Incorrect charging of a maintenance-free battery often results in its failure. And all because batteries of this type do not forgive mistakes. Especially those due to which the electrolyte boils and irretrievably evaporates. It is impossible to replenish its level without the barbaric disassembly of the battery. And 9 out of 10 attempts to top up water through a broken body end up with the purchase of a new battery.

Features of maintenance-free batteries

Maintenance-free batteries are usually called batteries that do not provide access to the electrolyte. In other words, they are without traffic jams. It follows that when charging a maintenance-free battery, it is impossible:

  1. Measure the electrolyte level.
  2. Make up for his losses if necessary.
  3. Measure the density of the electrolyte.
  4. Adjust it if necessary.
  5. See the boiling process.

But the above manipulations are familiar and very useful when charging conventional, serviced batteries. For example, they yawned with current or voltage, boiled the battery - and did not even see when and how it happened. The electrolyte level has dropped - it is impossible to top up. There is nothing to say about density at all.

Contrary to common misconceptions, which are briefly discussed below, maintenance-free batteries are made that way for convenience and safety. After all, any inept actions with electrolyte are always fraught. Both for the health of the user and for the battery life. Acid can burn the skin, poison, spoil vision. And if you add electrolyte or even water to the battery without understanding what is happening, you can also take away his health.

Maintenance-free batteries are an attempt by manufacturers to free motorists from the inconvenient and dangerous fuss with electrolyte when charging. But do not think that everything is simple - they did not supply the battery with plugs, and you're done. Keep a maintenance-free battery. No. Everything is much more complicated. At a minimum, since the manufacturer does not provide access to the electrolyte, he makes sure that it is more difficult to boil a maintenance-free battery by inept charging.

This can be achieved by applying special alloy coatings to lead plates. The most common defense against boiling is calcium. Due to its presence on the surface of the plates, the electrolysis process when charging the battery begins at a much higher voltage than without such a spread. For example, if earlier ordinary batteries easily boiled already at 14.5 V, then modern calcium ones do not boil until you apply 15-16 V to them.

Other solutions are also being implemented. For example, a well-thought-out system for relieving excess pressure and balancing it between all compartments. Some manufacturers are getting good results by implementing their proprietary agitation systems to prevent electrolyte stratification. Etc.

From the foregoing, it should be clear that automotive AGM and GEL batteries should also be considered maintenance-free. To learn how to properly charge them, read the links hidden in the names of technologies. Looking ahead, we note that both AGM and GEL batteries have their own nuances regarding charging. And they are different from batteries with bulk electrolyte. That is, the algorithm provided below for gel and AGM batteries is not suitable.

7 myths about maintenance-free batteries

Before you charge a maintenance-free car battery, it is useful to familiarize yourself with the myths and misconceptions about them. Ideally, this should help avoid fatal errors during operation. And for some motorists, the information reviewed will be a good reason to refuse to buy a maintenance-free battery. As well as vice versa - someone even more strongly wants one for himself, or rejoices at what he has already bought.

Inveterate motorists are "afraid" of maintenance-free batteries - it's like a car without a pop-up hood
Inveterate motorists are "afraid" of maintenance-free batteries - it's like a car without a pop-up hood

Myths and misconceptions:

  1. Maintenance-free battery is absolutely maintenance-free. Maintenance of a car battery is not only about the manipulation of the electrolyte. At a minimum, as needed, it must be recharged from a stationary charger. And ideally, it is also desirable to monitor the cleanliness of the case. It is also useful to clean terminals from oxides and salts. In general, he is not so unattended ...
  2. Maintenance-free batteries are a conspiracy. There is a conspiracy theory that manufacturers removed access to the electrolyte not for convenience and safety. And so that the batteries die faster. If you are inclined to believe in this, then it is better to immediately refuse to buy a maintenance-free battery. From other achievements of scientific and technological progress - too. You never know. All of a sudden, it's all a conspiracy.
  3. Maintenance-free batteries have plugs, but they are hidden. One of the branches of conspiracy theory in the world of batteries. If you shine a light on some maintenance-free batteries, you can see through the light something very reminiscent of traffic jams. But if such a battery is dismantled, then the “plugs hidden by cunning manufacturers” will turn out to be nothing more than the passages of a gas circulation system and pressure balancing.
  4. Without vandalism, it is impossible to find out the electrolyte level in a maintenance-free battery. Can! Sometimes in two ways at once. First, there are maintenance-free batteries with an indicator. The second is the electrolyte and its level can be seen through the light.
  5. Maintenance-free batteries are not afraid of tilts and flips. Gel and AGM batteries are really not afraid. But as for models with bulk electrolyte, they cannot be tilted or turned over. This is usually written in the instructions for them. But who reads them.
  6. Charging a maintenance-free battery can result in an explosion, as there are no plugs to remove. To relieve excess pressure in the process of charging a maintenance-free battery, it has a special system of channels and valves. And sometimes they are inflated not from gas pressure, but from overheating and deformation of lead plates.
  7. You can replenish the level of lost electrolyte in a maintenance-free battery through the control indicator. In one of the six compartments - really possible. If you manage to extract this very indicator. In the remaining five "cans" in this way, adding nothing will work. They are separated by partitions.

These are not all myths. But the task was to consider those that relate to the issue of charging maintenance-free batteries.


Proper charging of maintenance-free car batteries requires knowing the answers to the following questions:

  1. Which charger is suitable?
  2. From what charge do maintenance-free batteries die most often?
  3. When should you charge?
  4. What voltage?
  5. What kind of current?
  6. How long?
  7. Do I need to boil a maintenance-free battery when charging to avoid stratification?

Here are the answers to all these questions.

1.      What charger to charge?

It's not worth overpaying for "smart" chargers
It's not worth overpaying for "smart" chargers

The most detailed information about the competent choice of a charger for charging a car battery is described in a separate material. You can find it at the link above. If you are too lazy or have no time to read another article, here is a short list of requirements that a charger must meet to charge a maintenance-free battery:

  • the presence of regulation or automatic limitation of the charging voltage;
  • the presence of setting the initial charging current;
  • control devices for measuring voltage and current.

Everything. The first two requirements are met by almost all memory devices costing more than $50. As for measuring current and voltage, you can use a multimeter for these purposes. There is no real need to buy a charger that is supposedly designed specifically for charging maintenance-free batteries. They are usually expensive. And the filling of many of them is more miserable than that of conventional models.

2.      How to kill a maintenance free battery?

There are many ways:

  • charge with voltage above 14.4 V;
  • or current greater than 10% of the real capacity;
  • boil;
  • operate in a semi-discharged state;
  • allow deep discharges.

Another battery of this type is easy to ruin by trying to add distilled water or electrolyte into it through the holes drilled in the case with a drill. As a rule, those who charged the maintenance-free battery incorrectly come to this.

3.      When should you charge?

Under certain favorable conditions, a car battery of any type is able to work out its entire resource without a single recharging from a stationary charger. This is extremely rare. Most often, the conditions necessary for the natural maintenance of a normal charge level are not met. Therefore, it is periodically necessary to charge a maintenance-free battery “manually” if:

  • your car has low on-board voltage;
  • the car is rarely used;
  • or for short trips;
  • if it is winter now, then the previous two points are aggravated ten times;
  • According to a number of signs, the battery is not sufficiently charged.

How to understand that the battery is not sufficiently charged? Measurement of electrolyte density is no longer required. There are two options left. The first is by the indicator on the case, if any. Secondly, the site has a whole separate material about battery voltage. It tells, among other things, how to determine the charge level by voltage, and how to measure it correctly.

4.      What voltage to charge?

With the charge voltage of any battery, you must be extremely careful
With the charge voltage of any battery, you must be extremely careful

At the first stage of charging a maintenance-free battery, you need to focus primarily on current. The voltage at the terminals during this period will be much less than the maximum allowable. When the battery recovers a little, the volts will creep up. They need to be limited at around 14.4 V. If this value is exceeded, the electrolyte will boil and evaporate irrevocably. The boiling of batteries during charging is described in detail in a separate material on the site.

5.      What current to charge?

At the initial stage of charging, the current will most likely have to be limited according to the standard 10% rule. That is, we take the capacity indicated in ampere-hours on the battery case, divide by 10, and charge it with this current. Do not confuse the capacity and the inrush current, which is usually written on the case in large print, and therefore catches the eye. The starting current is indicated in amperes, and the capacity in ampere-hours. The first indicator is usually ten times greater than the second.

And one more important point regarding the charging current of a maintenance-free battery (any other too). If you calculate the current strength according to the 10% rule, then take into account the natural wear and tear of the battery. That is, if a capacity of 70 ampere-hours is indicated on the case of your battery, and you bought it a couple of years ago, do not rush to set the current to 7 A. Most likely, the battery that has served a couple of years no longer has the declared capacity. Accordingly, the charge current must be chosen less.

If it is not possible to measure the current real capacity, discard 10-15% from what is written on the case for each year of operation. For example, if the battery says that it is 65 ampere-hours, then set the charge current to 6.5 A in the first year, 5.5 A in the second year, and so on.

6.      How much time to charge?

The time it takes to fully charge a maintenance-free battery depends on the following factors:

  • the degree of discharge before charging;
  • battery capacity (real);
  • charge current;
  • ambient temperature.

If your charger is not automatic, then you need to focus on time last. Too many variables. It is much more correct to judge the completion of the charge by voltage and current. Namely, if at a charge voltage of 14.4 V the current did not decrease for two hours in a row, then the maintenance-free battery should be considered fully charged.

7.      Boiling to avoid stratification

Stratification is a problem of all batteries with bulk electrolyte, which consists in the stratification of the latter. As a result, water accumulates in the upper part of the battery, and acid, due to its greater density, settles at the bottom. Boiling is one way to prevent stratification. However, this adversely affects the resource of lead plates. What kills the battery faster is an unanswered question. No one has yet compared two batteries, one of which was boiled, and the other was the same - protected in this way from stratification.

The manufacturer is fighting with all his might so that we do not boil. But we keep boiling as best we can
The manufacturer is fighting with all his might so that we do not boil. But we keep boiling as best we can

Many manufacturers write about this nuance in the instructions for charging maintenance-free batteries. If there is not a word of such information in the piece of paper attached to your battery, then it is better not to boil. There will be no fatal harm from this.


As a result, we get such a brief instruction for charging a maintenance-free car battery:

  1. Determine the need for charging.
  2. Check over pressure relief valves (they must not be clogged with dirt).
  3. Limit the charging current on the charger in accordance with the actual battery capacity (10%, taking into account the service life).
  4. When the voltage at the terminals reaches 14.4 V, make sure that it does not rise higher.
  5. Charge until the charging current is stable at 14.4 V for two hours.

If, nevertheless, you decide to boil the battery in order to mix the electrolyte in order to avoid stratification, do it like this. At the very end of the charge (see paragraph 5 above), raise the voltage to 15-16 V, and charge in this mode for no more than 30-40 minutes. At the same time, do not move away from the battery - if the case is heated above 40°C, a whistling or hissing sound appears - immediately unplug the charger and complete charging.

It should also be clear that maintenance-free batteries such as GEL and AGM are not afraid of stratification. Accordingly, you should not try to boil them.


VIDEO: about maintance free car battery