Charging a battery is like filling a water tank, how long depends greatly on the size of the hose. Fill the tank with the garden hose, your going to be there a while, and may not have the right stuff for the job. Fill the tank with the fire department's equipment, and you'll be done in a few minutes, they always have the right tools. Same thing goes for battery chargers, there is a right tool for the job, and size matters.
The right tool for the job is important, no matter what you do. With a battery the same applies. There are predominantly 3 types of battery chargers:
1. One stage battery chargers - aka no brains, hook battery directly to generator
2. Two stage battery chargers - electrical computer brains to monitor charge cycle.
3. Three stage battery chargers - higher tech computer brains to monitor things.
Long story short, if you aren't as attentive for 24 hours as the computer will be, buy the better charger. A two or three stage charger is mandatory if you don't want to harm those batteries. Remember that your batteries weren't cheap, and you are now protecting that investment. A computer chip is basically your insurance. To understand the nuances between 2 and 3 stage chargers click here.
Long story short, both 2 and 3 stage chargers flip through a series of stage, cycles, or modes as they charge the battery. Each mode has pros and cons, and so by using a symphony of these modes, you get the best results. Bulk mode fills the battery as fast as the sticker on the side of the charger allows, until it gets about 80% full. The bigger the charger, the faster float mode runs up to 80% full. Next a charger flips to float or absorption. Absorption finishes the battery quickly at higher voltages. Float finishes the battery slowly (24-96 hours), and compensates for sulfation in a prolonged storage scenario. For a complete explanation of battery charging modes click here.
Batteries and the stuff inside of them are not inherently stable. The electrons they contain are desperately trying to move. As those electrons are held in check by the chemicals in the battery, a bit of power is lost, perpetually. We call this loss self discharge, or the self discharge rate for a battery. Trickle chargers compensate for this phenomenon. Click here to understand more about trickle chargers.
Battery Charger Warnings:
Always read and follow the manufacturer's battery charging instructions prior to
connecting your battery, or trying to charge a battery bank. Do not attempt to charge batteries
in a confined environment. Explosive and hazardous gases are an inherent byproduct of
battery charging, do think ahead. Batteries contain sulphuric acid, and lead, both of which are hazardous material if removed from the battery, or disposed of improperly, do take care to be
environmentally responsible. Batteries are useful, just be safe.