If a battery can't supply enough current, does voltage read lower?

In summary, current ratings for batteries are simply a conservative estimate of the maximum current the battery can tolerate before it begins to take damage.
  • #1
overtak3n
27
0
What happens if a battery cannot supply enough current?

Let's say we're using a 10V battery capable of supplying 1A of current.

A single resistor is then connected so that the current is flowing through it.

Using V=IR, if the resistance is 10 Ohms, current is 1A.

What if the Resistance is 5 Ohms, you'd need a 2A current for 10 V, but the battery is not capable of supplying that current.

Does that mean the 10 V battery is only giving 5 V? And would the reading with a multimeter show 5 V?
 
Physics news on Phys.org
  • #2
There is not a single value of current that a battery "can supply". A battery behaves as though it has an 'ideal' voltage source and an 'internal resistance' inside it, in series with the voltage source. You cannot make a connection to bypass this internal resistance, btw, because it's all part of the chemistry that's going on.
The internal resistance is only an approximation, though, and a very heavy current can cause the internal resistance to rise and damage the battery, through over heating etc. That's more advanced!
Until you overload the battery, there will be a steady drop in terminal volts as you take more current That will be due to Vdrop = I X (internal resistance).
Your description is a bit 'back to front' but contains some of the right ideas. Bearing in mind that it is the basic voltage of the battery (the emf) that you start with. If 1A drawn causes a reduction in terminal volts of 0.1V, then the internal resistance is 0.1Ω. Taking 2A will drop 0.2V etc.

In your model, putting a 5Ω load is causing a 5V drop so the internal resistance must have been 1Ω. The battery is 'not really suitable' for that load. lol.

OH yes and the rating of a battery will refer to the current that it will deliver without the volts dropping by a certain percentage - say 5%. Battery suppliers (AA and D types etc.) can be a bit cagey about this. Car starter batteries are often specified in terms of 'stall current' which it the maximum current it will deliver to a starter motor that is designed for 12V and is prevented from rotating (i.e. stalled). By most standards, that will be the nearest thing to a short circuit that you will come across in every day life. You can only subject them to this treatment for a few seconds or they destroy themselves.
 
Last edited:
  • #3
Heavy current would mean the internal resistance would need to lower to maintain same voltage?

Ok, so if there is current overload, and the battery can't handle it that means the voltage would be less because the battery supplies only the current it can handle?
 
  • #4
The battery is described by the voltage ##U_0## which is measured when no current is flowing (i.e. for a voltmeter with ideally infinite resistance) and the internal resistance ##R_i##. If you connect an external resistance R to the battery, the voltage drop on the external resistor will be ##U=R/(R+R_i) U_0##, i.e. less than the nominal voltage of the battery.
 
  • #5
So if there is a resistor with low resistance, the internal resistance of the battery will limit the current?

I'm still not quite understanding what happens if current exceeds what the battery can handle.
 
  • #6
The current will never exceed what the battery can handle, as the battery is what is providing the voltage to move the current. However, the chemical reactions that go on inside the battery can only take place at a maximum rate, so there is a maximum current the battery can supply. This maximum current would most likely be equal to the current flow if we short out the terminals. Of course doing so would probably lead to the battery exploding from the heat buildup by all those reactions.
 
  • #7
Thanks for the response.

I was just curious why batteries have current ratings. I'm now understanding that's the max current it can tolerate before the battery begins to take damage.
 
  • #8
overtak3n said:
Thanks for the response.

I was just curious why batteries have current ratings. I'm now understanding that's the max current it can tolerate before the battery begins to take damage.

Not necessarily "damage". It would depend on the particular size and technology of the battery to decide on the 'current rating' (a rather arbitrary, single - number value). You could be more interested in getting the lifetime as long as possible- as with hearing aids and watches. In that case, the rated max current would be stated very conservatively and far below the 'damaging' value. Indeed, I should imagine that many of those low power batteries couldn't actually 'damage' themselves.

There's a lot more to this than they hint at in the elementary courses.
 
  • #9
overtak3n said:
I was just curious why batteries have current ratings.

Not many batteries have their current rating written on them. Can I check you are looking at the current rating in a data sheet (typically specified in in mA or A) and not the capacity (in mAH or AH)?
 
  • #10
Batteries are a bit like generic pharmaceuticals. Each manufacturer will test their own product in a way that makes them come off better than the competition. Apart from Lead Acid car batteries, I am not aware of a published current rating. As you say , CWatters, it is the charge capacity that you read on the side of (only) some batteries. They are mainly re-chargeables and that's largely for the benefit of knowing how long to charge them for!
 
  • #11
overtak3n said:
I was just curious why batteries have current ratings. I'm now understanding that's the max current it can tolerate before the battery begins to take damage.
European standards for batteries require defining the capacity of the battery in amp hours (or milli-amp hours), but not a maximum current rating.

The rechargable batteries for radio controll models often include the maximum charge and dicharge rate as a multiple of the capacity / hour, called C. A battery with a 1 amp hour capacity and a 20C maximum discharge ratiing could handle a 20 amp load. If that battery had a 4C charge rating, the maximum charge currrent would be 4 amps.
 
  • #12
I didn't realize that was the charge capacity. Woops. Thanks for all the help.
 

1. What happens if a battery can't supply enough current?

If a battery can't supply enough current, it will not be able to power the device or equipment it is connected to. This can result in the device not functioning properly or not working at all.

2. How does a low current affect the voltage of a battery?

A low current can cause the voltage of a battery to decrease. This is because the voltage of a battery is directly proportional to the current it can supply. If the current is low, the voltage will also be low.

3. Can a low current damage a battery?

Yes, a low current can potentially damage a battery. When a battery is unable to supply enough current, it can become overworked and overheat, which can lead to damage or even explosion in extreme cases.

4. What are the consequences of using a battery with insufficient current?

Using a battery with insufficient current can lead to a variety of consequences. These can include decreased performance of the device, shortened battery life, and potentially damage to the battery or the device it is powering.

5. How can I prevent a battery from not supplying enough current?

To prevent a battery from not supplying enough current, it is important to use the correct type and size of battery for the device or equipment. It is also important to regularly check and replace old or worn out batteries to ensure they can supply sufficient current.

Similar threads

Replies
1
Views
769
  • Electromagnetism
Replies
3
Views
742
  • Electromagnetism
Replies
4
Views
1K
Replies
4
Views
815
Replies
5
Views
2K
Replies
22
Views
8K
Replies
14
Views
2K
Replies
3
Views
789
  • Introductory Physics Homework Help
Replies
4
Views
210
Replies
17
Views
1K
Back
Top