# Curiousity: Voltage Drop

Something I and a friend have wondered about for a while. In the theoretical circuit, voltage is equal at resistances in parallel, and on that note, the voltages we see should be equal to the battery’s voltage (after loss to wires.)

As motors draw large amounts of current, the voltage often drops considerably (indeed, enough to shut the RC before it manages to pop the breaker). What’s the science of this phenomenon?

I don’t know what the “science” of the phenomenon is, but a rather simple explanation is that electrical power is produced by a chemical reaction, and under high current load, the reaction cannot happen fast enough to keep the voltage up. Too many electrons leaking out, not enough being generated to replace them, so the voltage drops.

I hope a someone with more knowledge of batteries can explain it to us…

I am guessing that it has something to do with Peukert’s Law. http://en.wikipedia.org/wiki/Peukert’s_Law
If I am interpreting correctly, Peukert’s Law says that a battery’s capacity (units: charge) decreases when it is being run at higher currents.
The battery does output a lower voltage as its remaining capacity dwindles, so perhaps Peukert’s Law implies that while the current is high, the battery ‘feels’ like it has less charge remaining than it really does and can only output the voltage it would if it did in fact have that much charge left.

http://autos.groups.yahoo.com/group/ev-list-archive/message/43491, asks if saying "expressing Peukert’s would be an expression that at “x”%
state-of-charge the battery can sustain “y” amps without dropping below
10.5v " is appropriate (for a car battery).

So if Peukurt’s Law is indeed the cause, squirrel’s explanation is, although a condensed version of the story, correct. Elsewhere in that yahoo thread, someone explains it as “What Peukert does is try to model the cumulative effects of polarization, electrolyte starvation, depletion of active material on
the surface of the paste(s), ohmic loss and probably a few other
things.”

Simply, it is a voltage drop across the internal resistance of the battery. At stall current of one of the larger motors, about 100 amps, this drop will be about 1.1 volts when measured at the battery. It will increase as you measure across the resistances of other devices and wiring.

Hmmmm…interesting…

does the batterie’s internal resistance account for the increased voltage drop as the battery becomes discharged?

To some extent…if you look at the battery discharge curves, you will get a better handle on the operation of the battery at different discharge currents.
Graph at http://www.mkbattery.com/images/ES17-12.pdf