I’m putting together a set of kits for children to make their own batteries, but don’t know exactly what sort of devices I can power with them. How do you test the strength of a battery? I have a multimeter that can tell me the voltage, but that doesn’t help much if the current is too low to drive any devices.
They’re being made out of aluminum and brass washers, rather than the traditional coins, to make them easier to stack without falling over. I know I can power up a tiny LED but would like to offer something more impressive.
there are incandescent flashlight bulbs that run on 3V (2 cell) and 1.5V (one cell). you measure the voltage of the battery while powering the bulb. you can find the power consumption rating for any bulb and from there know the current that might be produced.
a bulb is good because the brightness will vary with the battery voltage. it is more meaningful to kids than a somewhat disconnected meter measurement.
get a second inexpensive multimeter and measure the current with one and the voltage with the other.
The question is - what is the internal resistance of the battery.
Easy to test. You need to measure the voltage produced by the battery into two different loads. One of these can be no load - so the voltage you have. Then use a known resistor as a load, and measure the voltage again. You can then solve for the actual current being delivered, and work out the value of the effective resistor inside the battery.
This gets you the Thevenian equivalent of the battery (where all devices can be modelled as a voltage source in series with a resistor). You can then work out what you can power, as this resistance is always there, and so limits the current the battery will deliver.
For you kit batteries the internal resistance will probably be depressingly high.
Thevenian equivalent, I looked that up and it seems to be what I’m looking for, I’ll still run tests with some small bulbs and motors, but this will get me part of the way there. I’m sure there isn’t much I can run with it, but hope springs eternal.
I’m coming up with 3.7V and a devilishly high 666 ohms. Turns out it is good enough to run an LED.
Keep me honest here, I see online a "low current’ incandescent bulb that takes 1.35V at .06A. That gives me 23 ohms, which would give the circuit 690 ohms total. 3.7/690 is a cool .005A, 1/10th of the current needed to light it up. I’m thinking an LED will have to be impressive enough for the kids.
I’ve done something similar, using copper, fiber, and zinc washers. Coca cola/salt was the electrolyte. A 6-cell pile would light an LED quite nicely; each cell generated about 0.6 volts.
There are a number of inexpensive LEDs on the market that are designed to operate at 2 mA, with a V[sub]F[/sub] of around 1.8 V. Assuming your Thévenin model is accurate, if you connect such an LED directly to your battery, the current will be around 3 mA, which is safe to operate at. (Typical max current spec for a “2 mA” LED is around 7 mA.)
666Ω doesn’t surprise me - I was guessing about 1kΩ. What determines the resistance is the surface area, and about the only thing that might help is some technique to increase the area. Obviously bigger washers, but also some sort of etching of the surface might help. Electrolytic capacitors are dependant upon quite extraordinary increases in surface area of aluminium foil obtained by (usually propriety) chemical etching. Even experimenting with sandpaper might gain some useful area.
Finally you can always parallel up a whole lot of cells.
For something much more cool that a LED - build up a ZN414 based radio receiver.
With these types of power sources, it is often better to use a “pulsed” load instead of a continuous load (such as a continuously-powered LED). For the former, the battery would charge a capacitor. Once the capacitor reaches a certain voltage, it quickly dumps its energy into something else, like a radio transmitter (beacon transmission mode), light (a bright but flashing light, obviously), solenoid, etc.