Why does the voltage of vinegar batteries in series not equal the sum of the individual voltages?
As well as demonstrating some basic electricity generation you will be demonstrating why we don't generate commercial electricity using NZ coinage and vinegar!







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Every battery has an internal resistance which causes a voltage drop as current increases.

The effect of internal resistance is to cause voltage droop as current increases. A simple demonstration of this will be to hook up as shown with the multimeter set to mA when in the AM1 position (make sure you have the leads in the right sockets) and set to DC V in the VM1 position.


Disconnect the LED. Measure the open-circuit voltage. There is no need to measure open-circuit current. It will be zero.
Reconnect the LED. Measure the current and the voltage. Using this information you should be able to calculate the total internal resistance of your battery. \$R = \frac {V_{OC} - V_{LOADED}}{I} \$.


It's hard to say why you're not getting four times 0.96 V when connected in series but a photo may tell us more.



Looking at the photo I suspect that you may have some leakage in your individual cells. If the sides of the washers or coins get wet there will be a partial short on the cell. To test this make four separate cells connected by wire rather than the stack.