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

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.)

• How are you measuring the voltage? With some 10kOhm input resistance analogue multimeter? – PlasmaHH Aug 18 '16 at 20:37
• A photo of the setup would be helpful. – Transistor Aug 18 '16 at 20:40
• Also, did you try measuring the voltage when the LED was connected? – Justin Aug 18 '16 at 20:40
• Ah, the good ol' done by kid project... I remember those. – Kzqai Aug 18 '16 at 22:47
• make sure that the cards don't touch between cells, and don't go through the hole in the washer, that would create an internal short-circuit. – Jasen Aug 19 '16 at 3:15

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 – 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.

• do you think that adding more cells is likely to increase the brightness? – Kem Mason Aug 18 '16 at 21:19
• Yes, more cells, more volts. Please un-accept my answer for a day or two to attract more answers. Then accept the best! – Transistor Aug 18 '16 at 21:31
• A used and rather grubby copper coin touching a zinc washer is likely to give some inter-cell resistance (internal resistance of the battery but not the cells). You may be able to clean things up to reduce that. – Chris H Aug 19 '16 at 9:07
• “There is no need to measure open-circuit current.” Right, and FTR, measuring the open-circuit voltage would not mean connecting an ammeter to both poles of the battery, that would be the short-circuit current. Commonly made mistake – I actually blew a fuse in one of my school's multimeters back then... – leftaroundabout Aug 19 '16 at 9:21

When any battery is connected to a load its output voltage will drop a bit due to the internal resistance of the battery.

If you are constructing a battery then there are four simple things you can do to reduce the internal resistance of the battery:

1. Make the electrodes wider.

• The resistance of the electrolyte is going to be inversely proportional to the cross section of electrolyte that the current travels through. Generally most of the current will travel in a straight line directly between the electrodes.
• The rate at which the chemical reactions can occur is proportional to the area of the electrodes.
• This is equivalent to wiring multiple batteries in parallel.
2. Reduce the spacing between the electrodes. The resistance of the electrolyte is going to be proportional to the distance that the current has to travel through it.

• Using thinner cardboard, or using paper towels should reduce resistance.
3. Increase the concentration of ions in the electrolyte. The conductivity will be somewhat proportional to the concentration of ions.

• For example, vinegar contains acetic acid. Using another brand of vinegar with a higher percentage of acetic acid could help.
• If you use citric acid rather than vinegar, you can purchase pure citric acid powder from your local grocery store. It is usually in the baking isle.
4. Use an electrolyte that has higher ion mobility. The resistance of the electrolyte is going to be inversely proportional to the ion mobility.

• Citric acid (found in lemon juice), and sodium bicarbonate (baking soda) both make good electrolytes.
• Thanks for all the tips, although I'm a bit confused about #4 -- what kind of electrodes would I use with baking soda as my electrolyte? I tried googling a bit but couldn't find a lot of good info -- just lots of tips on cleaning batteries with it. – Kem Mason Aug 18 '16 at 21:43
• @Kem Mason You can probably still use zinc and copper with baking soda. In the copper zinc vinegar battery the zinc is oxidized, but does not react directly with the vinegar. Therefore citric acid, or baking soda should work fine. I have made several aluminum-copper batteries that worked by converting between aluminum-oxide and copper-oxide an the same electrodes worked fine using salt, baking soda, and other acids. Although aluminum does sometimes form gels if the wrong electrolyte is used. – user4574 Aug 21 '16 at 0:56

In addition to internal resistance phenomenon which lowers the battery voltage under load, you should pay close attention to @Jasen comment about internal short circuits. Take a look at industrial-grade batteries, and you will notice that individual cells never share electrolyte.

I suggest you try to put a non-wettable film (like pieces of a plastic bag or candy wrap) between each coin from one cell and the washer from the next cell. Since you still need them to be connected electrically, put a small wire across your insulation layer. That should improve your battery output and bring the total voltage closer to the sum of voltages from individual cells, at least without load.