A common rule of thumb we teach in Virginia RACES which works well in practical experience is to plan for a minimum battery in field deployments of one amp-hour per watt of transmitter output, for each 12-hour duty shift.
Your 17ah battery will power an HT, a TNC and a laptop for a portable packet or APRS station all day for a public service event. It would also work well in powering a mobile rig in portable use on low or medium power, but you would have to severely limit higher power transmissions of over 25 watts.
It would work fine for QRP rig, but if using it for a higher powered HF rig such as IC706 or FT100D, you should reduce your SSB output to 25 watts to enable the battery to last for a useful amount of time. This will work fine on 10 through 20 meters where background noise levels aren't as much of a problem. An RF output of 25 watts is only about an S-unit in signal strength below 100 watts, but if the background noise level is high, as often is the case on 40 and 75 meters, you may not be able to maintain reliable contact on SSB at this reduced power, particularly on a compromise antenna such as a mobile whip.
A battery I can generally recommend for ARES ands RACES auxiliary power is something in the same class as the Yuasa NP65-12. This is a sealed lead-acid 12V battery rated for 65 amp-hours at C/20 (3.25A over a 20 hours) discharge rate at 77 degs F. It weighs about 50 pounds and has retractable handles. You can see pictures of the battery or place an order at http://www.buck.com/yuasa.html
Tony Day, KC4AUF of Buckmaster is a battery supplier to Virginia RACES and we are very grateful for his support.
73 de KE4SKY
Virginia State RACES Training Officer