I can't seem to find any helpful info online about the internal batteries for the Medela Symphony. The battery doesn't seem to hold a charge at all. Even if I charge it all night, as soon as I switch to battery power it says Low Battery and doesn't last through a single pumping. I thought it was a defective pump and exchanged it at the place I am renting from, but the new one has the same problem. I travel for work and need to be able to pump while travelling, which was a total nightmare on my last trip. Has this happened to anyone else? Any ideas on what I might be doing wrong? All help is much appreciated!