In reply to M0ZZA:
Hi Tony,
As has been suggested I would recommend only using a power level necessary to make contact. I have used a Yaesu FT897D,(basically the same inside as a FT857D), on several activations & a 7Ah Slab was more than sufficient to last several hours with careful control of transmit power levels. Of course, If I had left the TX power at 100 Watts, it would have drained much quicker.
It makes good sense to only use whatever power is necessary for a QSO, although sometimes when conditions are very poor, even 100 Watts will not be enough.
I have found that a starting power level of between 10 & 20 Watts (SSB)is normally more than sufficient to get yourself noticed on an open band under average conditions, from there you could alter your power up or down accordingly & make efficient use of your battery power, which is after all, a finite resource on a summit.
If the HF bands are not playing, no amount of power on a particular band will get you a contact, so the ability to move around the spectrum is very useful.
Since I acquired an FT817 I have only used that radio for activations. I must admit I was a little wary at first taking no more than 5 Watts up a hill, but with the added confidence that if I had to use CW, I could, I have found it to be very rewarding, & somewhat easier to carry. HI!
I have activated the same summit twice this year, with the same radio under both very poor HF conditions,(G/SP-012 22nd May 2010) & under absolutely superb HF conditions, (G/SP-012 24th September 2010). In May I only managed one contact on HF with the other 21 contacts being on VHF. HF conditions were not very good at all that day & 22 contacts was my total for several hours on the summit. I as not complaining though, as the weather was glorious 
On 24th September with much better conditions on HF, I managed 48 HF qso’s in 59 minutes, with a final 2 qso’s on 2 metres FM using a handheld. HF conditions that day were superb with both 60m & 40 in fine shape.
I had found a 7Ah slab to be quite a bit more capacity than I would need for the FT817, so I took a 2.2Ah slab which still had plenty of life left in it when I got home.
Your 7Ah Slab should be more than sufficient for a single summit activation with your FT857, but for more time, you will need to drop the TX power down to only what is necessary.
More activators do seem to be looking at Li-Po batteries due to their smaller size & higher capacity. As mentioned, these have been discussed in great detail on the reflector, & I am sure that a search of past topics would be very informative.
With regard to the 7Ah Slab’s durability in a rucksack. Apart from accidentally shorting the terminals, you should not have any problems. Most Slabs used by activators have a gel type electrolyte, which should not leak, although the battery will vent any excess gas it may produce.
Best of luck with your future activations, & don’t be afraid to experiment
Best 73,
Mark G0VOF
