"The battery is only a few months old, and the Altima still wouldn't start this morning." When a San Ramon driver tells us that on the phone, the second dead battery is what sends us past the battery to the charging side. When the alternator falls short, a new battery runs flat again because the charging side isn't replacing what starting and the accessories take out. Finding the reason starts with knowing what that charging side includes on this car.

On a 2007-2018 Altima, that charging side includes a small current sensor clamped on the negative battery cable. It measures the current flowing in and out of the battery, then sends that reading to the engine computer. Swapping the battery leaves that sensor and the alternator exactly as they were. At Driven Auto Care of San Ramon, we measure alternator output and the voltage lost across the cables, so the repair goes to the failure point.

The Flicker At Idle Comes First

Alternator output is lowest at idle, because the engine turns the alternator at its slowest speed there. That's why weak output shows first at a red light, where the headlights dim and then brighten as you pull away. At the same stop, the battery light flickers and goes out as engine speed rises. A whine rising with engine speed belongs on the call too, since worn bearings or a failing diode inside the alternator can cause it. Each of these signs shows up while the Altima still starts every morning.

Once output falls far enough, the car runs on the battery alone. Electrical systems then drop out one after another, and the engine stalls once the battery runs down. If the battery light stays on and the lights or gauges start dropping out while you drive, pull over safely and have the car towed.

Why Does The New Battery Die Again?

Every start draws on the battery, which the alternator refills while also running the lights and the blower. When alternator output falls short, each drive takes a little more out of the battery than the alternator puts back. Fresh from the store, a new battery covers that shortfall for a while on its full charge. Then one morning there isn't enough left to turn the engine over. That new battery was never the part that failed. Putting in a third battery leaves the shortfall in place, because nothing on the charging side has changed. Finding that shortfall takes more than one quick reading on this car, because of how its charging voltage is set.

Altima Charging Voltage Is Programmed To Rise And Fall

On 2007-2018 Altimas, the engine computer is programmed to raise and lower charging voltage as conditions change. It sets a target voltage from the current sensor's reading and lowers output when the battery needs less, which takes load off the engine. So a low reading while you cruise can be normal on this car. One reading at a steady speed proves little about the alternator. The reading worth trusting is the one taken with the headlights and blower on, when the system needs everything the alternator can give.

Faults in the current sensor store their own codes in the engine computer. Those stored codes are the first reading we take on the car. With a stored sensor code, the search goes to that sensor and the wiring at the negative cable. Without one, we move on to the alternator and the cables under load.

The Load Test And The Cable Check

We run the engine and switch on the electrical loads a night drive puts on the car. With headlights and blower switched on, we raise engine speed above idle and read the alternator's output. Healthy units hold their output under that load, and weak ones fall away from it.

Next we measure voltage drop on the main positive cable and on the ground, meaning the voltage that never reaches the far end of the cable. Resistance on the positive side causes undercharging that looks exactly like weak alternator output. The repair then goes to whichever part gave the reading that was out of line.

What To Tell Us About The Second Battery

Call Driven Auto Care of San Ramon at (925) 830-4701 and describe the flicker and the age of the battery. We'll run the load test at our San Ramon shop with the headlights and blower on, and the repair goes to the part that came up short. After the repair, the same test runs again, so your Altima goes back on the commute with its charging side measured under a dark evening's load.