automatic charger is best, but if you want to do it manually:
For wet-cell batteries, supply a constant 14.4 volts DC to the battery (14.8 volts for a "maintenance-free" battery).
AGM and Gel-cell battery charging voltages vary by manufacturer; if you have one of these, you're on your own.
The initial current should be limited to 25% of the battery's amp-hour capacity. If you don't know the AH of your battery, 10 or 12 amps is a good starting current.
Don't let the battery temperature exceed 125º F (51.5º C); if it does you need to reduce the current.
As the battery recharges, it's internal resistance increases, which reduces the charging current.
It's done when the current reaches 2% of the amp-hour capacity, or else when the charging current stops decreasing. Turn it off or else it'll overcharge. If you don't know your AH capacity, use 1.5 or 2 amps as a rule of thumb.
It's normal for gassing to start at 80% to 90% of full charge (kind of a hissing or bubbling sound).
Time to completely recharge depends on the capacity of the battery, its state of charge, and the charging current.
An 80AH battery that is 50% discharged would take 4 hours at 10 amps (40AH divided by 10 amps), but since the current is decreasing, it would actually be around 8, maybe 10 hours.
To determine the battery's state of charge, use a digital voltmeter to measure the voltage across the terminals (analog meters are too imprecise). 75% state of charge is 12.45V, 50% is 12.24, 25% is 12.06, and 0% is 11.89. Those figures are at 80ºF (26.7ºC), and vary slightly with temperature (about half a percent for every 50ºF).
After charging, dissipate the surface charge by waiting 5 or 7 hours, or else turn the headlights on high-beam for 5 minutes then turn 'em off & wait 10 minutes. Measure the voltage across the terminals; should be at least 12.65.
If it's self-discharging too quickly, you have a few choices: test the vehicle's charging system, and/or measure the parasitic drain, and/or get a hydrometer (few bucks at a car parts store) and test each cell of the battery individually using the directions on the package (this won't work with a sealed battery).
