Hot car battery and a rotten egg smell: signs of overcharging and how to test for it

After a long drive, you open the hood to check the coolant. Instead you get a sharp whiff of rotten eggs, and when your hand gets close to the battery, it feels warm, much warmer than usual. For the past few days the interior lights have also seemed brighter, and now and then they flicker.

Together, these symptoms point to a possibility that is often missed: the car's charging system is overcharging the battery. That does not only ruin the battery; it can damage other electronic components too. Here is what is going on and how to confirm it with a simple tool.

The smell is hydrogen sulfide gas. It is released when a battery is overcharged or damaged internally. Excess current overheats the electrolyte, and it boils off.

Overcharging is not the only cause. A battery with an internal short can also run hot. So the question to answer is whether the problem is the battery itself or the charging system.

• Lights that are too bright or flicker. The headlights and dash lights can get brighter than usual and then drop back, so they seem to flicker.
• Warning lights. The battery light often comes on, and the check engine light may come on too, with code P2504, Charging System Voltage High.
• Bulbs burning out early. Bulbs fail long before they should.
• A swollen battery case. Excess heat can make the case bulge. A swollen battery must not be used again.
• Electronics acting up. Audio glitches, jumping gauge needles, or seat heaters running too hot on cars that have them.
• Rotten egg smell and a hot battery after driving.

The alternator generates electricity while the engine runs, and the voltage regulator caps its voltage so the battery is not flooded with current. A failed regulator is the most common cause of overcharging. On most cars the regulator is built into the alternator, so the fix is usually a new alternator rather than just the regulator.

What about a bad ground cable? A poor ground usually causes undercharging, not overcharging. So if the measured voltage is too high, the regulator and alternator remain the main suspects. A P2504 code does not automatically mean a new alternator either. According to Mechanic Base, it can also come from the voltage regulator, corrosion on the battery or an electrical short. So measure the voltage and check the connections before buying parts.

An inexpensive digital multimeter is all you need. Work carefully: keep your hands away from the fan and belts while the engine runs, and keep battery acid off your skin, because it is highly corrosive.

1. Set the multimeter to DC volts, on the 20 setting.
2. Touch the red probe to the positive battery terminal and the black probe to the negative terminal.
3. Measure with the engine off. A healthy battery reads around 12.4 V to 12.7 V.
4. Start the engine and measure again. A normal charging system usually puts out about 13.5 V to 14.8 V. Some sources use a tighter upper limit of 14.5.
5. Switch on electrical loads such as the high beams, the audio system and the hazard lights, then measure once more. The reading should stay above the engine-off battery voltage.

Reading the result: above that upper limit with the engine running (about 14.7 to 14.8 V, depending on the source) means overcharging. Below 13.5 V means the alternator is not supplying enough power. If your reading sits near the upper limit while the battery is hot and smells, have a technician check the battery and the alternator together.

Excess charging energy does not just vanish. In a lead-acid battery, it splits the water in the electrolyte into hydrogen and oxygen gas, a process called gassing. The gas escapes through the vents, the water level drops, and the remaining electrolyte becomes more and more concentrated.

What that does depends on the battery type:

• Flooded (wet-cell) batteries can still be topped up with clean water to restore the balance. Add water, not acid electrolyte: adding electrolyte upsets the specific gravity and speeds up corrosion.
• Sealed (VRLA) batteries are more vulnerable. Electrolyte loss from overcharging is a recurring problem with this type, and adding water has only limited success. With sealed batteries, preventing overcharging matters far more than repairing the damage. Battery University names correct charge-voltage regulation and moderate operating temperatures as the ways to reduce this failure.

Prolonged overcharging also speeds up corrosion of the plate grids inside the battery, especially in sealed batteries. For perspective, Battery University notes that even a starter battery held at a constant 14.40 V while driving can be overcharged, while the recommended float voltage for lead-acid batteries is 13.60 V. That is why charging that runs only slightly high, but every day, still shortens battery life.

Sometimes the multimeter reading is in the normal range, yet the battery still feels warm and goes weak quickly. One cause is a soft short inside the battery. Lead shed from the plates, or crystals growing at the plate edges, gradually bridges the plates. Such a battery looks normal right after charging, but after resting 6 to 12 hours its voltage is lower than a healthy battery's. A battery case that feels warm during charging can also point to a soft short.

Another easy-to-spot sign is white powder on the terminals. That is oxidation, and if left alone it can break the electrical connection. Clean it off carefully, or have a mechanic do it, because the residue is corrosive.

If the regulator is confirmed faulty, the alternator is usually replaced, or its regulator section is repaired. Before the alternator comes out, the negative battery cable is disconnected.

Do not forget the battery. A battery that has been overcharged for a long time is often damaged as well, so sometimes both the alternator and the battery need replacing. Putting a new battery on a charging system that is still faulty will only ruin the new battery.

If the charging voltage turns out to be normal but the battery stays hot and smelly, the battery itself is the likely source, for example an internal short. It needs to be tested and replaced if it is in fact damaged.

Do not keep driving if the rotten egg smell is strong, the battery feels hot or looks swollen, or the battery light is on. Driving while the alternator is overcharging can make the electrolyte boil out and damage sensitive electronic components. Do not touch a battery that is hot or smoking, and open the hood slowly. In this situation, having a mechanic check the battery and alternator where the car is parked is safer than pushing on to the workshop.