How long a battery lasts in losing charge

In this mechanical girl article, she explains how to calculate the time it takes a battery to discharge and can no longer turn on the car.

How long does a car battery discharge?

This is the question that many people ask themselves, and they really search the internet and nobody gives a clear and verifiable answer of how long a battery can last to be downloaded. So in this article I will try to answer this popular question.

The battery

The battery is simply an accumulator of electrical energy in direct current, by means of a chemical process.

So, as I said it is a continuous electrical energy accumulator, it has two important work variables, which are:

Capacity: Expressed in Amper hour (A/H) o (Ah)
Working voltage: Expressed in Volts (V)

The working voltage is self-explanatory, it is the voltage that the battery can supply.

The Amper-hour is the amount of current that a battery can supply for one hour until it is discharged. For example, a battery with capacity of 10 Ah, means that theoretically it can give a current of 1 A for 10 h, or 10 A for 1 h.



I say theoretical because in practice it is not so because the faster a battery is discharged, the more energy is lost by the internal resistance of it. This is why the load capacity in Ah usually goes hand in hand with a time limit and a final voltage that is related to the minimum conditions needed by the battery system. So, for example, for a cell phone the final voltage would be 3.1V, for a car it would be about 10.5V.

How long the battery lasts?

I'm going to give you a fairly simple equation:

Time (hours) = (Battery charge in Ah) / (Consumption in Amperes)

Now to explain this equation with data…

We have a 1000 mAh battery, we want to know how long it will take to discharge up to its working voltage limit if 200 mA current is being used. We do the following:

Time (hours) = 1000 mAh / 200 mA = 5 h.

Important units. If we use milliampere, both above and below we must use milliamperes.

Here is another example: Battery capacity 3000mA, consumption 1A

Time (hours) = 3000 mAh / 1000 mA = 3 h.

As you can see, I took 1A to milliamps so that the equation is coherent.

And what about the batteries for cars?

Many will ask ... but the car batteries are not expressed in Ampere hours, but rather in Maximum Amperes. Of course, because usually the battery for the cars are used only for the start-up and for current peaks that the alternator can not supply (sound equipment with ground floor). The batteries in our vehicles are designed to withstand those enormous peaks of current at the moment of starting, which a normal stationary battery could not supply.

But in these cases we do not have the load data in Ah, how we calculate the download time?

Booking capacity in minutes

Usually with the model and brand of the battery we can go to the manufacturer's page, which will provide us with the following information:


Voltage (v)

start 27º C

Capacity of
Reserve (min)

22MR - 800




24M - 1100




The capacity of reserve in minutes is our data of interest. It is the amount of time that the battery can support supplying 25A without the voltage between its terminals falling below 10.5V.



Knowing this data, we can calculate the capacity in Ah of our battery using this equation:

Ampere hour = (Capacity of reserve in minutes * 25 A) / 60 minutes

An example: for group 22M-800 12V

Ampere hour = (90 min * 25 A) / 60 minutes = 37.5 Ah.

Now with this battery, if we leave our sound equipment on, which uses on average about 8A (normal volume, music without many bass).

Discharge time = 37.5Ah / 8A = 4.68 h => 4 hours 40 minutes

Then in just under 5 hours, we can not hear good music, since the player with less than 10V starts to fail, let alone we can turn on the car.

Capacity of reserve in amps

Some pages can give us is the capacity of reserve expressed in amps, and we can use it directly without doing the transformation. But we must be clear that it is for a final voltage of 10V.

Be very careful with this, since they also usually include the maximum capacity (Voltage 0V) and C20 tests that are at 20 hours. This is the nominal capacity of our battery. These data are not valid for our study.

And well, as you will see, it is very simple to know how long it takes the battery to discharge using the equations that I mentioned. Of course, remember that it is not an exact science because of the problem of the internal resistance of the battery and that it changes with the use of it. But these equations give us a quite acceptable error.

Questions or suggestions

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Written by:

Dennis García

Chica Mecánica.


muy bien

Gracias chica mecanica por este aporte

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    How long a battery lasts in losing charge

    How long a battery lasts in losing charge

    In this mechanical girl article, she explains how to calculate the time it takes a battery to discharge and can no longer turn on the car.

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    2 Question(s) answered

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    • Asked by Alberto
      on 06/30/2019
      Si tengo un carro sin usar, cuánto le dura la batería(batería de gel), lo único que tiene es es la alarma puesta del carro!! Answer:
      Alberto, con alarma y consumo de computador te durará solo de 2 a 3 semanas.
    • Asked by Jose
      on 11/14/2019
      Cuánto me puede durar una batería de 60 amp x 12 v, utilizando un portalámparas de 12v leds 60, la pregunta es cuántas noches de 3 hs de encendida me dura la carga de la batería, es para ir de camping, gracias Answer:
      Que tal Jose, la cuenta es algo como: los 60 led te consumen como 1,2A/h + perdidas asumamos unos 1,35A/h de consumo. Hacemos la cuenta: 60A/h / 1,35A = 44.4 horas de autonomía, casi dos días. Lo que si consigo raro es el dato de 60A/h. Tiene que ser una batería enorme para que dé los 60A/h. Si es de las de 12V de moto o de carritos eléctricos, esas dan desde 6A/h hasta unos 9A/h. En ese caso, debes rehacer los cálculos con ese dato.

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