hi depends on how many pirs bellboxes keypads expanders you have on but i normally fit 2 x 7.0ah batterys just put two sets of battery leads into the panel or you can buy a battery splitter.
As you're going down the route of 2x7Ah Batts, please do it the correct way as recommended by Texecom for the Premier.
Premier Battery Splitter
Allows connection of two 7.2Ah batteries to a single set of terminals on a Premier Series control panel. Compliant with EN 50131-6 the Premier Battery Splitter independently monitors batteries, performing presence and load tests, signalling any faults to the control panel."
http://www.traderswarehouse.co.uk/Home/Batteries-Power-Supplys/Premier-Battery-Splitter
Tadaaaaaaaaaaaaaaaaaaaa
or stuck in a 17 Ah one
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