Gday, I bought an Ashton Busker amp yesterday so that I could go busking (duh). It has an internal rechargeable battery which I charged for the required 8-12 hours. HOWEVER, when playing it today with the battery (meaning it was unplugged), when I turned it up too loud then the sound 'fluttered' - as if I was turning the amp off and on and off and on. This didnt happen when it was plugged in to a power supply, so I think the problem is with the lead-acid battery failing to supply constant power. Has anyone had experience or got advice for this?