It will “work” but it won’t last for multiple nights, and depending on how much fast slewing you do around the sky vs finding one target and letting it track all night, it may run out of juice after 4 hours or so. If you plan to run anything else off this supply, like dew bands, lights, laptop, motorised focuser, CCD then you need to think again.
It will need recharging after every session, and I suspect you will discharge it quite deeply so over the coming year or two you may well notice it retains less and less charge.
If this is just to power the mount, then I would just get someone who is OK at electronics to knock up some polarity safe-terminals on the end of an old car battery (40-60 amp hours); most garages will let you have one for £5 as they just go to the scrap man for the same price. Take a multimeter and pick one which has a resting voltage of >12.5v if possible, but at least >12v. You’ll need a trickle charger like this (https://www.amazon.co.uk/12v-Motorcycle-Trickle-Charger-SWTBC2/dp/B006B3MI8W/ref=sr_1_sc_1?s=electronics&ie=UTF8&qid=1503046011&sr=1-1-spell&keywords=charger+12v+streetwise). Stick it in a box, terminals on the outside. Job’s a good un, and all for under £40.
Deep cycle batteries would be better if you are using dew bands and other stuff, but an old car battery in ok condition will last many years if taken care of. I’ve only just recycled the ones we used at our observatory, and apparently, they were obtained 12 years ago.