Q: How are voltage and current related to battery life? What is the difference between batteries with the same voltage, but different shapes or sizes? What about capacitors?

Physicist: Chemical batteries use a pair of chemical reactions to move charges from one terminal to the  other with a fixed voltage, usually 1.5 volts for most batteries you can buy in the store (although there are other kinds of batteries). The chemicals in a battery litterally strip charge away from one terminal and deposite charge on the other. In general, the more surface area the chemicals have to deposit charge onto, and take charge away from, the higher the current the battery can produce.

The best way to represent the way a real battery works is to replace the battery in a circuit with an ideal voltage source (which is what we usually think of batteries as being) and an imaginary resistor called the battery’s “internal resistance“.  The internal resistance can be used to describe why an AA battery is incapable of generating an arbitrary amount of power; the more current that the battery creates, the more the voltage across the internal resistor drops according to Ohm’s_law (V=IR). You can picture this as being a little like pushing a cart; if the cart isn’t moving you can really put your shoulder into it, but as the cart moves faster it becomes harder and harder to apply force to it.

A car battery only produces 12 volts, which is the same as 8 ordinary batteries in series.  That voltage is so low that you can put your dry hands on the terminals of a car battery and feel nothing (please don’t trust me enough to try it; I don’t even trust me enough to try it).  And yet the internal resistance is so low that if you connected the terminals with a normal wire, the current in the wire would be so high that the wire would melt or explode.

You can model the way a battery dies by increasing the internal resistance.  A nearly dead battery still provides 1.5 volts, but has a very high internal resistance so that drawing even a trickle of current zeros out the voltage gain.

The voltage across a capacitor on the other hand is always proportional to the charge presently stored in the capacitor (this is the defiition of capacitance). You can think of a battery as being like a water pump, always providing the same pressure, and a capacitor as being like a water ballon, the pressure increasing the more water is in the ballon. The amount of energy in a capacitor is much easier to measure because of this (if you can measure the voltage across it, you can know the energy immediately).  But, because the voltage supplied by a capacitor changes dramatically as it drains, special adaptive circuits are needed to step down the voltage to a fixed, consistent level in order to power a device.  Alternatively, the device can be made to work over a wide range of voltages, but that tends to be more difficult.

It’s only fairly recently that capacitors have become small and powerful enough to store energy on par with chemical batteries. A few decades ago electrical engineers would prank the new guy by asking them to go into the backroom for a 1 Farad capacitor, which at the time was ludicrous. The poor sap would be back there forever (electrical engineers think they’re so funny). However, that joke has run its course, because today you can buy an over-the-counter, several-thousand Farad capacitor, that’s small enough to fit in your hand (and they pack a punch)!

So, as a general rule of thumb, batteries have a fixed voltage but:

big or new batteries tend to have a low internal resistance, so they can deliver a high current

small or old batteries tend to have a high internal resistance, so they can’t deliver much current

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10 Responses to Q: How are voltage and current related to battery life? What is the difference between batteries with the same voltage, but different shapes or sizes? What about capacitors?

  1. kakio says:

    dude that doesn’t help

  2. Valro says:

    It doesn’t really answer the question. Do try to explain in layman terms and make it as simple as possible.

  3. Gift says:

    Pls I did get the explanation pls explain in the language I will understand

  4. Anonymous says:

    You can absolutely touch both terminals of a car battery at the same time. Your skin’s resistance is high enough that you won’t feel the minimal amounts of current that will pass through you. I’ve done this many, many times safely.

  5. Crafty says:

    Ok, but I don’t see any mention to how current is affected. Does the current stay the same and only the voltage changes due to internal resistance? Does a dead battery still put out the same current?

    Anonymous should try standing barefoot in a puddle and wetting both hands while touching the car battery terminals for science.

  6. Engineer says:

    Crafty – The current decreases. Voltage stays the same. Resistance increases. A dead battery produces no current.

    Equation -> V=IR
    Where V -> Voltage (volt)
    Where I -> Current (amp)
    Where R -> Resistance (Ohm)

    In layman’s terms…
    Over the life span of the battery, the voltage basically remains the same. However, the internal resistance increases. Which means, the current must decrease, in order to balance the equation.

    This is proven with your car battery. Your battery powers the engine starter. When there isn’t enough “juice”, the engine wont turn over. It is because you do not have enough “cranking amps”. That is why, in colder climates, you will see batteries with higher “cranking amps”.

  7. Anonymous #2 says:

    Any battery which is used commercially is basically an electrochemical cell. It converts the energy released during a chemical reaction to electric current. While the cell runs, the chemicals within the cell are used up, this in the words of “Physicist” and “Engineer” (Colourful names btw) increase the internal resistance of the cell. In theory the cell is supposed to have the same voltage, but IT MAY NOT (There’s a equation called the “Nernst Equation” which governs the voltage of these cells at varying concentrations of the reactants). As for the current, it is not possible for the current to stay constant, in fact even when you start using cells the current isn’t constant.

  8. Anonymous #2 says:

    And to the main issue initially raised, take for example a AAA and a AA battery. Both of them put out 1.5 volts. However their capacities are different (which “Physicist” has explained brilliantly with the balloon example). Thats the basic difference between cells of different sizes with similar voltages.

    The relation between the voltage or the current with the battery life is very vague. The battery life is dependent on how long the chemicals last and how they can be replenished (for a rechargeable only). There’s no doubt that the current reduces with battery use.

  9. John says:

    Please I need to know, do battery powered devices turn off due voltage starvation or current starvation? or both?

  10. mbusoki says:

    how do we apply the use of battries in our everyday life?

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