Battery Internal Resistance: What It Tells You About an EV

A used traction battery raises two questions, not one. The first is familiar: how far will the car still go? Capacity answers that, usually quoted as SoH. The second is rarely asked: how much power does the battery still deliver, and how quickly can it be charged? Internal resistance answers that one.

The tank and the pipe

Picture the battery as a water tank. Capacity tells you how much is in the tank. Internal resistance tells you how wide the pipe is that the water comes out of. A narrow pipe does not make the tank smaller, but less comes out at once, and the pressure drops when you draw hard.

Electrically it works the same way. With the car standing still, the battery shows its full voltage. As soon as current flows, the voltage sags, because part of it is used up inside the battery. The more current, the bigger the drop. Charging works the other way round: there the current pushes the voltage up.

The resistance sits in everything the current has to cross: the connections between the cells, the metal parts, and the liquid inside the cells themselves.

What this means day to day

Power. The car may only load the cells down to a certain voltage. Because high resistance makes the voltage sag sooner, that limit arrives earlier and the controller takes power away. A car that accelerates more meekly than it used to at half charge may be running into this.

Charging speed. Charging is the same story with the sign reversed: the upper voltage limit arrives sooner and the car accepts fewer kilowatts. A disappointing charging speed is therefore often not a question of capacity.

Heat and losses. Whatever is used up in the resistance turns into heat. Firm acceleration easily draws 250 amps; with a typical internal resistance that costs around 12 of roughly 350 volts and produces about 3 kilowatts of waste heat, as much as a fan heater. This is why the battery warms up during rapid charging and motorway driving, and why a little more electricity always goes in at the socket than comes back out for driving.

Cold makes every battery sluggish

Temperature is the biggest lever. In the cold the battery becomes sluggish inside and resistance rises sharply. Every winter driver knows the consequences: less power, less range and noticeably slower charging.

Rapid charging works best within a moderate temperature window. Below it the car has to pull the charging current back, and below freezing, charging at full power would damage the cells permanently. Modern EVs therefore preheat the battery while driving when you enter the rapid charger in the navigation system. Without route guidance many models do not do this, and you arrive at the charger with a cold battery.

From this follows the most important rule for buying used: never judge a battery on a cold morning. What looks like weakness there is usually just temperature. For the same reason a serious capacity estimate has to account for temperature, otherwise the same car scores worse in January than in July.

Reserve and power age separately

Battery engineering distinguishes two kinds of ageing: the loss of reserve, meaning capacity, and the loss of power. The two are related, but they do not move in lockstep.

For a used purchase, the combination is the interesting part. A battery with normal capacity that nonetheless delivers noticeably less power and charges more slowly than usual is a warning sign: aged cells or a poor connection inside can sit behind it. Conversely, a battery can have lost capacity and still stay strong.

Worth keeping in mind: in modern batteries, internal resistance stays fairly stable for many years and rises markedly only late on. So it is a warning sign, not a fine scale you could read percentage points off.

What you can observe yourself

Two things work without a garage and without instruments:

• Charging power at a rapid charger, with a warm battery and a low state of charge. If the peak stays well below what this model normally manages, that is a reason to look more closely. With a cold battery the observation says nothing.
• Power at half charge. A car that feels noticeably more reserved than it used to may already be working against its limits.

Both are clues, not diagnoses. A single poor charging session can have many causes, from the charge point to the weather.

What only becomes visible across many measurements

A single observation fluctuates, for capacity as much as for power. A statement only becomes robust once many measurements in different weather and at different states of charge come together.

Akkubrief builds exactly that as a battery history: connect once online, and it then builds itself while you drive and charge as usual, with no box and no appointment; that costs nothing. What comes out of it, and where the limits are, is set out openly in our methodology.