Battery and charging system test (2024)

OP

System test for slow boot

A lead-acid battery has certain defined characteristics, making it easy to assess the health of the charging system without expensive testing equipment. A test in the car is actually better than taking parts off. I hope this helps people avoid wasting money on parts. Here's how we can check a charging system with a few simple devices. As a starting point, all you need is a test light and a small test meter.

I have a complete diagram of the 1989 Mustang here1989 ford mustang wiring diagram

Generator

The generator converts mechanical energy into electrical energy. There are unavoidable power losses in a generator. Some losses are mechanical, mainly heating the generator bearings and the drive belt. There are also electrical losses. The diodes drop in voltage slightly, which causes the diodes to become hot. The generator windings and internal wiring have resistance and this causes power loss and heat. The changing magnetic field also causes some losses.Keep in mind that most of the load from the alternator on the crankshaft comes from the electrical load effect pulled by the alternator.

Contrary to the myths and what marketing departments and salespeople would have us believe, using an underdrive pulley system isdoesn'tunleash horsepower while racing. It can even do the opposite! It frees up horsepower when idling, but puts more strain on the system when you rev ​​the engine as the alternator tries to make up for the lack of battery charge!

When the alternator shaft speed is reduced, the voltage regulator increases the field current. The governor, as long as the shaft is turning fast enough, increases the field current and pulley torque requirements until the alternator is back and pulling exactly the same engine power as it would have consumed at normal speeds! Because efficiency often decreases at lower rotor speeds, the generator sometimes dragsonlyengine power and runs hotter with an underdrive pulley system than with standard shaft speeds!

The only way to reliably and significantly reduce dynamo drag is to turn off the dynamo while racing, although turning off lights and electrical accessories while racing certainly helps. Please note that when the vehicle is moving, the alternator is trying to supply all the charging energy. At reasonable to maximum engine speeds, usually from 1500 rpm. to redline with heavy loads, and from idle to redline with light electrical accessory loads, the battery just lasts the ride. It doesn't really do anything other than wait for the generator to drop below operating speed. A battery only consumes significant engine power when the battery is almost empty and the missing charge is replenished. A battery, when charged, is really just an electrical storage tank.

NEVER pull on the battery cable to check the alternator.This very crude testing method was somewhat acceptable when we had car radios with vacuum tubes and point ignitions, but now it is a very bad idea. The battery stabilizes the electrical system and charges the alternator, preventing high surges or surges as the alternator adjusts the magnetic flux to produce the same average voltage at different current requirements. If you rev ​​the engine and pull on a battery cable, the alternator voltage may rise to 100 volts or higher before the alternator dies down enough to reduce the voltage to about 14 volts. This can damage the car's computer and other expensive electrical components. I have seen headlights go out when someone opened a battery switch while the engine was running. If you hear someone telling you this is a way to check an alternator in a modern vehicle, stop them!

TEST the generator, is it good or bad?

To charge the battery, the generator output voltage must be higher than aminimumcharging voltage. This minimum charging voltage is 13.8 volts DC across the battery terminals or at the output of the generator. A single lead-acid cell starts charging at just above 2.25 volts. Because a 12-volt battery consists of six cells, any 12-volt lead-acid battery requires at least 13.8 volts to function.Beginto charge. This voltage is sufficient to fully charge the battery or to keep the battery at a maintenance charge, but the charging time will be very long at 13.8 volts.

To fully charge at reasonable times, the alternator output should be 14.2 V to 14.5 V as measureddirectly over the battery terminals. Above the charging voltage of 14.5 volts, batteries have a greatly increased tendency to release excessive acid fumes and hydrogen gas and to corrode things around the battery. The battery terminal charging voltage should beLess than14.7 volts to prevent excessive gas formation. Charging voltages above 14.7 volts can cause the battery to dry out prematurely due to the boiling out of electrolyte, which increases the risk of a hydrogen gas explosion in the battery.

In this case, the battery charging voltage is 14.61 volts while the engine is at high idle. 14.4 volts is the gas limit. The above battery will throttle a little, but not enough to be harmful, and the battery will be fully charged quickly after starting. 14.8 would be a concern (there could be fluid or corrosion on the battery) and 15 volts would be a big deal, but 14.6 is fine. Less than 14.3 would be a "weak" alternator or regulator. Much less than 14.2 at fast idle is bad wiring, a bad alternator or regulator, or a bad connection or fuse connection. During normal and slow driving, the voltage across the battery terminals should remain above 14.3 volts, even at full load, e.g. lights, heater fan and anything else that is working. If this system was in a restored 1966 GT coupe, I would probably adjust the regulator to lower the maximum alternator voltage. This prevents deterioration of the metal around the battery due to excessive charging fumes. In my daily driving it's fine, as long as I keep an eye on the battery for cleaned acid deposits.

If you measure the battery voltage and it is somewhere above 14.2 volts and below 14.8 volts while the car is running at low engine speed and maximum load, you already have a larger alternator than you need. If the voltage is higher than 14.2 under maximum load at speed. minute it is a waste of time and money to buy a bigger generator or a new generator.

Turn off the engine under no load (headlights, etc. all OFF) and read the battery voltage.

With the engine just off, the battery voltage should be 13.2 volts to 13.8 volts. The exact voltage varies depending on the battery, how quickly you read it and the battery's state of charge. This voltage is not that important because the battery will slowly and steadily drop to a new voltage which indicates the true state of charge of the battery, but the voltage measured immediately upon stopping the engine is a very clear indicator of whether the alternator or the charging system is charging. If the voltage is higher than 13.2, the battery was just charging.

So what happens if your battery drains all the time, but the alternator works fine?

Measurement of leakage current in the electrical system

To test the electrical system for unwanted loads that are consuming power, turn off everything in the car. Do exactly the same as when you park the car at night. make sure all lights and accessories are turned off.

Remove the negative terminal wire and check the current draw with all electrical loads turned off using a test light. (I made a test light from an old taillight bulb.)

The dull glow in the light filament indicates a current drain problem. At this point I dodoesn'twant to connect a current meter to check for leakage, because short circuit can damage the test meter! If such a small, bright test light does not illuminate, it is generally safe to measure the current draw directly with a test meter.

Measurement of parasitic current drain

With all electrical loads turned off, connect the meter to a low current scale of approximately 1 ampere in series with the negative battery terminal to ground. The positive meter cable is connected to the chassis of the car and the negative meter cable to the negative terminal of the battery.

Proper battery drainage from the electrical system

This is measured on the 20mA scale. The mA scale indicates thousandths of an ampere. My 1989 Mustang LX, after replacing a bad alternator diode, now has a battery draw of about 1.73 mA. This drain comes entirely from the EEC-IV computer memory. Different radios and different computers may have different standby drains, as can accessories such as clocks, but in no case should the "night off" leakage exceed 25 mA or so. 100 mA is like turning on a small dome light indoors!

My Kenwood stereo draws 1.5 mA when plugged in. If you have a digital clock that stays on, an alarm or some other load, this current will be higher. At 75 mA, leakage can compromise battery life in vehicles that are not driven frequently. mA is milliamperes or thousandths of an ampere.

The meter above has a 20 mA scale and reads 1.73 mA. That is nothing. The battery charge would probably last for months.

Bad battery parasitic drain

If the test light is on, you need to find the wire that charges the battery. First make sure all lights are off. You can do this by having someone open and close things with light, for example using lamps. trunk and look for a defined major load change. You should see a noticeable change in load when you close doors with light, for example the glove box.

Connect the test light in series with the negative terminal and begin running power wires. The first thing to check is the heavy duty alternator charging cable. A bad or leaking diode in an alternator is a common cause of battery drain during the night.

Connect the wires one at a time to see which wire is drawing current. In my case it was the alternator cable! Although the alternator charged fine, it also drained the battery. My problem was a faulty alternator diode. There could be a number of other problems, such as a bad voltage regulator or a stuck relay switch.

Download a wiring diagram

I downloaded this fromT. Moss's website, which I find much more useful than other resources. Tom Moss goes out of his way to help people and is a very nice guy. AutoZone and others also have some free charts.

T.Moss's diagram (linked above) showed me that the heavy dark green wire from my starter relay that was causing my "drain problem" went directly to my car's alternator output. In my case, one of the diodes (the little black "arrows") in my car's alternator was defective. This loss of power also made my alternator feel a bit warm, even when I was outside for several hours.

Other useful voltages

The battery voltage may be higher than 12.6 volts immediately after charging.

Any open terminal voltage below 12 volts is considered a complete discharge or dead battery.

Starters can sometimes come out of the car well, but can also be bad. A common problem with cheaply built or defective starters is the loss of starting torque when hot. This usually happens because the iron loses its ability to hold magnetic flux (starting current shoots up when hot), or the wire is too small and goes up in resistance (starting current drops when hot), or the starter gets stuck (also causes high current). .

The best way to test a starter is to measure the voltageIngift.

To test a starter and wiring with a simple meter:

• Clamp the positive lead from the meter to the jumper cable that goes into the starter
• Cut the black negative wire from the meterENGINE BLOCK
• Make sure the meter is on volts and set it to the lowest possible voltage scale, reading at least 15 volts. In other words, if your meter has a scale of 2.5 volts, 25 volts and 250 volts, use the 25 volt scale. The 25 volt scale is closest to 15 volts, but not below 15 volts.
• Hold the gauge over the starter and watch the gauge as you start the engine.

Make sure the battery is good. There is a voltage table above for battery charging. The battery terminal voltage must be a minimum of 12.6 volts and a maximum of 13.8 volts on no load (all off).

If the starting voltage falls below 9-10 volts, you have a problem with the starting current, the engine ground or the battery.

Measure across the entire battery by probing directly into the battery terminals (NOT the terminals that clamp to the posts, but directly into the lead posts coming out of the battery) and see how much the battery drops when rotated. If it drops and you are sure the alternator is working, take the battery to an auto parts store that tests the batteries. Unlike starters, battery testing is VERY easy and very reliable.

If the battery remains on the posts and the starting voltage drops, you likely have a faulty jumper cable, ground cable, or other wiring problem. If the battery voltage tests fine, but coincides abnormally with your starter, you probably have a starting problem. You must have the starting current checked.

Cheap or poorly made starters usually appear when the starter is very hot. Starters often cannot be bench tested accurately because they often only fail when very hot. I see very few tractors, cars and trucks that start fine when cold and won't start when hot, and have problems other than a starter! My diesel tractor was difficult to start when hot, but ran like a dream when cold and it was the starter. My tractor also has no headers. The block heat alone was enough to stop the starter. I have the same experience with cars. When cold, the starters work and test fine! Marginal starters may have enough power to start well when the system is cold, but fail when hot.

Bad alternators or batteries usually become apparent when the car is very cold, but both alternators and batteries can be reliably tested to see if they are good.

Installing a larger alternator will not fix a bad starter, bad battery, or bad wiring.

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