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I recently learnt that the voltage of the battery (for example, a 9V battery) is constant at their bounds, whereas the battery current depends on the load that is connected to it.

If I connect a 12V car battery to a smartphone in cigarette lighter socket my phone will only draw for example 50 mA but my computer a little more, for example 100mA.

If the current passing through in the circuit is a variable, why does my battery have a limitation of current that it can supply? After all I could more quickly discharge my car 12V battery by connecting it to a 1000 W heater.

Why do I have a current limit?

Is a battery a source of current and also a source of voltage so a source of 2 things ? Or is a battery only a potential difference between their 2 bounds ... plus and minus... and the current depends of the plugged loads resistors and the like?

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  • \$\begingroup\$ If you want to learn about batteries, it looks like batteryuniversity.com has a lot of information. (I have no connection to that site.) \$\endgroup\$ Commented Jun 24 at 18:51
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    \$\begingroup\$ It is like, your car only goes at the speed you tell it to go by pressing the pedal, so why does it have a limitation on the speed it can go? \$\endgroup\$ Commented Jun 25 at 12:05
  • \$\begingroup\$ You can discharge it really fast if you drop a wrench across the terminals.... There's circuitry between the cig lighter and the battery preventing your cell phone from becoming a similar pile of plasma. With the over 600 cold cranking amps inside a car battery, there isn't a noticeable 'limit' when something goes wrong. \$\endgroup\$
    – Mazura
    Commented Jun 25 at 15:42

2 Answers 2

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The electrochemical reactions in the battery can only take place so fast. With some batteries the current should be artificially limited to protect the battery from self-destruction. It may be able to produce a high current for a short time and then chemical products build up that limit the current ("polarization").

The electrolyte and connections will have some resistance and that limits the current. From this CR2032 '3V' lithium coin cell battery datasheet, consider these curves:

enter image description here

The voltage is roughly constant with a light load for most of the useful life - between 2.8 and 2.9V. The internal resistance (IR) starts off at around 10Ω and increases to more than 20Ω (2:1) during that portion of the useful life. So the more current you draw (pulses) the lower the voltage drops, and it gets much worse as the battery is depleted.

The graph tells us that if we apply a 10Ω load we'd get less than 1.5V out and thus less than 150mA. This is also the most power this battery can deliver when fresh- 0.23W - half the power is provided to the load and half is wasted in the battery. This battery is primarily used in applications where the current draw is rather low, far from the maximum power transfer point.

It's possible to design batteries that can supply extremely high currents for short periods of time, for applications such as power tools and electric vehicles. The current will always be limited, of course.

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  • \$\begingroup\$ Hi, what does the horizontal axis of the chart represent? Is it total removed charge during one discharge cycle? \$\endgroup\$ Commented Jun 25 at 0:15
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    \$\begingroup\$ @FabioBarone Yes, total removed charge from fresh. This is a primary cell, so there is only ever one cycle. \$\endgroup\$ Commented Jun 25 at 1:46
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    \$\begingroup\$ The 12V car battery in your (@user381936) Q is another example of a battery designed to deliver high currents briefly when cranking, as well as low continuous currents (w.r.t. the last paragraph). The internal resistance is low enough that for small loads it can be treated as zero (like the phone charger in the question) but when the starter motor runs it's certainly noticeable, leading to effects like the radio in my van rebooting from low voltage \$\endgroup\$
    – Chris H
    Commented Jun 25 at 8:22
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    \$\begingroup\$ On the other end of the design spectrum, a deep cycle battery designed for an RV might be rated to store 50-100% more power in the same space, but would significant wasted power if you used it as a starter battery. By adjusting the internal geometry to change the structures involved in the reactions, you get very different properties. \$\endgroup\$ Commented Jun 25 at 18:51
  • \$\begingroup\$ @user1937198 worse, drawing current too fast from deep cycle batteries degrades them. Even in parallel with the vehicle's starter battery and with series resistance from a long cable if you make a habit of it \$\endgroup\$
    – Chris H
    Commented Jun 26 at 8:14
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An ideal voltage source can supply whatever current the load wants, unlimited.

But a battery is not an ideal voltage source. So, it can't.

A battery can be modeled as a voltage source plus a series resistance. The current results in a voltage drop across that resistance which manifests itself as a voltage sag. So, a 9 V battery may read 9 V when not used, but may read, say, 8 V under load. The remaining 1 V is across that series resistance and it wastes power as heat.

Cell equivalent model

{Source: me}

This effect prevents the battery from providing unlimited current. Indeed, the most power you can get out of a battery is into a resistor whose value equals the internal resistance of the battery. If so, the battery voltage is half of the nominal (e.g., 4.5 V) and the power wasted in heat is equal to the power doing work in the load (the efficiency is 50 %). This is called the "Maximum Power Point".

Additionally, there are physical effect that limit the current even further, such as the mobility of the ions inside the battery cells. Battery cells are permanently degraded when discharged at a high current. Which is why manufacturers specify a maximum current rating. Its value is not a hard limit: degradation occurs even if the current is less than the rating, just not as fast.

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