I suggest you get a battery hydrometer and check it periodically until it is charged. Overcharging depletes water from the battery. Undercharging shortens the battery life.
It takes four hours of charging
The battery time can vary from notebook to notebook. On average, a HP notebook battery can last anywhere from four to eight hours on a single battery charge.
With a charger specifically designed to charge this battery.
I have learned that when i get new batteries for my hobby cars you want to fully charge the battery to the recommended hrs, you want to charge the battery fully because it will improve how long your battery will last on one charge and how long the battery's life will be.
How long does it take for an 18v battery to charge
At the lowest screen brightness, the Nintendo DS can get up to 18 hours of battery life from a full charge. For most practical applications, however, at highest brightness, the actual average battery life is between three and four hours at full charge.
I assume you mean how long to charge the battery. You should never let the alternator charge a dead battery if possible. The alternator is designed to keep the battery charged but not to charge a dead battery. You put a strain on the alternator and it can cause a diode to burn out. Drive home and then put a battery charger on the battery to fully charge it.
Indefinitely.
2 hours
Depends on the condition of the battery. Most will hold a charge for at least 6 months if they are not connected to anything.
As long as both batteries are the same voltage, you can jump start with the battery. If you want to charge the battery then use a battery charger.