Having forgotten to regularly charge my powertank it now only lasts around the 3 hour mark and I am only using it to power my Celestron 8SE. I tried searching on the forum for suitable replacement batteries or measurements as I just wanted to replace the battery. I couldn't find this info anywhere so I took the side off (8 screws on the 7ah version—see image—the only hidden screw was in the accessory tray under the lid—although you do need a long-bladed (?) Philips to reach some of the screws). Once off I measured the battery and it is Length 150mm, Height 100mm, Width 65mm. I thought at first that I would be able to fit a larger capacity battery but soon realised that you can't unless you started modifying the case. The largest capacity battery I could find with the right measurements was a Yuasa Yucel Y9-12 which is a 9ah model priced at £15.99 from Hardwareexpress. I hope this will help others with the 7ah powertank who want to change the battery and who can use a screwdriver.