Not all voltages are created the same...

Thread Starter


Joined Jun 28, 2018
You can have a car battery and a motorcycle battery, both of which are 12V, but say the car battery can move more charge than the motorcycle battery. This amounts to the car battery being able to transfer more energy than the motorcycle battery. In saying that the car battery can move more charge, is this the same thing as saying the car battery can output a higher current than the motorcycle battery?

Next, if you place both batteries in circuits with identical resistances, it would seem that by Ohm's law they should output the same current. Is this where internal resistance comes into play? Is it the physical properties of the two batteries that would result in different internal resistances? The car battery, being larger, having more chemical capacity can move more charge or output more current than the motorcycle battery and we describe this in terms of a smaller internal resistance for the car battery than the motorcycle battery. Correct or no?


Joined Jul 1, 2009
The size of the battery determines CAPACITY (how much current it has intrinsically by whatever internal process is being used to generate current). Voltage is the force, or the potential of the current to want to move, by design.

Both batteries have the same potential, so both batteries will move current at approximately the same rate, given the same external resistance between the poles (aka load). Internal resistances are a slightly difference discussion than the example you're providing.

In the case of your example, both batteries having the same external resistance (aka 'load')-- the larger battery will output the same level of current at the rated voltage for a LONGER period of time- This is why batteries are rated in Ah, not just A.

If you remove the external resistance and short the poles, the larger battery will output more current because it has nothing resisting that transfer- so it will attempt to equalize the difference between the poles as quickly as possible, just like the smaller battery. As crutschow stated, the larger battery has larger plates (ie. greater capacity), so it will dump more current.

Remember that Ohm's Law is a reciprocal equation. It has 3 components- 2 real, and one conceptual. Voltage is the conceptual component. It is the value that represents the relationship of a fraction or ratio between the flow of electrons (current), and the impedance (anything resisting that flow) limiting their ability to flow/move. Furthermore, voltage usually references the amount of potential lost/dropped in order to get current to flow, not what is left. In other words E = IR is how we figure out how much voltage it took to get current to flow through the resistance.
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Delta Prime

Joined Nov 15, 2019
Hello there :)
The potential difference of a typical car battery is 12 V. The potential difference is set up by the electromotive force generated by chemical reactions. The electrolyte used in automotive batteries is a strong solution of sulfuric acid and distilled water. The chemical reaction inside a car battery can be reversed


Joined Oct 2, 2009
Yes and yes.

The design of the battery and all its physical properties, size, number of plates, chemistry, temperature, age and level of charge will determine its capacity and current delivering capabilities.

Two different batteries may output the save voltage and current. The battery with a higher capacity (units of Ah) will maintain the voltage longer than one with lower capacity.

An aging battery may still show the proper nominal output voltage with no load but fail to deliver any appreciable current into a load. As a battery ages its internal resistance increases. Internal resistance plays a significant factor in the ability of the battery to supply current at the nominal voltage.

schmitt trigger

Joined Jul 12, 2010
Taking the example to the extreme, you can compare a hefty auto battery with an A23 remote control battery.
Both will be roughly 12 volts open circuit, but the car battery will deliver massively higher short circuit currents.


Joined Sep 24, 2015
Is it the physical properties of the two batteries that would result in different internal resistances?
I know a bunch of people have given good answers. You probably got your answer already. Nevertheless, here's mine:

Assume the electrolyte is just a bunch of parallel resistors. The larger battery has more electrolyte than the smaller battery. Ohms law predicts how the resistance is going to be lower in an environment such as a bunch more parallel resistors. Hopefully the illustration will explain it. The four sets of resistors and their associated plates represent the smaller battery. This is true of all batteries that I know of; smaller batteries have less energy and higher internal resistance for a given chemistry. Different chemistries will have differing properties, nevertheless, a larger battery will have lower internal resistance.