Another car battery was admitted for diagnostics with complaints about the refusal to turn the starter after a long period of inactivity. Charging with a primitive charger during the day did not give a positive result. Took the battery to check. It turned out that two banks were shorted in the battery at once. How such a diagnosis was made and what to do in such cases - you will learn from this material.

More about symptoms

The failed battery was about a year old in a car that was parked on the street. The engine did not start, as the car was waiting for repair. No one was engaged in periodic recharging of the battery. Moreover, the battery hibernated under the hood, in the cold. Hands reached her already in the middle of summer. Quite expectedly, the engine could not be started on the move...

Next, the owner put the battery on charge. The charger is the simplest - it consists of a transformer, two diodes (half-bridge) and a pointer ammeter. The latter throughout the entire process consistently showed a charge current of 2 amperes. Charging was carried out for a whole day. This is usually enough to revive a healthy battery.

After charging, the battery was installed on the car. From the first attempt to start the engine, it became clear that the battery was dead. After that, the "patient" fell into my hands for diagnosis.

Since I knew the history of this battery, I first made two possible diagnoses - fatal sulfation or an internal short circuit. The owner immediately said that in the first case there is a meager chance to revive the battery a little with the help of desulfation. If the second diagnosis is confirmed - a closed battery bank - you will have to buy a new battery.

Battery check algorithm

For quick battery diagnostics, there are special devices - modern analogues of a load plug. They measure internal resistance, voltage without load and with load. As a result, a brief information about the state of the battery in percent is displayed on the screen.

Detailed material about the internal resistance of the battery

I don’t have such a device, since I don’t professionally diagnose batteries. But some equipment, nevertheless, is available. Including, there are:

  • advanced laboratory power supply based on the RD6006P module, which allows you to monitor the exact voltage and current of the battery charge;
  • electronic load based on the DL24M module, with which I usually measure the real capacity of batteries;
  • multimeter;
  • hydrometer for measuring the density of the electrolyte.

Looking ahead, I note that a multimeter or hydrometer is quite enough to determine a shorted can in a battery.

I usually perform battery diagnostics according to the following plan:

  1. Battery cleaning.
  2. Visual inspection.
  3. Measurement of the self-discharge current on the case.
  4. Charger.
  5. Measurement of real capacity.
  6. Electrolyte density measurement.
  7. Voltage evaluation after charging and idle.

Further actions depend on how the diagnosis went. In this case, there was little hope for desulfation and a strong suspicion that the battery bank had shorted out.

Battery cleaning

During operation, two troubles accumulate on the battery - dust mixed with electrolyte and oxides. There is a separate detailed article about the oxidation of terminals on the Auto without service station website. As for the pollution of the case, the dust with electrolyte is a current conductor, due to which the battery is discharged on itself.

Therefore, the first step is to put the battery in order. To clean the terminals from oxides and salts, you can use fine sandpaper. The case is best washed with water, to which ordinary soda is added. The latter neutralizes the acid contained in the electrolyte. After that, the battery is thoroughly wiped with a damp, clean cloth or dry wipes.

At this stage, it is useful to check the self-discharge on the battery case. All you need for this is a multimeter. The device enters the voltage measurement mode in the range from 0 to 20 volts. One probe is attached to the positive terminal of the battery. The second probe needs to "walk" along the body. If the multimeter shows some voltage, then the battery cleaning needs to be repeated.

Visual inspection

You need to inspect the battery in good light. The task is to find cracks, swelling and other damage.

In this case, a swollen side wall of the hull was found. This symptom indicates two possible problems - the battery either overheated or froze.

The freezing of the electrolyte occurs because in the discharged state it loses its density. In the liquid state, only water or a weak acid solution remains. Such an "electrolyte" freezes at minimal frost, turning into ice. The latter greatly expands in volume, destroying the lead plates inside the battery. Most often this ends with a closure in the cells.

Obviously, our "patient" froze in a discharged state. This is evidenced by the history of the operation of the battery. Accordingly, already at this stage, it is possible to make a preliminary diagnosis - a closed battery bank.


The battery is 12-volt, so it is recommended to charge it with a voltage not higher than 14.4 V. The charge current is defined as 10% of the capacity. And here, many make a mistake, calculating the charge current according to the nominal capacity, which is indicated on the battery case. But in the old battery, it is guaranteed to be below par. Especially if the battery does not show signs of life. Accordingly, in such cases, I set the minimum charge current. Usually it is 1 amp.

In the process of charging, the "patient" immediately showed himself inadequate. The charge voltage at a current of 1 amp rose to 14.4 V after about an hour. This is a bad signal, usually indicating strong sulfation. But not only.

This particular battery has plugs that have been previously unscrewed. This made it possible to confirm the earlier diagnosis - a short circuit. This was indicated by the abundant "boiling" of the electrolyte in four cells, and complete "silence" in the remaining two. At a voltage of 14.4 V, the electrolyte in a healthy battery should not "boil" at all. The maximum that should be - a moderate release of gasses in the form of single bubbles.

If the electrolyte in healthy cells at a voltage of 14.4 V seethes intensely, this means that not all cells are "live". When one bank fails, the nominal voltage of the battery is no longer 12 V, but only 10 V. If there are two banks at once, then it is already 8 V, and so on. Accordingly, when we apply a voltage of 14.4 V to such a battery, electrolysis begins in the "live" cells. The electrolyte "boils", decomposing into hydrogen and oxygen, which in the gaseous state leave the battery forever.

We draw conclusions. In two banks, the electrolyte remained immobile, while in the remaining four, intense bubbling was observed. This is already enough to finally confirm the diagnosis - an internal short circuit.

All further actions are aimed only at ensuring that the diagnosis is 300% correct. It will also come in handy when nothing suspicious can be detected during the charging stage.

Measuring the real capacity of such a battery is pointless. Therefore, I skip this step so as not to waste time.

Electrolyte density measurement

The procedure is only available for serviceable batteries that have plugs. Measurement of electrolyte density is performed using a hydrometer (hydrometer vs refractometer). Additionally, at this stage, it is possible to evaluate the appearance of the electrolyte in the cells.

Very often, a cloudy electrolyte indicates an internal short circuit of the battery. Especially if the cause was freezing. The expanding ice destroys the plates, contaminating the electrolyte. It becomes opaque, cloudy, brown or even black. If the electrolyte in one of the jars is opaque, it is most likely that it is shorted.

In our case, the measurement result is quite expected. In four banks, the density of the electrolyte is normal. Two shorter ones contained not an electrolyte, but muddy water. This means that the cells did not participate in the process of charging the battery, which was established earlier.

Battery voltage after charging

The last method for determining a short circuit in battery banks is to measure the voltage at the terminals immediately after disconnecting the charger. On a healthy battery, the voltage will be above 13 volts. After a few hours of inactivity, it will drop to 12.7-13.0 volts. If such readings of the device are recorded 8-12 hours after a full charge, then the battery is conditionally charged by 100%.

In our case, the picture is radically different. In just a few minutes, the voltage dropped from 13 volts to 10 volts, and continued to decline. Usually, if only one bank is shorted, the voltage drops sharply to 12 volts, and then gradually decreases to 10 volts. If the device showed 10 volts a few minutes after charging, then the short circuit is not in one bank.

Possible causes of a shorted battery

Now you know how to check the battery for shorted cans. This information will save you a lot of time. Especially if you decide to desulfate batteries with shorted cells. Unfortunately, this procedure is effective only for healthy batteries. And that doesn't always help.

We briefly list the possible causes of a shorted battery:

  • mechanical damage to the battery;
  • electrolyte freezing due to low density;
  • long-term idle battery in a deeply discharged state;
  • shocks, falls, strong vibrations, traffic accidents;
  • battery overheating;
  • incorrect charging;
  • frequent "boiling" of the electrolyte.

In my practice, there were about a dozen batteries with shorted banks. There were only two reasons. The first, and most common, is the freezing of the electrolyte and the destruction of the plates by ice. The second is a long idle battery in a highly discharged state. The remaining reasons from the list are taken from specialized literature and personal considerations.

Comprehensive battery check.

Shorted battery bank - what to do?

There are many articles and videos on the Internet that show how to recover shorted batteries. Unfortunately, this information is completely useless. It is technically impossible to restore a battery with a shorted can. Don't waste your time on this. Especially if the cause of the short circuit was the freezing of the electrolyte or a long downtime in a deeply discharged state.

However, such a battery can still be used. But not by car, and very carefully. The fact is that the remaining serviceable cells may well perform their functions. Shorted elements in this case will play the role of jumpers. There are two things to keep in mind when using such a battery. Firstly, the nominal voltage of a battery with a shorted bank will be lower than 12 V. Secondly, such a battery should be charged with an appropriate (reduced) voltage, which will require a laboratory power supply.

When operating batteries with a short circuit, it should be borne in mind that at any time the remaining cells can “die” one by one. You can skip this moment by continuing to charge the battery with a high voltage for it. This threatens with harmful electrolyte fumes, a decrease in its level, as well as overheating of the battery and destruction of the case...

It is best to take the battery with shorted cells to the store where you will buy a new one. The old one will be accepted from you at a fixed rate, and the resulting amount will be subtracted from the price of the new battery. You won’t get much, but you will definitely save 5-10 dollars.