Re: Switching batteries to start the motor.
Battery capacity and battery "ability to hold a charge" are two very different things.
All lead-acid batteries (flooded, gelled, AGMs, TPPLs, etc.) begin losing capacity shortly after leaving the factory. While there are many reasons for this, the principal one is the reduction of available plate surface due to sulfation. Sulfation is the formation of PbSO4 crystals on the surface of the plates and, over time, the embedding of these crystals in the plates themselves, thereby reducing available surface for storage of energy.
The more time a battery is left undercharged, and the warmer the ambient temperature, the faster sulfation will occur. Full charging at proper voltages and periodic "controlled overcharging" are ways to slow down the sulfation process and to prolong battery life.
Consider the lowly battery in your automobile. It begins its life will something near "full capacity", i.e., it can deliver the rated CCA or CA needed for engine starting. Throughout it's miserable life, being tucked under the hood next to a hot engine in sometimes exceedingly high or low temperatures, its capacity decreases.
Maybe when new it can deliver 1,000 CCA. A year later it may only be able to deliver 500CCA. One more year and we're down to, e.g., 250CCA.
But, you don't notice this, because it still cranks your engine. After all, it only takes LESS THAN 0.5 AMP-HOURS to start your engine, and a few minutes after the engine starts the alternator has replaced the energy used for starting.
Finally, after three or four or five years there comes that morning when the battery won't crank your engine. Voltage may be good...12.6VDC resting...but it simply doesn't have the capacity needed to start your engine.
House batteries in boats lose capacity, too, and you're more likely to notice the loss because things don't run as long without charging, and because voltage drops faster than it used to.
How much loss of capacity is tolerable? How much is too much?
That depends on the application, and your preferences and your cruising style. If you're mostly at the dock or do overnites or weekend sailing, with a bunch of motoring to boot, you'll be able to tolerate large reductions in capacity. After a few years your spiffy 400AH battery bank may only have about 200AH capacity, or even less. But, because of the way you use your boat, that's OK. And, you can continue along until, presto, you decide that the batteries are just too old or "too weak" and need replacement.
For one sailor this point may come when the batteries have lost half their capacity. For another it might be 75% loss. For still another it might be 90% loss is needed before replacement is considered.
And that, in a nutshell, is why statements like "I got 12 years out of my batteries" are virtually meaningless, since they say nothing about real capacity of the batteries or cruising style.
How do you know how much capacity is left? Only a 20-hour controlled discharge or measurement with a good (and expensive) inductance/conductance tester like the Midtronics series will tell you. The difficulty in measuring true residual capacity is why so many of us are willing to seek proxies, like voltage measurements, which in reality tell you almost nothing about residual capacity.
Last edited by btrayfors; 10-12-2012 at 06:55 PM.