How can I tell if my battery is charged or not?
The simplest method is to test your battery with a voltmeter to measure the voltage between the positive and negative terminals or posts. While the battery is not being charged and not powering anything, you can use your voltmeter and get a reading. For a 12-volt lead-acid battery, the voltage should be between 11 and 13 volts. The closer it is to 13 volts, the closer it is to being fully charged. If it's closer to 11 volts, the battery needs recharged. The exact voltage will differ depending on the battery voltage, style, and age.

How long will it take to charge my battery?
There's no definitive answer to this question, but you can make a pretty good guess with some simple math.

(Battery Capacity) / (Charger Amps) = Hours

Our ETX-30L Big Crank battery has a capacity of 26 Ah. You plan on charging this with a 2 amp charger. Using the above equation, you would have [26 Ah / 2 Amp = 13 Hours]. This is a rough estimate and usually tells us how long it will take to recharge the battery to about 80% of its capacity.

To complete the recharge all the way to 100%, it will probably take an equal amount of time to recharge the last 20% of capacity. Some chargers may get it there a little faster, but there's no absolute guarantee.

Why should I use an automatic charger?
The short answer: Overcharging batteries can kill them.

If you leave a manual charger connected continuously, even at low amps, the battery will eventually die. When a battery gets overcharged, the electrolyte inside gets hot and gasses are created. If you have an older vented battery, the electrolyte could evaporate and your battery plates will be exposed and eventually ruined. Overcharging a sealed battery will cause the gasses to build up, until the battery balloons out and bursts.

An automatic charger will only charge the battery as necessary, shutting itself off until the battery voltage drops again. Great chargers from brands like BatteryMINDer and Battery Tender can keep a battery charged without the risk of damage.