# Why doesn't the voltage drop across the resistors of my battery charger follow Ohm's law?

Here's the setup, the three items below are in series:

1. Battery at 215V

2. Power supply at 250V (It is a 5A constant current power supply that will drop voltage down to 200V minimum before failing)

3. 8 ohm resistor

When I flip on the charger, I am pushing 3.5A and reading 35V across the resistor. Shouldn't an 8 ohm resistor put through 4.375A? The voltage across the power supply remains at 250V so I don't think it is limiting the current. Or is it?

I have had this issue before and can't explain it. How can I use Ohm's law to size my resistors in this situation?

• Can you add a drawing of your circuit? Commented May 1, 2019 at 20:28
• What is the tolerance of the resistor? Have you tried measurung it with an ohmmeter?
– dim
Commented May 1, 2019 at 20:32
• If the power supply is constant current, then the voltage will adjust to the current you set. So, why would you expect a different current across the load resistor than the 3.5A you are pushing? If you expect 4.375A, then you are using a constant voltage supply, which will admit as much current as necessary to keep the voltage regulated. Commented May 1, 2019 at 20:34
• Drawing added to the post Commented May 1, 2019 at 20:49
• @dim I measured the resistor at 8 ohms Commented May 1, 2019 at 21:07

I don't seem to have enough reputation to comment, but something seems off. You're reading 35V across the 8 ohm resistor?

This would mean from ohms law 35V/8ohm = 4.375A. so something is off if you're producing too different measurements. in other words, for 3.5A to flow through the resistor, the drop would be 3.5A*8ohm=28V, but you're seeing a drop of 35V.

So for some reason I can only think the current measurement is faulty... Can you maybe explain how you're getting the 3.5A and 35V reading. Could you place a multimeter in series with the circuit and measure the current through the resistor if the 3.5A reading is from the PSU.

• producing too different measurements should be two I think? (Cant edit it as it is less than 6 characters) Commented May 1, 2019 at 20:54

Thanks for the help everyone.

Turns out I have crappy resistors. They are "rated for 100 watts" (I said 50W in the diagram but even that was too generous). They warmed up pretty quick; I read up 250C (!!!) with a reasonably accurate infrared thermometer after letting it sit for just a few minutes.

After disconnecting power I measured resistance again while the resistors were still warm and saw a value closer to 10 ohms.

I thought I could get away with convective cooling if I ran the resistor at only 1/3 its rated power... next time I'll get a heat sink and/or better resistors.

• Next time only use 2/3 of rated W or use 300W halogen bulbs Commented May 2, 2019 at 1:22

One of your readings is incorrect if it does not match this,

• So the error was in the calibration of the R value. Debug Rule #1 know your assumptions and then validate them. (-2 to those who disagree) Commented May 2, 2019 at 16:54