I recently left my rear hatch unlatched and this drained the battery.
I didn't know this and the next day the car did not refuse to start.... instead it didn't recognize either key fob. Since one had given me a low battery warning I figured it was the same for both. New batteries in the key fobs of course failed to work.
My brother found the problem, I had dragged a 4x4 post out the night before. This had also dragged one of my off season rug protectors over the latch. hatch didn't latch and apparently left some lights on.
He said the battery still had 5 volts.
So, if you try to start your car and it doesn't recognize your key fobs try a jump start. Or maybe turn on the headlights and check them for brightness. Usually we discover these problems with no time to do anything. I was on my way to an appointment and had no time to figure out the problem

 

You must have a good vehicle-battery to receive any signal and start the vehicle. There is no hard limit that I know of, but I have been successful with 10V at the terminal. Keep in mind voltage is just one factor and the other key one is the current. System needs both [voltage and current] to spin the starter.

Rule #1 ... If anything is wrong, check the battery voltage and condition of the clamps. Without this being right, any thing that you get from sensors can not be relied upon [this is true for the on board computer too].

Salim

 

I may have said this in a confusing way. The car didn't start. But it didn't start because it didn't recognize the key fobs. It never got the chance to try to start the engine, which mine doesn't do on start up anyway.

I would never check the battery if my dash was telling me that my key fobs were not being recognized. I would figure that a low car battery would not have anything to do with key recognition.
I had no time to figure this out at the time but thought a warning to others that just because the car doesn't recognize your keys it might be something else.

 

On the contrary, it was my poor description on what you faced.

KeyFob -- sends the right signal -

Receiver sensor in the vehicle did not amplify and capture the signal [low vehicle battery voltage] -- sends garbled message to ECU
ECU itself does not work properly with low supply voltage. [cant say for sure but it is a combination of this and the previous statement]
You get a "key not recognized"

Solution .. provide 12V to the vehicle [jump]

[leap of faith -> If your battery discharge state was not as bad, you might have gotten 'Key detected', but the battery would not have had the oomph to start the vehicle]

Salim

 

If your car battery "still had 5 volts" than it was dead, with no chance of starting your car.

So, did you charge the battery before trying to start it? Or did you try to start it with the 5 volts remaining?

The car shouldn't "forget" the key fobs when losing power. The batteries go dead all the time, and the cars start normal when the battery is charged or replaced.