Allan is correct regarding the deep discharge of Lead-acid batteries, it kills them very quickly.
Regular 50% discharge though will give you a reasonable life from a battery.
At shallow discharges, like 20% or less, they can last 20 years or more if maintained.
This ofcourse reduces the effective Ah rating. With a 50% max discharge, your 17Ah is effectively an 8.5Ah battery.
Another thing that most salesman will never tell you(many of them probably have no idea) the Ah rating of a Lead-acid battery is calculated with a 20hour discharge, or... 17Ah/20h = 0.85 amp draw for 20 hours. This is unique to Lead acid's as they have very high internal resistance and a 1hour discharge figure doesn't look as good for sales purposes
Other battery chemistries (NiCd, MiMH, Lithium etc) have very low internal resistance so are calculated with a 1hour discharge.
Higher current draws will derate this figure, at 2 amp draw the battery could probably be considered a 15 or 14Ah, then at 50% max Discharge, ~7Ah.
At 5A draw, maybe 10Ah, at 50% ... 5Ah
For these reasons, I used to tell my customers(portable solar setups) to come up with a $$ figure they would like as a limit, and then spend all of that on the largest battery they could afford.
Edit: The depth of discharge is determined by measuring the batteries open circuit voltage(no load) after resting for a while.
A 100% full battery should be approx 12.7 or 12.8v, at 50% discharge approx 12v. Going deeper is getting into low-life territory(regular replacement) but you won't be doing it daily so it may be okay. Anything below 10.5v(loaded) regularly will kill it. All of this does depend on cell chemistry and design, but generally is pretty close for all lead-acid types.
Edit2: haha
Attached a couple of images to show what I'm talking about, these are lifted from the datasheet for a Panasonic 17Ah AGM battery.
Which reminds me, try to steer clear of the GellCells, they are fussy little so and so's, go for the AGM type.
hehehe
).
