The indicator built into some batteries is designed for quick and convenient control of its condition. It is able to work in 3-4 modes: the charge is normal; the battery is low; deep discharge (not on all models); insufficient electrolyte level. To save battery life, you must strive to ensure that the indicator is green. This is usually easy to achieve. But what if the "peephole" does not turn green even after charging from a stationary charger? You will find the answer to this question in this material.

Important clarification! The green color of the indicator means that the battery is 65-100% charged. However, it is believed that long-term operation of a battery charged by less than 75% significantly reduces its resource. Some manufacturers are cunning, indicating on the body that the green color is more than 75%. But in practice, it is physically difficult to make an indicator that would work in such a narrow range (75-100%).

Therefore, it is impossible to focus only on the indicator. From time to time, you need to evaluate the condition of the battery in other ways. For example, check the density of the electrolyte with a hydrometer or measure the voltage at the terminals (the article on the link tells how to do this correctly).

The device and principle of operation of the battery indicator

At its core, the battery indicator is a primitive hydrometer - a device designed to measure the density of the electrolyte (hydrometer vs refractometer). Technically, this is a small flask, inside of which there is a transparent stick and two balls - red and green. They are made from different density materials. Due to this, they have different "buoyancy", depending on the density of the electrolyte.

Design feature. It is important to understand that the indicator "monitors" the state of the electrolyte in only one battery cell. Moreover, the balls are always only in the upper electrolyte layer. This means that the indicator readings refer to only one point in the accumulator. In other cells (and even in the lower part of the same cell in which it is installed), the level and density of the electrolyte may be different.

The principle of operation in the main modes:

  1. Green indicator with a red dot in the middle. This state means that the electrolyte density is higher than 1.25 g/cm3. The battery is charged at 65-100%. Technically, both balls float, with the green one below the red one. In this position, they are pressed against a transparent stick, which gives the described picture.
  2. White indicator with a red dot in the middle. The density of the electrolyte is 1.23-1.25 g/cm3. The battery is half discharged or more. Technically, the red ball is still buoyant, but the green one has sunk to the bottom of the flask. This mode is not available on all batteries.
  3. Red indicator with a black dot in the middle. Indicates a deep discharge of the battery. Technically, the density of the electrolyte decreases so much that even the red ball drops, setting the background of the indicator. Since nothing rests on the end of the tube, the user sees a black dot.
  4. The indicator is red with a white dot in the middle. Indicates insufficient electrolyte level. Technically, all the balls have sunk to the bottom of the structure, since there is no liquid in which they could float.

Structurally, there are two main types of indicators. In a simpler version, both balls are located one above the other in one vertical flask. In more complicated options, each ball has its own inclined bulb, which allows you to get an additional mode of operation of the device.

Why does the indicator not turn green after charging the battery and what should I do?

When the battery is used correctly, there are usually no problems with the indicator. His testimony is clear and logical. But sometimes users are faced with the fact that the indicator does not turn green, although the battery has just been charged from a stationary charger. This can be due both to the carelessness of the user, and to the peculiarities of the operation of modern batteries.

There are 5 possible reasons why the indicator on a charged battery does not turn green:

  1. The battery is not actually fully charged.
  2. Low electrolyte level.
  3. Uneven electrolyte density.
  4. The indicator is stuck.
  5. Strong sulfation.

Let's consider each of the reasons - why the indicator does not turn green when it seems to be, and how to "make" it work correctly.

Cause 1: The battery is not fully charged

This reason is often faced by owners of cheap chargers. Especially automatic ones, which themselves "decide" when and under what conditions to stop the process.

The problem is that some chargers are physically unable to fully charge the battery. In manual models, this happens due to primitive circuitry and extremely inaccurate ammeters and voltmeters. A bad automatic charger may misjudge the current parameters of the charging process, terminating it earlier than necessary.

What to do? Charge the battery fully using a normal charger. There are several ways to find out if the battery is fully charged. For example, according to the charge current or electrolyte density - if at the end of the charging process they did not change for two hours, then the battery no longer accepts a charge.

Reason 2: Low electrolyte level

Often the indicator is not looked at very carefully or in poor lighting. With a discharged battery, or with an insufficient electrolyte level, the background of the indicator is red. Only by the point in the middle can one understand the difference between these modes. If it is white, then there is not enough electrolyte. Even if the lead plates are covered with an electrolyte layer, this does not mean that there is enough of it. There are two ways to check the level.

The first way is with a transparent tube. It must be lowered into the electrolyte to the plates, pinched with a finger on the other side, and carefully pulled out of the battery. The length of the liquid in the tube should be 25 mm.

The second way - if the design of the battery allows, you can look at the current-carrying bridges that connect adjacent cells. If they are not immersed in the electrolyte, then its level is below normal.

Important! If the level is insufficient, in no case should it be replenished with electrolyte. Top up with distilled water. In this case, the same level should be achieved in all cells.

Immediately after adding water, the indicator will not turn green. This is due to the following reason. However, if the level was insufficient, then the color of the dot in the middle after adding water should change from white to black.

Reason 3. Uneven electrolyte density

Due to the design features, the indicator evaluates the level and density of the electrolyte only at one point of the battery. After adding water at this point, it turns out to be almost pure. But this problem exists even in cases where the electrolyte level is normal, and you did not add water to the battery.

This is due to the design features of modern batteries. The fact is that lead plates are placed in separators. When the battery is charged, the density of the electrolyte in the separators increases. If the charge is completed without "boiling", the denser electrolyte from the separators does not mix with that which is in the upper layers, above the plates.

How to solve this problem is clearly shown in the master class on charging a calcium battery.

Reason 4. Hanging indicator

In order for the balls in the indicator bulb to move freely, the electrolyte in the battery must be clean. If it is cloudy and with suspensions, then the flask may become clogged with solid debris that blocks the movement of the balls. You can check this at the stage of measuring the electrolyte level. In a new battery, it is crystal clear. After 2-3 years of operation, it may be slightly cloudy, which should not affect the performance of the indicator. If the electrolyte is dirty, then this can cause a malfunction not only of the indicator, but of the entire battery.

At the initial stage, this problem can be temporarily eliminated by lightly tapping on the indicator.

Reason 5. Strong sulfation

Sulfation of lead battery plates is a separate big topic, which will not be considered here. Let's just get the gist. Sulfation is normal and irreversible. Abnormal sulfation occurs every time the battery is discharged. Lead sulfate forms on the plates. When the battery is charged, it disintegrates, due to which the density of the electrolyte increases.

With irreversible sulfation, not all lead sulfate participates in the reaction, remaining on the plates forever in the form of solid gray crystals. Physically, this means that part of the sulfur is not returned to the electrolyte, which is why the normal density is not achieved. Accordingly, the green ball cannot float in such an electrolyte and the indicator does not turn green because of this.

If the problem is noticed at the initial stage, it can be partially eliminated. What is desulfation and how to perform it, read the material at the link.


The built-in indicator allows you to control the current state of the battery without tools. Depending on the color, it may indicate a normal or low battery level, as well as an insufficient electrolyte level. If the indicator does not turn green when charging from a stationary charger, there is a simple reason that is easy to identify and eliminate.