Electronic – Why does the voltage of vinegar batteries in series not equal the sum of the individual voltages

battery-chemistryled

I'm working on an in-class demo / hands-on for my sons primary school class, and I've made some small batteries with New Zealand 10 cent (copper coated) coins, and zinc washers, and vinegar soaked cardboard. Each individual cell is measuring about 0.96 volts, but when I put 4 of them together, I only get out about 2.6 volts. I'm wondering if there is something I'm unaware of about the nature of these batteries that makes them not add up.

Also, even at 2.6 volts, the same voltage as I'm getting out of a pair of eneloop AA's, the LED is not very bright at all — compared to hooking it up to the eneloop AA's, where the LED is quite bright. Is this because of the low amperage of the vinegar battery? Would putting more in series make it better (or do I need to make a second one and hook them up in parallel?).

I'm a bit of a noob with electronics, mostly learning it now as my son is very interested, so having some fun learning it with him.

Thanks for any tips.

I've attached a picture below showing the intended final product (I squeeze the top and bottom of the led wires to complete the circuit, as a simple switch). On the right is what I'm using for my cells (minus the vinegar, and without the cardboard being cut to fit the coin.)

4 cells enclosed in electrical tape with led and single cell construction

Best Answer

As well as demonstrating some basic electricity generation you will be demonstrating why we don't generate commercial electricity using NZ coinage and vinegar!

schematic

simulate this circuit – 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.

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