A quick and correct answer to the question is what current to set when charging the battery - 1/10 or 10% of the capacity. This information is available in all articles about battery charging, including on the Auto without a service station website. However, I was very surprised when I tried to look for a more detailed answer to this question on the Internet. Why exactly 1/10? What happens if you charge with more or less current? Why sometimes the charging current “does not obey”, does not rise or decrease? What does this depend on? I was unable to find answers to all these questions in one material. And you found such material. He is in front of you.

What determines the battery charging current?

If an automatic charger is connected to a discharged battery, the actual charging current will depend on the following factors:

  1. Battery charge level.
  2. Internal resistance.
  3. Charger settings.
  4. Charger capabilities.
  5. Battery charging stage.

Note. I do not support automatic “smart” battery chargers, as I prefer to control the process myself. How to choose a device that will allow you to do this, I described in detail in the material about the criteria for choosing a charger.

Battery charge level

The charging current depends on how charged or discharged the battery is. If you connect an automatic charger to a battery charged 70% or more, the current will be significantly less than 1/10 of the capacity. This is because a charged (or nearly charged) battery consumes less energy. The charging current drops to almost zero as the process approaches the end.

A discharged battery can consume a lot of energy. Therefore, when you connect a “smart” charger, the battery will take as much current as you apply to its terminals. The maximum current is held until the voltage reaches the limit set by the settings. Then it will gradually begin to decline. In a working car battery, this “turning point” occurs after about 8-10 hours of charging. Provided that the total battery charging time is 16-20 hours.

Internal resistance

The internal resistance of a battery is the resistance that the battery cells provide to electrical current. The higher the resistance, the lower the current, and vice versa. All else being equal. Internal resistance is an important characteristic of any battery. It depends on the design, workmanship, type, age.

The battery charge level also affects the internal resistance. For a fully charged battery it is minimal. For a discharged battery - maximum. During charging, the resistance decreases.

During operation, the internal resistance of the battery increases due to sulfation and natural wear of the lead plates.

I spoke in more detail about the internal resistance of a car battery in the hotel material on Auto without a service station.

Charger settings

Almost all chargers have a charging current limitation. In automatic models it is limited by software. In devices with manual adjustments, it is set by the user at his own discretion. At the same time, you need to understand that when charging there will not always be the current that you set in the settings. It will depend on the other four factors.

Charger capabilities

Not all chargers are capable of delivering the optimal current to charge the battery. This is due to their low power. Many cheap models produce a current of about 2-4 A due to the weak “filling”.

For example, my homemade “harvester” for battery maintenance based on the RIDEN RD6006P module is theoretically capable of charging with a current of 6 A. However, when assembling the device, I did not have a suitable transformer and switching power supply. Therefore, the “combine” is capable of delivering only 2.5 A, which is not enough to optimally charge a standard starter battery.

Battery charging stage

The most efficient battery charging algorithm is the CC-CV method. First, the battery is charged with direct current, which is set at 1/10 of the capacity. When the set voltage is reached (14.4-14.8 V), charging continues at a constant voltage. The charging current gradually decreases. Until this stage, the battery accepts maximum current. Technically, a battery is considered charged when the charging current drops to 1% of capacity.

Note. Reducing the charging current to the specified value does not always guarantee 100% charge. Additionally, you should check the density of the electrolyte. This can be done using a hydrometer or refractometer. If the density at this stage has not reached the norm, then this is one of the signs of irreversible sulfation. I recommend that you familiarize yourself with the materials to which I provide links. This will help you learn the battery topic much better.

Optimal battery charging current

Let's return to the question - what current to set when charging the battery. If you haven't done enough research into battery maintenance, I highly recommend filling that knowledge gap. My site will help you with this. In the meantime, use the golden rule: the battery charging current is equal to 10% or 1/10 of its capacity .

A charging current not exceeding this value will allow you to charge any acid battery with an optimal balance between safety and charging time. That is, by setting the current to 10% or 1/10 of the capacity, you will charge the battery as efficiently as possible, without greatly reducing the resource, and without wasting a lot of time.

Note. There are several exceptions to this rule, which I will definitely share with you right now.

Is 10% of the capacity the ideal charge current for all occasions?

The optimal charging current for a car battery is not always suitable. This rule can be unquestioningly followed in the following cases:

If the battery is no more than 2-3 years old and has been used correctly, then use the golden rule. I will tell you about cases when the battery charging current of 10% of the capacity may be insufficient, ineffective, useless, unsafe, and detrimental to the battery life.

High current charging

Charging a battery with high current is not fatal. But not advisable. If we take AGM technology, then these batteries have a design designed for charging with a high current. Read more in the material at the link. Here I will only briefly say why this is so.

AGM batteries have lower internal resistance. This is due to the fact that the electrolyte present in the fiberglass mats contacts the lead plates more effectively. Thanks to this, such batteries can be safely charged with high current without overheating. That is why AGM is the best option today for cars with a start-stop system.

GEL batteries, on the contrary, are not designed for high current charging and are very sensitive to increased voltage. Because of this, they quickly overheat, swell, and collapse. The gel-like electrolyte dries out, cracks, peels off from the lead plates and is no longer in contact with them. As a result, the internal resistance jumps, the capacitance and cold cranking current decrease. If you have such a battery, pay close attention to the charge parameters and monitor the voltage and current with a multimeter whenever possible.

Classic lead-acid batteries with liquid electrolyte can be charged with high current. But it is extremely undesirable. Firstly, such a charge may be ineffective, and the battery will accumulate much less energy than it can. Secondly, high current can cause the battery to overheat, which is guaranteed to reduce its resource.

Set a high charging current only as a last resort. For example, if you are in a hurry, and the starter turns sluggishly or clicks and does not turn due to a discharged battery. When using batteries older than 3-4 years, I categorically do not recommend experimenting with high currents.

Low current charging

Low current charging actually has only one significant drawback - time. The lower the current, the longer it will take for your battery to charge. However, by wasting time, you increase charging efficiency and battery life. I have repeatedly compared the charge at optimal and low current - in the second case, the battery almost always accumulates more actual energy.

Low current charging is useful in the following cases:

  • there is time;
  • you do desulfation;
  • the battery is in storage;
  • The battery is more than 3-4 years old and you want to extend its life;
  • you have a gel battery;
  • the battery overheats when charging;
  • you want to “push” more ampere hours into the battery.

It is advisable to reduce the charge current in another case, which for some reason few people talk about. I've read a lot of books about batteries. I regularly watch professional battery bloggers. I know all the standards. Almost everywhere there is not a word about this important nuance, in my opinion.

Real and nominal battery capacity

I mention real and nominal capacity in almost every material about charging batteries. I consider this aspect important, as well as understanding the difference between these two parameters. In the article about charging current, I simply have to say this again.

The nominal capacity of the battery is the capacity indicated on the battery label. It is declared by the manufacturer and, as practice shows, does not always correspond to reality. Almost everywhere, when choosing the current for charging a battery, this is the capacity that is meant. That is, literally like this: look at the battery case, divide the ampere hours indicated there by 10, and charge with this current.

At the same time, the same instructions often say that the charging current should not be exceeded. I explained above what this threatens. But what's the problem? The problem is that the nominal capacity is just a number on the label. No more. The battery may well not have the same capacity, but...

The actual battery capacity is the actual capacity, the amount of electricity that a particular battery can actually accumulate. As a rule, the rated capacity is equal to the actual capacity in new batteries. Which, however, is also not a fact. If the battery is operated correctly, under normal conditions, then this equality may well be maintained for 2-3 years.

However, life is not ideal, so the actual capacity may be less than the nominal value on the label in the following cases:

  • new battery is defective;
  • or frankly low quality;
  • the manufacturer exaggerated the denomination in order to sell it at a higher price;
  • the new battery is not entirely new, but has been sitting in the store window for many months;
  • with each year of operation, the actual capacity of the battery inevitably decreases;
  • calcium batteries rapidly lose capacity during deep, long-term discharges;
  • irreversible sulfation;
  • short circuit;
  • low electrolyte level;
  • and a million more cases...

What's the result? You look at the label, set the charge current to 10% or 1/10, and charge your battery with a high current with all the ensuing consequences. If the battery is “healthy” and the stated characteristics correspond to reality, then it’s okay. And if not? As a result, the battery prematurely refuses to turn the starter, does not last more than 3-4 years, the charge indicator does not turn green, the density does not increase to normal, and so on. The Internet is overgrown with myths and misconceptions that confuse not only ordinary car enthusiasts, but also professional battery technicians.

How to control the charging current and why is it needed?

Control allows you not to exceed the optimal charging current for your battery. Even if your charger has an ammeter, check it at least once. If no indication is provided, then you definitely need control. Moreover, it is very simple and accessible.

All you need to monitor the battery charging current is a multimeter. Switch the device to the mode for measuring direct current up to 10 A. Do not forget to move the red probe into the socket corresponding to this mode. Connect one clamp of the charger to the corresponding terminal of the battery, as usual. Connect one of the multimeter probes to the second clamp. When you connect the second remaining probe of the multimeter to the free terminal of the battery and turn on the charger, the device will show you the true charge current.

Note. If you are a novice battery enthusiast, be careful when working with a multimeter. It must be configured and connected correctly. To measure current, it is connected in series with the charger and battery. When measuring voltage, be sure to switch the mode, move the probe to the desired socket, and connect the device in parallel to the battery. In case of an error, both the multimeter itself and the charger may fail.

Why doesn't the charging current rise to 10% of capacity?

If the car battery charging current does not rise to the required value, there may be several reasons:

  • the charger technically cannot deliver the required current;
  • incorrect settings;
  • the battery is worn out or damaged;
  • the battery is very deeply discharged and the charger cannot detect it;
  • the ammeter built into the device is lying;
  • the battery is more than 70% charged.

First of all, check the actual charge current using the method described above. If the current is indeed too low, look for the cause among the listed problems.

Why does the charging current not drop when charging the battery?

During the first 8-10 hours of charging a completely discharged battery, the charging current remains stable at the same value. This is normal operation for most chargers. When the set voltage is reached, the current should gradually decrease. If this does not happen, the problem is most likely in the charger and meters.

At the very end of the charge, the current should drop to 1% of the capacity. If the ammeter shows a higher value for two hours in a row and the current does not decrease, there is most likely a problem in the battery. The cause may be increased internal resistance or irreversible sulfation.

Important! If you use “boiling” to force the electrolyte to mix while charging the battery with increased voltage, the current should not decrease. The initially set value may decrease slightly in the first few minutes. Then the current stabilizes and will not decrease. This is normal for the boiling mode.

With what current is the battery charged from the car's generator?

The battery charging current from the generator completely depends on the five factors that I talked about at the very beginning of the article. Additionally, the current strength in this case depends on the power of the generator, engine speed, voltage in the on-board network and included consumers. Immediately after starting the engine, the charging current can reach 10-30 A, since a significant voltage drop occurs during starter operation. However, after just a few minutes (and even seconds), the charge current rapidly decreases as the battery is restored.

Note. Often, large battery charge currents from a generator cause bewilderment, because the numbers far exceed the optimal values. However, it is worth noting that this happens for a short time and does not cause significant harm to the battery. Especially if it is an AGM battery.

5 Common Misconceptions About Charging Current

In conclusion, I simply cannot help but address common myths and misconceptions about the charging current of a car battery.

The battery is charged not by current, but by voltage

In the process of charging the battery, useful work is performed. And only the current can do work. It is this that passes through the battery, facilitating the electrochemical processes necessary for charging. Voltage is just a potential difference. It doesn't do any work. Open the circuit and the voltage will remain. But current cannot flow through an open circuit.

Note. I understand that the roots of this myth are much deeper, and have more to do with confusion in the constant current and voltage charging modes. However, this does not change the essence. The useful work—charging the battery—is performed by the current. Tension only makes this work possible.

Charging the battery with low current is useless and even harmful

I have tested this in practice many times. Yes, there is an opinion that due to low current charging the process is incomplete and even “kills” your battery. However, this is a far-fetched misconception. The proof is in my batteries, which last much longer than the 3-4 years for which they are designed by the manufacturer. Even my charger is capable of delivering only 2.5 A. Why then do 60 ampere-hour batteries charged with it (optimal charge current 6 amperes) last so long?

Charge with low currents if there is time for this or another reason mentioned above.

The battery will not take more current than it needs

This misconception is usually reported by users of low-quality chargers that do not provide any adjustments or stabilization circuits. In fact, if the battery is “healthy” and severely discharged, it itself will not be able to limit its charging current to the required level of 10% of capacity. It will take 10, 20, and even 50 A. Of course, if the current source is capable of this. This is why it is so important that the charger has the ability to limit the charging current.

In a car, the battery is charged from the generator with a current of 100 amperes

This misconception is quite easily refuted in the presence of current clamps. If you don't have them, take my word for it. If the battery is severely discharged, and it is in normal technical condition, then after starting the engine, the charge current from the generator will indeed be high. However, if you observe this parameter for some time, you will notice that the current is rapidly decreasing.

The battery can be fully charged only with pulsating current

This myth was invented by developers and sellers of chargers that can work miracles - resurrect dead batteries. Pulsating current is really useful if you are doing desulfation. However, during normal charging of a healthy battery, no pulsations are needed. On the other hand, this myth is at least harmless. If you have a ripple charger, good. If there are no pulsations, you won’t lose anything worthwhile.


Charging current is the most important parameter of the battery charging process. In most cases, the golden rule of 10 percent of capacity is sufficient. However, remember the nuances and exceptions to this rule that I described. In any unclear situation, charge the battery with low current. But not big. High current is only for AGM and those in a hurry.