well as a clock radio. As this vehicle sets it is continually using current or amps which is the volume of electrical energy (electrons). It is a rather small amount and we will get into that later on. The point in these four pictures is to example what should be expected when looking at a battery that has been charged by a working circuit in a mobile or DC electrical system using the conventional 12 V battery.

NOTE: It is rare and not normal to find a 12 volt DC battery that has been in service with a voltage reading of more than 12.7 Vdc. If you get a Vdc reading of more than that take some surface charge off for 15 seconds. Keep the meter attached and note the voltage dropping by a hundredth of a volt at a time. A battery that is good will always come back up to voltage within a few hundredths when the load is stopped. A battery which has damaged insides will recover voltage slowly and never reach the original primary voltage after the surface charge is removed. An example in this second picture of 13.07 Vdc that after the surface charge is removed the meter should read around 12.7 Vdc, after that reading or close would be considered base primary voltage of that battery. When a load is started by turning on something to draw current the meter will slowly go down in the number and when it gets to 11.5 Vdc or so then turn off the load and watch the meter slowly rise until it gets to the stabilized primary voltage of 12.7. I will have examples of those tests following.

If your primary voltage at the battery posts is good and there is no action then go directly to the starter motor solenoid. At the starter motor is a junction connection, follow the positive cable from the battery post + to where it is attached. Most all battery + cables will end up at the primary input post of the starter solenoid. Put the negative lead of the DVOM onto the negative battery terminal, not the post the cable terminal on the battery - post. Use the red lead or + from the meter probe and touch the cable end attached to the inlet lug on the starter solenoid. This should read the exact same as the primary voltage reading and if it does then try to start the equipment with the key switch as the meter is attached, if the meter does not change by dropping voltage then we will need to look at the main fuse of the equipment we are working with and clean the terminal spade ends. If you have a spade fuse box (plastic fuse with 2 terminals spades) there are two hollow points that allow you to test them without removing them.

In the above picture 1 there shows a variety of spade fuses that are different current ratings. The A is for amps which are a volume of electrons passing a single point in the conductor (wire) in one second.

The number of the electrons is huge but they are just considered energy with no weight. This passing of current (electrons) is translated into what is called amperage. Approximately 6.25 billion/billion electrons past one point in one second is equal to one ampere.


A regular tail light bulb (1157) will draw around 3 amps with all elements lit in the bulb. The colors are specific to the amperage rating of the fuse, they are accepted as generic in all forms of spade fuses. Picture 2 shows the two open ends where a DVOM meter lead probe can be lightly pressed into. This allows a person to quickly find the problem in an electrical circuit without taking the fuses out and looking at them.


NOTE: A fuse is able to run below the ampere rating however at the exact moment the current in amperage reaches it's rated point the fuse will blow. Unlike the breaker which we will cover on another page after this basic testing is completed, the breaker will run at it's rated amperage capacity and will trip after that rating has been exceeded.

Picture 1 of the set of 5 above shows we have identified where the power fuse is for this mower, it's a John Deere D 110. The fuse color appears yellow so using the chart above of the fuses shows a 20 ampere current rating. Also notice the battery cable coming from the battery post, the cable is red and it goes directly to the one of the large lug posts on the starter solenoid. We will explain in detail about the workings of solenoids and the multiple variations that all have two large lug posts and two smaller spade male tabs. Also notice the smaller red wire attached to the battery cable terminal end and going down and into the right side of the spade fuse holder. There are two other red wires coming from the left lower side of the spade fuse holder and these each supply the main circuits with electrical energy. For example the key switch, the hour meter and so on, they go back into the dash panel and supply positive dc current. This red wire from the battery is always at the supply battery voltage.

Picture 2 shows now the meter lead points are on the terminal ends of the cables attached to the posts of the battery being tested. I am sure you noticed in picture #1 the corrosion between the red cable end that is attached to the battery post. With the DVOM the meter reading picks up hundredths of a volt so you can clearly see that the battery cable terminals read 12.75 Vdc and in the next picture 3 the red (positive) meter lead point (probe) is lightly pressed into the open end of one side of the fuse reading a voltage drop to 12.74 Vdc. In picture 4 the red lead probe from the meter is pressed lightly into the other open end of the spade fuse and again there is another .01 Vdc drop leaving the total of 12.73 Vdc at the fuse. This is a really good reading for voltage because it's only a couple hundredths of a volt lower than at the cable terminal ends on the battery posts in the preliminary and the secondary voltage tests for this battery system. Right now to this point of testing we have established what a good system should look like on the meter readings. We have established that the battery has sufficient voltage for further testing the circuit with the DVOM set on the Volts D.C. setting 20 V.   

Picture 5 is a test for the dash key switch back to the starter solenoid. As viewed in Picture 1 the red large battery cable from the battery connects to one large lug post of what is called the starter solenoid. The other large lug post on the starter solenoid has another red cable attached that continues on to the starter motor input lug post protruding directly from the starter motor case housing. Most all solenoids are very similar with 2 large lug posts and two small male spade terminals. We will view and compare those later and for now we have un plugged the white plastic female spade terminals with the purple and black wires. These two wires are a plus (+) & (-), the black is negative and the purple comes from the ignition switch. With the operator on the seat and the meter lead probes attached as seen when the ignition switch is rotated to the start position it produces a reading of 12.17 Vdc and again the drop is picked up because the voltage current is passing through a switch and wiring that shows a drop in voltage. This is normal and considered good unless the voltage drops to much. A volt or two is not good and indicated a rotted connection point in the wiring system. This starting circuit wiring from the battery to the ignition switch and on to the starter solenoid looks good and the readings are normal. 

Picture 1 shows a primary voltage of 11.99Vdc and it may look pretty good to someone who does not understand how much difference one volt means to a starting system. This is the lowest voltage of all the tests. The key switch was left on all night and the hour meter was energized. This took all surface charges off and clearly as seen in the second picture a voltage drop to 5.58 Vdc as the starter switch is depressed and the meter lead probes are on top of the battery posts as the starter is being turned. This is to low voltage to rotate the engine completely to start so you can see how important a volt is when checking a battery when in use. In the picture 3 there is shown a battery voltage reading indicating one cell in the battery is bad.

The battery picture above shows there are 3 cells, 3 caps on the lead acid battery means 3 cells. These cells are separated apart within the battery itself but have a common acid passage on the top side under the top cover. A 12 Vdc lead acid battery has 6 cells and the voltage rating for any battery is the sum of each cell added to the previous cell. So our battery would read 2.2 volts per cell times the number of cells which is 6. This battery brand new with a 100% optimum charge would read 13.2Vdc on our meter. The 3 cell battery would read 6.6 Vdc with the same new condition. Our batteries pictured were in rare excellent condition according to the volt meter readings however there was no load applied to any of them except for the last picture of the 11.99 Vdc discharged battery and 5.58 Vdc with the starter motor rotating slowly.

The most average battery was the 12.59 Vdc truck battery so most likely when you check a good battery with the surface charge taken off the reading on the meter should be between 12.49 to 12.8 Vdc. It can vary slightly but not more that .1 Vdc. We have shown some good batteries with almost the same voltages.  

Important to understand is that one cell will only optimally produce 2.2 volts direct current (Vdc) in a common lead acid battery. A single cell battery like a solar system battery will only produce 2.2 Vdc no matter if it's as small as a watch or the size of a large building. Volts in an electrical system is like pressure in a bladder water tank and is referred to as Electromotive Force (E) in electrical standards and (V) on a meter.


Volume of electricity is like the size of a bladder water tank, a large one may hold 100 gallons and a small one holds 10 gallons and electrical volume is referred to as Current or (I) in electrical standards and (A) on a meter. The pressure can be the same but the volume is increased. Volume is what keeps a load moving in time and pressure is the potential force to operate it or to start movement. A 12 Vdc system needs 12 volts dc to operate efficiently. So your equipment will start on 12.5 Vdc and run until the voltage drops around 2.2 volts which would be 10.3 Vdc and then problems would become evident.


NOTE: Batteries are damaged by heat over 120 degrees F. When charging at any rate do not get the battery hotter than a touch can handle.

Here are some things you may experience on your equipment prior to and or when electrical failure like a no start situation becomes evident.


A smell of acid in the equipment cab or around the battery is a sign that the charging system is overworked and is mostly caused by running the equipment with a poor battery with low voltage or a bad cell. This makes the charging system charge as much as it can and overheats the battery. This is different than a smell of rotten eggs which is the catalytic converter in a modern automobile.


Seeing the lights fluctuate in intensity called Lumens or brightness is a sign that there is a regulator problem in the alternator as well as high corrosion on the battery cable terminal connectors. It is difficult to write all possible scenarios for what a problem may be however this example is to show what a good system will read on a Volt Meter (DVOM), any deviation from these readings would indicate a type of problem starting however if the battery primary voltage tests come out close to these exhibited and the readings are taken in the places shown and you still have a problem go to our Electrical 2 page that picks up at the starter solenoid testing.