What is battery internal resistance?

Published by Mooch in the blog Mooch's blog. Views: 2186

Inside each battery there are two things that can interfere with the flow of current in or out of the battery. The two of them together are called the internal resistance of the battery.

Why is internal resistance important? It's what causes your battery to heat up and the voltage of your battery to sag.

So what are the two things that add together to create the the battery's internal resistance?

First, it's the actual resistance of the metal contacts and the internal structure that carries current through the battery (the electrolyte, separator, etc.). This resistance is typically only a few milliohms (thousandths of an Ohm) to a couple dozen milliohms.

Second, it's the efficiency of the chemical reactions and flow of the ions through the battery. These ions can't be transported through the battery at any rate we want. As the current level rises there is a difference in the density of the ions in different parts of the battery. This change in the density and distribution of the ions results in a voltage difference between different points of the battery. We see this effect as a voltage change as soon as current flows. Knowing the voltage change and how much current is flowing we can use Ohm's Law to determine the equivalent resistance that would cause the same voltage change.

These two resistances added together (one actual and one equivalent) give you the internal resistance of the battery...or the IR. The "DC IR", direct current internal resistance value, is the one we want to use. Since we pulse our batteries for up to several seconds we want to use the IR value measured when switching between two steady current values, one of them being zero amps in our case because we pulse our batteries on/off.

The "AC IR" value often quoted in battery datasheets is lower but this is something we would use only when measuring performance in an unregulated PWM device. It's measured by pulsing the current at 100Hz or 1000Hz.

The typical DC IR (which I'll just call IR) of a new Samsung 25R battery at room temperature is roughly 0.022-0.025 ohms. For a high-capacity 5200mAh 26650 battery the IR can be as high as 0.06 ohms. This is what causes the large voltage sag when we try to vape with these high-capacity 26650's at higher power levels.

The IR of a battery affects how we vape by causing the voltage to sag during discharging and the voltage to rise during charging. Since these voltage drops or rises are just temporary, and aren't the true voltage of the battery, this can be a problem.

So how does IR affect us when vaping? What other problems does it cause?
We'll cover this in a future article.
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