If batteries are arranged in series in a closed circuit (I.e. with a load, which is constant) why doesn’t the current increase when voltage does?

Question

I guess people have asked slightly similar questions to mine before, but I still haven’t been able to find a complete answer as to WHY the current doesn’t increase even when the voltage in a circuit does.

When batteries are connected in series, their voltages add up. Now let’s imagine we put these batteries in a circuit with a load, but we do this by first connecting the load to ONE battery and finding the I (current,) then after that, adding the second battery in series BUT keeping the load and everything the same. By V=IR, wouldn’t this mean that since we’ve doubled the voltage in this case that the current would also be doubled, considering we keep the resistance (ie load) the same? I’ve done a sketch of an example, attached below:

Please provide a straight-to-the-point answer if you can. I don’t know why this has been bugging me so much but if I can understand this then I’m confident that I’ll be able to grasp pretty much everything else about electronics.

Answer 1

They mean the maximum current doesn't increase. If you have a battery labeled as 9V 1A and you put two in series in a box, the label on the box should say 18V 1A, not 18V 2A.

But you don't have to use the full current that the battery is designed for. If you only take 0.1A from your 9V 1A battery, and you put two of them in a box and put the same resistor in series with the battery box, you get 0.2A, but that doesn't mean the batteries can support 2A.

Note that the full current is going through each battery. Battery 1 has to support the full current and so does battery 2.

Answer 2

The current through the load certainly increases when you put a second battery in series with the load - you've used Ohm's law to prove it.

What people mean when they say "current doesn't increase when batteries are in series" is that the maximum current you can get from the batteries doesn't increase.

All batteries have a limit to the current they can deliver. If you have two batteries that can each deliver one ampere of current and you put those two batteries in series, then you can still only get one ampere out of the batteries. The voltage adds, but the maximum deliverable current stays the same.

If you put those same two batteries in parallel, then the current from the batteries adds. You can get double the maximum current out of them - but the voltage is only the voltage of one battery.

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Look at it like this:

A battery is an ideal voltage source with a resistor in series with it:

^{simulate this circuit – Schematic created using CircuitLab}

When you put two batteries in series, you also get two of those series resistors in series:

^{simulate this circuit}

You've got twice the voltage, but also twice the internal resistance. That keeps the maximum current the same as with just one battery.

Answer 3

It completely depends what the load is. There are different kinds of loads.

If you have a 12V battery and 12 ohm load which draws 1 A, adding another battery will double the voltage to 24V, and a 12 ohm load will draw double the current, 2A. No surprises there for purely resistive loads with constant resistance.

If you have a device with switch mode power supply, and it draws 1A from 12V battery for 12 watts, adding another battery to achieve 24V will make the device draw only 0.5 A, because it still needs only 12 watts. So doubling the voltage will halve the current, for a constant power load.

A constant current load would take 1A regardless of if you apply 12V, 24V, or maybe 6V.