The average normal charging time for a car battery is 15…20 hours. But this is under ideal conditions. More than 10 factors influence the duration of battery charging, and each of them can both reduce the average indicator and increase it. For the average user, time is a status indicator. If, other things being equal, charging has accelerated, this means that the battery life has been reduced. When the process is completed in just 2...3 hours, this indicates the unsuitability of the battery for further operation on the machine. From this material on the site ABS you will learn: what factors affect the charging time; how to determine the end of charging; is it possible to speed up the process; what conclusions can be drawn from the actual battery charge time.

What determines the charging time of a car battery?

Charging time is a very informative indicator of the state of the battery. However, it is not worth focusing on it as the main one. You can not charge the battery strictly according to the timer. If you control the process only on the basis of the clock, then there will be either an undercharge of the battery or a useless waste of time. What is really worth focusing on in order to assess the degree of charge of the battery is described below.

The following factors can potentially affect battery charge time:

  1. Capacity.
  2. Current charge level.
  3. Charging current.
  4. Charge voltage.
  5. Process efficiency.
  6. Temperature.
  7. Battery type.

What are ideal conditions? If the battery was charged from 0 to 100% with the same current, and received energy, then the time could be calculated elementarily. Example. A battery with a capacity of 70Ah would be charged in 10 hours with a current of 7 A or 70 hours with a current of 1 A. In real life, this does not work. And that's why.


Battery capacity is the energy that the battery is able to accumulate during the charging process and give to the load. Expressed in ampere-hours. For example, a battery with a capacity of 65 Ah is capable of supplying a load of 6.5 A for 10 hours. The more amp-hours, the longer the battery can power the same load. It also works in the opposite direction - the more ampere-hours, the longer it takes to charge the battery.

It must be understood that the battery capacity is of three types:

  1. Nominal - declared by the manufacturer, indicated on the case, significantly affects the cost. Technically depends on the amount of lead used to make the plates. The technology by which the battery is made also affects.
  2. Real - the energy that the battery is actually able to accumulate and give. As the operation progresses, it gradually decreases due to sulfation, degradation and shedding of lead plates. When the battery is working under normal conditions, the actual capacity is close to the nominal capacity for 1-2 years. In 5 years, the potential may be reduced by 20-40%. When the actual capacity of the starter battery is reduced by 60% of the nominal capacity, it is considered unsuitable for further operation on a car.
  3. Reserve - this is the time during which a charged battery is able to provide a load of 25 A. It is indicated on the battery case in minutes (not always). The reserve capacity is used when the generator fails.

Now we are only interested in the actual capacity of the battery. It directly affects the charging time.

Current charge level

The more the battery is discharged, the longer it takes to charge. The dependence of these two parameters is not linear. A 100% charged battery will not take twice as long to charge as a 50% charged battery. The reason is the charging current.


This is one of the main factors that affects the charging time. The higher the current, the faster the process. However, it is impossible to infinitely increase the charging current, as this will lead to a violation of the charging technology. It will be ineffective. The battery life will be reduced. Most modern batteries can be charged with currents up to 20 A. When we start the engine from an almost discharged battery, it consumes about the same current from the generator (the first few minutes).

When charging from a charger, the optimal charging current is 10% of the capacity. For a 70 Ah battery, the recommended charging current is 7A. Some chargers limit the current according to the program. Others are manually adjusted. In unregulated budget devices, the current is limited by the power of the transformer.

The battery takes the maximum current only at the very beginning of the process. After a few hours, the internal resistance of the battery increases. The current gradually decreases. Because of this feature, we cannot accurately calculate in advance how much the battery will be charged.

Not all chargers are capable of delivering the maximum recommended charger. Cheap models cannot deliver more than 2...3 A. The charging time for such devices increases significantly. However, a large current is needed only in the first stages of charging. When the maximum possible charging current is halved, the charging time is not doubled, but only one and a half times.


Charging a battery obeys Ohm's law. Therefore, the voltage also affects the charging time. In order for the battery to start charging, a voltage slightly higher than its own must be applied to its terminals. As the battery saturates, the voltage has to be increased, as the internal resistance of the elements increases. Also, for the charging process, the rule is fair: the higher the voltage, the faster the charge will pass (since the current will also increase).

Detailed material about the internal resistance of the battery

Impossible to increase the charge voltage indefinitely. The limit depends on the technology by which the battery is made. For example, hybrid batteries need 14.2 V to fully and safely charge. GEL and AGM batteries require 14.8 V. Calcium batteries charge normally at 14.4 V (but never charge to 100% at that voltage).

Why can't a battery be charged with an infinitely high voltage? In the case of batteries with liquid electrolyte, exceeding the maximum allowable voltage leads to the start of electrolysis. Water (the basis of the electrolyte) begins to decompose into hydrogen and oxygen. The charge actually stops. The battery "boils", heats up, and loses water from the electrolyte.

Charging efficiency

Technologies for the production of car batteries are constantly being improved. However, there are still no 100% effective models. To store 70 amp-hours of energy in a battery, it needs to give 30% more energy. This is spent on heating, self-discharge and current leakage.

The efficiency of a battery gradually decreases as it is used. Dirty case, sulphation, shedding of plates and other factors - reduce efficiency. However, the charging time does not increase, but rather shortens. This is because the actual capacity of the battery decreases over time.


The optimum ambient temperature at which a car battery charges most efficiently is +25°C. When reduced to -5°C, the battery can be charged normally. But time will increase. When the temperature is significantly below 0°C, then the battery practically does not take a charge at all for the first 30 minutes. This is one of the reasons why batteries often fail in winter. They simply do not have time to recover from the generator.

High ambient temperatures do not speed up battery charging time. On the contrary, the battery begins to overheat, which significantly reduces efficiency. AGM batteries are especially vulnerable to overheating. They begin to work poorly and irreversibly degrade at temperatures above +40°C. Batteries of other types "do not like" temperatures above +55°C.

Car manufacturers take this into account when choosing a place to install a battery. There are even models in which the batteries are located in the cabin or luggage compartment. This approach is not widely used for a number of reasons. First, not all batteries are safe. Secondly, the farther the battery is from the starter, the thicker wires are needed to connect them.

Battery type

Battery type affects:

  • Maximum recommended charge current.
  • Battery efficiency when charging.

Batteries on cars with a start-stop system must necessarily be designed for high currents (fast charging). These include AGM batteries, which can be systematically charged with currents up to 30% of capacity. Much better in this regard are lithium-ion, lithium-polymer and lithium-iron-phosphate batteries. They can be discharged and charged with very high currents without consequences. They are not widely used yet due to their high cost.

How to assess the battery charge level?

Total: it is impossible to accurately determine the level of charge of the battery by time. What then should be the focus?

There are as many as three very accurate indicators:

  1. Charging current.
  2. Electrolyte density.
  3. Rested battery voltage.

Charging current

This indicator is suitable for chargers with digital voltage and current indicators. These indicators can also be measured independently. With a multimeter. The current in the circuit is the same everywhere. Therefore, measure it where it is convenient. Voltage must be measured directly at the battery terminals.

The battery can be considered 100% charged if the measurements show the following values:

  1. Voltage 14.4 V.
  2. Current 0.1 A.

It makes no sense to wait until the charging current drops by more than 0.1 A. After reaching this current, the battery either accumulates nothing at all, or accumulates very little energy. That is why almost all automatic and "smart" chargers are turned off at this stage.

Electrolyte density

This method is only for batteries with liquid electrolyte, to which there is access. AGM, GEL, maintenance-free batteries with liquid electrolyte are not suitable.

About charging maintenance-free batteries.

The density of the electrolyte is determined by a hydrometer (hydrometer vs refractometer). An important advantage of the method is the ability to assess the charge level directly in the charging process. The concentration of acid in the electrolyte increases synchronously with the accumulation of energy.

When the density of the electrolyte reaches 1.26...1.28 g/cm3, the battery can be considered 100% charged. Note that the density of the electrolyte near the lead plates may be normal, and in the upper layers, the indicators are almost always lower. To equalize the density, it is necessary to mix the electrolyte. There are several ways to do this. For example, forcibly "boil" the electrolyte, charging the battery with high voltage.

The “boiling” of the electrolyte is described in more detail on the website ABS in the following materials:

I recommend to take a look.


It is a simple, affordable and accurate assessment method. However, the battery terminal voltage cannot be measured at any time. Especially while charging. To obtain reliable readings, it is necessary to wait 12 hours after charging is completed, and only then measure the voltage. If you do this earlier, the voltage will be overestimated and has nothing to do with the level of charge of the battery. Measure the voltage the next day, and compare the readings with the data in this table.


Quiescent voltage (V)

Charge (%)

< 11.90























Incorrect measurement


Can the battery charging time be shortened?

Yes. But this should not be done unless absolutely necessary. To speed up the process, you will have to increase the current and voltage. And this will negatively affect the battery life. Use fast charging only in exceptional cases. For example, when you are in a hurry.

Video: how long to charge a car battery


It is not worth focusing your attention on the battery charging time. Keep track of voltage, current, electrolyte density (if possible). However, if you notice that a deeply discharged battery is charging too quickly, this may be a bad sign. Perhaps it's time to think about choosing and buying a new car battery.