Battery Charging

Poor charging practices will kill batteries fairly quickly.  Leaving them discharged for much time at all "sulfates" them, temporarily or permanently reducing battery capacity.  Overcharging them drives off water and dries them out, permanently damaging the batteries.  There are also slow killers  Leaving them on charge continuously oxidizes the positive plates, which eventually fall apart.  Deeply discharging your batteries physically causes the plates to crumble sooner rather than later.  Then there is the matter of getting batteries charged as quickly as possible without damaging them.  If you get back to the dock every few days this is not a big problem -- use a modern marine two or three stage electronic charger.  If you live on your batteries every day like a cruiser, then everything becomes more challenging.

This is the bad news right up front: "Fast charging" is an oxymoron.  It is only possible in a very limited sense.  Fully charging batteries takes time, like the better part of a day.  Fully-charged batteries sit at 12.65 volts.   At room temperature.  Raising the voltage above that point will cause the current in the battery to reverse.  The charging lead-acid battery has a negative temperature coefficient.  This means that the voltage required to pass a given current through it drops with increasing temperature.  At 32 degrees Fahrenheit, it takes 12.85 volts to reverse the current through the battery.

A completely discharged battery will sit anywhere from little above 11 volts to zero.  Reversing the chemical reaction in the battery simply requires reversing the current through the battery.   Once the voltage across the battery is raised high enough to reverse the current through the battery charging takes place.  The higher the voltage, the more current flows at any given state of charge.  If the voltage is maintained at 14.0 volts, currents above 50 amps can flow through a battery.  Aside from requiring a monster charger, running very high current through a battery produces enough heat to raise the battery's temperature.

If you apply 13.5 volts to a partially discharged battery it will charge, very slowly and not completely.  If the battery is fully charged it will "float."  At about 13.5 volts the amount of charging equals or slightly exceeds the self-discharge.  The battery neither charges much nor discharges much.  The battery will not last forever in this state because parts of the lead structure in the battery that ordinarily serve as support or conductors will oxidize, sulfate or be otherwise slowly attacked and will physically fail.  So the concept of maintaining batteries by "floating" them has a big asterisk by it.  No lead acid battery lasts forever no matter how it is treated.  On a boat, between 5 and 10 years between replacements is doing somewhere between very well and spectacular.  Having to replace batteries every year indicates you have a problem, but some people live with it.

Wet lead-acid batteries self-discharge on the order of 10% per month or more.

If the charging voltage is raised to around 13.8 volts the battery will charge moderately well when it is fully discharged.  The more fully charged the battery becomes, however, the more it resists being charged.  The last 20 percent of charging will take a fairly long time, like a day.  So called "ferroresonant" chargers put out about 13.8 volts at no load over a reasonably wide range of input voltage.  When they were developed they were a blessing.  The completely unregulated chargers that proceeded them had to be physically monitored because changes in AC line voltage could turn what was a gentle charge to a hard charge or no charge.  The process was to set the unregulated charger to some reasonable voltage, say 13.8 to 14.2 and come back a day later and turn the charger off.  By looking at the battery voltage and the charging current you can make some conclusions about when the battery is charged.  By definition, when the charging current changes by less than some small amount with the same charging voltage around 14 volts, the battery is considered fully charged.  At 13.8 volts plus or minus very little, the charger could be left connected for weeks on end.  The battery must be maintained above the float voltage to charge completely.  Once it is charged, however, it "overcharges" slightly.  The concept of "overcharging" is a soft and muddy one.  If you continue to pass more current through the battery than is required to "float," which is a bit of a soft concept anyway, the water in the battery is broken down into hydrogen and oxygen and these gasses escape into the atmosphere.  The result is that the battery electrolyte level falls and needs to be "topped up" with pure water.