I refer you back to: How "fast" you can reliably charge your battery; and "Understanding a model for battery charge acceptance" both threads appearing under this subject heading (Power Equipment
and Electricity). Some of the details are not addressed by Nigel Calder and certainly not generally known in the cruising community in general.
Just to clarify the terminology no battery stores "Amps". In addition, no battery stores "Amp-hours". No battery is rated in "Amps". There is no such thing as "Amps per hour" in this technology. Storage
batteries are rated for energy storage
period. Just how that energy storage is rated depends upon how the energy is to be absorbed by a load. In the case of lead-acid batteries (flooded, AGM
, gel-cell, or whatever) the amount of energy DELIVERED to a specific load will vary according to rate at which the energy is depleated and the temperature of the electrolyte, hence the Peukert effect and formula descriptive of the inability of a battery to deliver an energy rated for a 20 hour discharge rate at a one hour discharge rate.
Storage battery energy is given in terms of Amps multiplied by time multiplied by voltage. An Amp-hour rating implies current
in Amperes multiplied by the time in hours and leaves out the variable voltage, which means that an Amp-hour rating is NOT an energy rating and is bandied about as though it is. That is why an AGM
battery having a normal standing voltage of 12.9V and a 100 Amp-hour rating has MORE stored energy than a flooded-cell battery having a standing voltage of 12.6Volts and also rated at 100 Amp-hours. Comparing the two is like comparing apples and orages without the voltage versus time Amp-hour discharge curve known. I have made many such measurements.
Some people say "Amps per hour" when they really should be saying, "Amps for the duration of one hour (or fill in the number of hours). Mathmatically the word "per" means divided by and, therefore, "Amps per hour" would be current
divided by time which is Coulombs per second per second which is the acceleration of electrons as they leave the source, not a meaningful concept
in battery terminology.
Although popular usage bastardises original usage it might be interesting to know that Amps is formally capitalized as is Volts because they are names of Ampere and Volta, the people for whom the terms honor. Naturally the adjective and adverb forms are not capatilized like voltage and amperage.
Storage lead-acid batteries designed as deep-discharge batteries are rated in terms of Amp-hours for a specified time. In the case of 12V nominal batteries the end of the discharge time is noted when the battery reaches 10.5 Volts at the rated load. For example, a 100 Amp-hour battery rated at a 20 hour rate (those that we are interested in because if they are rated at a 1 or 8 hour discharge rate we cannot translate that energy meaningfully into a once per day charge/discharg cycle like many of us do living on the hook) is rated at a 100 divided by 20 equals 5 Amperes load. At the end of 20 hours with a 5 Amp load expect the voltage to be 10.5 Volts regardless of the starting voltage.
Like Euro Cruiser wrote, if the battery is specified by cranking current you can assume that it is NOT a true deep discharge battery. It may be a compromise (translation: shitty at one thing and shitty at the other yet it will do both) design.
If you do not know assume that lead-acid batteries can deliver 1 to 1.5 Amp-hours per pound of weight. In my experience most Amp-hour ratings are optomistic yet in one case I actually measured an AGM 4-D case size battery at over 200 Amp-hours. In another case I measured a flooded battery in an 8-d case at 110 Amp-hours. You just don't know until actually making the measurement.