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

1. Jul 24, 2014

### overtak3n

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?

2. Jul 24, 2014

### sophiecentaur

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: Jul 24, 2014
3. Jul 24, 2014

### overtak3n

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. Jul 25, 2014

### DrDu

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. Jul 25, 2014

### overtak3n

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. Jul 25, 2014

### Staff: Mentor

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. Jul 25, 2014

### overtak3n

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. Jul 25, 2014

### sophiecentaur

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. Jul 25, 2014

### CWatters

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. Jul 25, 2014

### sophiecentaur

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. Jul 25, 2014

### rcgldr

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. Jul 25, 2014

### overtak3n

I didn't realize that was the charge capacity. Woops. Thanks for all the help.