Battery Keeps Running Down

Is it the Battery, Alternator, or Voltage Regulator?

Basic Circuit2

It could be any one of the three, or an undetected voltage drain caused by a trunk light, under hood light, or glove box light that does not go out when the lid is closed.

Battery states

An alternator is based on the rotation of a magnet inside a fixed-loop conductor. The output circuit and the field circuit make up the automotive charging system.

Alternator basics

The first thing that should be checked is the battery state of charge. If it has a built-in hydrometer (charge indicator), a green dot means the battery is 65% to 75% charged and okay for use or further testing.

If the charge indicator is dark, the battery is less than 65% charged and needs to be recharged and load tested.

On 1985 and later model Chrysler vehicles, the charge indicator on some batteries also contains a red dot which shows if the battery is less than 50% charged.

If the charge indicator is clear or yellow, the level of electrolyte inside the battery has dropped too far to give a reading. It also means the battery will need to be replaced soon. Once water level drops below the top of cell plates, they dry out and lose their ability to hold a charge.

Never attempt to jump start or charge a battery with a low electrolyte level. It may explode.

The state of charge of a sealed top battery without a built-in charge indicator can be determined by measuring its open circuit (no load) voltage:

Open Circuit Voltage State Of Charge
12.6v 100%
12.4v 75%
12.2v 50%
12.0v 25%
11.7 or less Discharged

A low charge level does not mean anything is wrong with the battery or charging system, it simply means the battery is low and needs to be recharged.

Performing a load test would be the next step. This checks the battery’s ability to deliver current. The battery must be at least 65% charged before load testing. If not, a good battery may fail the test.

A conventional load test is performed with a carbon pile battery tester. The load created by the carbon pile is adjusted according to the battery’s cold cranking amp (or amp/hour) rating. The carbon pile is usually set to one half the battery’s CCA rating (or three times its amp/hour rating).

Temperature compensation is also important because a cold battery puts out fewer amps than a warm one. The load is then applied to the battery for 15 seconds while voltage output is observed. If voltage remains above 9.6 volts, the battery is good. If it drops below 9.6 volts, the battery can be recharged and retested, or given a three-minute charge test.

A three-minute charge test checks for a sulfated battery. Slow charge the battery at 40 amps for six minutes, then check voltage across the terminals with the charger on.

If the voltage is above 15.5 volts, the battery is not accepting a charge. Slow charging for 20 hours can sometimes reverse the sulfated condition, otherwise the battery is junk.

If the battery check is okay, the next item to check is the charging system. A properly working system produces a charging voltage around 14 volts at idle with lights and accessories off (refer to a shop manual for exact charging specs).

When the engine is first started, charging voltage should rise quickly to about two volts above base battery voltage, then taper off and level out at the specified voltage.

Exact charging voltage will vary according to battery state of charge, load on vehicle electrical system, and temperature. The lower the temperature, the higher the charging voltage. The higher the temperature, the lower the charging voltage.

On a GM application, for example, accepted voltage charging range is 13.9 to 14.4 volts at 80 degrees F. At 20 degrees F below zero, charging range is 14.9 to 15.8 volts. At 140 degrees F, the charging voltage is 13.0 to 13.6 volts.

Charging output can also be checked with an adjustable carbon pile, voltmeter and ammeter. The carbon pile is attached to the battery and adjusted to obtain maximum output while the engine is running at 2,000 rpm.

Basic Circuit

If charging voltage is low, the alternator or voltage regulator could be faulty. To find out which component is bad, a procedure called “full fielding” can be used to bypass the regulator.

If the alternator produces the specified voltage or current output after full fielding, the problem is in the regulator (or wiring) not the alternator.

The exact procedure for full fielding an alternator varies from vehicle to vehicle depending on how the alternator is wired. Basically, the regulator is bypassed by connecting a jumper wire between the field (FLD or “F” terminal) and battery positive (BAT) terminal on the alternator.

On older GM applications with Delco integral regulator alternators, inserting the tip of a screwdriver through the D-shaped hole in the back of the alternator full fields the unit.

Either voltage or current output can be compared against manufacturer specs to determine if the alternator is functioning at full capacity. Generally speaking, alternator output should fall within 10 amps or 10% of its rated capacity at 2,000 rpm.

For several reasons, it is important to follow full fielding test procedures exactly. If only one diode or stator winding is bad, for example, the alternator may still make enough electricity at high rpm to keep the battery charged, but not at idle or low speed. The alternator and/or regulator can also be damaged if the wrong test procedure is used.

On Chrysler externally regulated alternators, for example, you do not apply voltage to the “F” terminal. This system is full fielded by grounding the green wire at the regulator connector. On externally regulated Ford alternators, the alternator is full fielded by disconnecting the four-wire connector from the regulator and jumping across the “A” and “F” terminals.

If charging output goes up when the regulator is bypassed by full fielding, but otherwise fails to produce voltage, check the regulator for a poor ground. This is especially important on Ford and Chrysler systems. Poor or open wiring connections between alternator and regulator can also cause a charging problem.

A slipping fan belt is one of the most common causes of under charging. A fan belt that holds at idle or low rpm may slip when the alternator is under load. Glazed or burned streaks on the belt are an indication of slipping.

If the battery and charging system are okay and the battery keeps running down, check for a voltage drain somewhere in the electrical system. To isolate the cause, remove one of the battery cables and connect a volt meter or amp meter between it and the battery.

A voltage drain will cause a reading on the meter. Disconnect fuses one by one until the circuit is found that causes the reading to disappear.

On-board electronics such as the computer, an electronic clock, etc., will draw a few milliamps all the time, but should not be enough to run the battery down unless the vehicle is not driven for long periods of time.

What size battery is needed?

A battery should be big enough to allow reliable cold starting. The standard recommendation is a battery

with at least one Cold Cranking Amp (CCA) for every cubic inch of engine displacement (two for diesels). CCA rating is an indication of a battery’s ability to deliver a sustained amp output at a specified temperature.

Specifically, it is how many amps a new, fully-charged battery can deliver at 0 degrees F for 30 seconds and still maintain a minimum voltage of 1.2 volts per cell.

A rule of thumb says a vehicle’s battery should have a CCA rating equal to or greater than engine displacement in cubic inches. A battery with a 280 CCA rating would be more than adequate for a 135 cubic inch four-cylinder engine, but not big enough for a 350 cubic inch V-8.

Battery manufacturers have been trying to outdo one another by introducing batteries with higher and higher cold cranking amp ratings. There was a time when a battery with a 550 CCA rating was considered a powerful battery. Now there are batteries with 650, 750, 850, and even up to 1,000 CCA available.

One reason for the “amp wars” between battery manufacturers is that bigger is definitely better. How much overkill is really necessary to assure reliable cold weather starting? Two amps per cubic inch of engine displacement? Three, four or five amps? The bottom line is bigger sells better.

The difference between a group 23 battery and a group 24 battery is 1/2″ in length, 1/16″ in width and 7/16″ in height. It does not sound like much, but it is enough of a difference that the longer battery might not fit the space provided for the shorter battery if a swap were attempted.

Since there is little or no effort on the part of vehicle manufacturers to standardize original equipment battery dimensions, aftermarket battery suppliers are faced with the task of trying to cram as many amps as they can into the smallest battery case that will fit the most applications.

Consolidation reduces the number of different batteries a jobber has to stock to cover the various vehicle applications. It also simplifies manufacturing by building fewer basic battery sizes.

The most powerful battery in the world will not be able to do its job properly if battery cables are not up to the job. One often overlooked source of cranking trouble is undersized battery cables. If the original equipment cables have been replaced with cheap ones with undersized wires, the cables may not be able to deliver the battery’s full amp load to the starter.


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All Around Automotive Repair Inc. is a full-service automotive repair and detail shop—but with a unique calling: to provide affordable service to the entire community regardless of financial circumstances.

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