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Most basic car battery chargers will charge at a rate of about 2 amps per hour with most batteries being rated at 48 amp hours. This means that your charger will take 24 hours to fully charge the 48 total amp hours if your battery is completely flat. Fast charging, while tempting, should be considered carefully. The heat created from fast charging can cause battery plates to buckle, or can cause other damage to your battery. These are relatively inexpensive when compared with the cost of replacing flat batteries over time and can be purchased at most automotive or hardware stores. Be sure you read all information provided with your charger so you have a thorough understanding of safe operating procedure. If your battery is flat dead, this step may be unnecessary. However, if you are unsure or want to take extra measures to protect your battery from overcharging, you should check the remaining charge with a hydrometer. A hydrometer is a turkey-baster shaped tool that measures electrical capacity as a function of specific gravity by: Carefully removing the battery cell cover as directed by your battery instructions. Squeezing the hydrometer bulb to force out air and maintaining this position. Inserting the nozzle of your hydrometer into the battery cell. Taking the reading of the electrolyte fluid drawing from the cell into your hydrometer. Comparing your reading to the normal reading for fully charged batteries, usually between 1.270 and 1.290. The reading for depleted batteries ranges from 1.130 to lower values. Be sure your charger is off before connecting it to your battery. Remove any cap covers from the positive and negative terminals of your battery. Your car battery charger should have clamps at the end of positive (+) and negative (-) leads. Clamp these to the corresponding terminal on your battery. The leads, clamps, and covers are almost always color coded to ensure the correct connection. For most vehicles, red will indicate positive and black negative. Now that the leads are connected, you can turn on your charger and begin charging your battery. Most car battery chargers come with an ammeter which reads the amperage used to charge the battery. Verify the reading on this to ensure you are not charging at too high of an amperage. The starter for most car engines will have enough energy to operate after about 10 - 15 minutes of charging. Once your starter can run, you can start your engine and allow the car to idle. This will allow your alternator to restore charge to your car without the sustained use of your charger. Once you have charged your battery sufficiently for your purposes, you can turn off the charger. With the charger off, you should have no difficulty removing the clamps from the positive and negative terminals of your battery. Remember to reattach the caps that cover the terminal ends of your battery. Corrosion to the terminal can ruin a good battery.
Know the consequences of high and low amperage. Purchase a car battery charger. Check the remaining capacity of your battery. Connect your charger. Turn on your charger. Prevent overcharging by minimizing your charging time. Turn off and disconnect your charger.