Originally Posted by southace
Basically I believe the amount of charge you take out of your battery should be replaced with the equal amount of charge that goes back into the battery.......that is the only way you will get 10years.
There is far more to it than that though.
There are many types of batteries. The common types are gel and water
lead acid. After that there are sealed lead acid and regular "water" lead acid. The voltages used to charge them are different.
However equally important is how the plates are made. A plain old car battery has many very thin lead plates with high surface area. The high surface area provides a lot of area for the process of creating current
and is used for STARTING an engine, which takes a MASSIVE current
draw for a very short time (usually a few seconds).
A "Marine" battery is often used to start outboards and thus also needs SOME thin plates to provide that high current for the short time the engine is starting, however it also has some thicker plates.
A "golf cart" or "true deep discharge" battery has a few very thick plates. Furthermore to that point, when you use 6V deep discharge batteries, the entire amount of lead is used for only half as many cells (6 v instead of 12 v) and thus you get even more lead per cell, more thicker plates.
This matters because the lead plates are literally eaten away over time. Thin lead plates are consumed faster than thick lead plates. Car batteries will simply die, VERY quickly, if used as a "deep discharge" battery. Marine
batteries will last longer, but nowhere as long as the true deep discharge batteries.
So starting batteries are designed to provide a huge current for just a few seconds, but NO long term current draw. Marine
batteries are designed to provide a huge current for a short time but still provide a medium current for a long period. But they are a compromised design as a result.
So a house battery is best server by pairs of 6V "golf cart" batteries, DESIGNED specifically to provide a medium amount of current for long periods.
IN ANY case, you should never draw more than 50% of your current before recharge, and you should absolutely NEVER leave a battery in a partially discharged state for long periods of time, and you should absolutely NEVER EVER leave a battery low on water, as the part of the plates exposed to air will very quickly be permanently damaged.
They actually make a little device that measures the current out and in to a battery. They use these typically for solar
systems so that you can know with some certainty what the "charge level" of a battery (or bank) is. IOW you drew X out and put Y back in, the charge state is Orig - X + Y.
BTW. trying to determine the true charge state by reading the voltage is not very accurate, or at least is difficult. After charging, the voltage remains artificially high, and requires a small current draw for a half hour or so, OR you must allow the battery to sit without charging or being used (no current in or out), for about 24 hours to really be able to use the voltage as a reliable indicator.
My point is that "replacing the charge" isn't as simple as it sounds. You could use 1/2 of the current (theoretically OK for deep discharge batteries only), leave the boat for a month and then put that 1/2 charge back in. You have "replaced the charge", but would likely severely damage the battery in the process.
And finally, a battery will "self discharge" over time. So you can damage a battery simply by leaving the battery in storage
for 6 months without a smart charger on it.
Believe it or not, You can also damage a battery by over charging it, which is why smart chargers were invented. They are designed to put out a "bulk charge" voltage (14.5 V for open lead acid) and MEASURE the current into the battery. A fully charged battery will not accept much current. The smart charger figures out the charge state by the current that can be "forced" into the battery at the bulk charge state and then either leaves the battery to bulk charge (if very discharged) or cuts the voltage back to a lower voltage to top off, or to trickle charge the battery. All the while monitoring the current going in so that it can decide when to adjust the charge voltage (up or down) to correctly charge and maintain the battery.
Perhaps more than most people want to know, but we need to know this stuff if we don't want to buy new batteries every three years (or less).