What would make the instrument panel brake & battery lights come on at the same time and stay on while driving my 2006 Nissan Altima 3.5? Brakes felt OK.
It could very well be your alternator has gone out. Open the hood and put the probes from a multimeter set to vdc onto the battery terminals. Observe the voltage of the battery with the vehicle running. If the voltage is slowly creeping down, your alternator is not functioning (it's not charging the battery, so the engine is absorbing the energy to keep running).
This happened on my 2012 Altima, and it turned out to be two things:
- The alternator died
- The connection from the alternator to the battery was loose at the battery terminal
When I tested with a multimeter, the voltage was always 14 or so. A local shop was able to put a proper alternator tester on my car, and that confirmed it had failed.
The loose connection was my own fault. I put on an aftermarket battery terminal, and the alternator wire connection worked loose. This may have cause the alternator to die.
The problem went away when I fixed the loose alternator wire connection, and replaced the alternator (with a new one from Amazon, for significantly less than auto part stores).
In my research I found that when the battery and brake warning lights come on at the exact same time, it is a Nissan way of indicating a problem with the Alternator. However, my sources were all 2nd-hand, and not from Nissan.