[shorod]

If the battery was down to 5 volts, it's toast. It should not see below 10.5 volts when completely discharged, and lead acid batteries don't like to be discharged that low. They can handle momentary current surges which cause the voltage to drop that low, but if it was that low steady state or under load, it's done. Even if they charge it and it appears to come back, that will only be a surface charge. The internal resistance will be too high to remain charged or provide sufficient current to start the car.

-Rod