Im horribly confused with a simple problem: wiring 8 1.25V cells so that they total 10, 5, and 2.5 volts?
Question by Riley Murray: Im horribly confused with a simple problem: wiring 8 1.25V cells so that they total 10, 5, and 2.5 volts?
I am doing a project on voltage effecting the efficiency of electrolysis for my H Chem class. I know very little about wiring but my goal is to, with the use of 1.25V NiMh cells, wire together three circuits, with voltages of 2.5, 5, and 10 volts. I need to use all 8 cells for each so that the amount of available energy to the chemical reaction ultimately stays the same (as my theory is that some voltages may be more efficient than others due to energy bing wasted in higher voltages).
I thought this could be done with resistors, but that is leaving me very confused and doubtful. I now thinking that this can be done by wiring the battery packs (each holds 2 cells) in series.
The 10V scenario would have all the battery packs in the same direction
The 5V would have 3 battery packs (7.5V) running in one direction and one battery pack (2.5V) running in the other direction, and from what I know (very little), these battery packs in a series circuit would subtract from one another, totaling 5V.
2.5V would be achieved by… uh… well… this method doesn’t work with 2.5V.
I could maybe do 5 cells in one direction, and 3 in the other (6.75V-3.75V= 3V = good enough for me)
But if that’s all crap then im at a loss.
So will resistors work? It doesn’t need to really be these voltage levels, the idea was to have a low voltage, and multiply by 2 to get the middle voltage and then 2 times that equals the highest voltage.
So could this be done in parallel? Or will I have to go with higher voltages? (5, 10, and 15V)
Hell, if voltage isn’t even the speed that electricity travels at then please tell me some way how to control amperage, or wattage, or- SOMETHING!
10 points and everlasting gratitude to best answer 
Best answer:
Answer by dis_orient_ed
parallel wiring, not just serial.
Give your answer to this question below!
You are correct that wiring some cells in reverse will subtract voltage, but the total amount of energy transfered to the electrolysis cell wont be the same, some of the energy will go to charging the reversed battery! For 5 volts, you could connect 4 in series, the other 4 in series, and then put these two banks of 4 in parallel. For 2.5 volts you could wire 2 in series, and have 4 of these pairs of seriesed cells , then connect these groups of 2 in parallel.
Using resistors wont work well for this project. The problem is to get a constant voltage across the electrolysis cell, the resistance of the electrolysis cell would have to stay the same, which its unlikeley to do. If you connect the battery directly to the cell, due to the very low internal resistance of the batteries, this will force the voltage across the cell to remain aproximately constant (which is what you want)
Not to mention, that some of the power will be dissipated in the resistor, rather than going into the electrolysis process!
Electrolysis itself depends on current, and the resistance of the cell will determine the corresponding voltage, a la ohm’s law. But the resistance of an electrolysis cell can vary hugely, and depends on the electrode shape, size, solution concentration etc. You will have to keep these constant between the seperate experiments. Also, if possible, meausre the current, not just the voltage, as it is the current that does the work in electrolysis.
Usually, electrolysis is done with constant current source power supplys (they maintain a constant current, regardless of the voltage across them, like a voltage source maintains a constant voltage regardless of the current you pull from it.) Batteries look more like constant voltage sources, so it will be important to measure the current. You can then actually calculate the power going into the electrolysis as P=I*V (I is the current entering the cell, and V is your battery voltage). Then of course total energy is the integral of power over time (or simply power multiplied by time if the power is constant.)
Very simple !
Join all 8 cells in series.
1. From top to bottom , it has 10 volts or more.
2. From middle to bottom ( count the cell starting from the bottom cell,the 4th.cell + ) , it has 5 volts or more.
3. From quarter to bottom (count the cell starting from the bottom cell,the 2nd.cell + ), it has 2.5 volts.