I am attempting to build my own Bluetooth speaker. I know i could buy one and save myself a headache but it gives me something to do. I want to mimic this version seen in this youtube video:http://youtu.be/IwuvarrwhvQ The difference is i want to charge the batteries in place and also be able to run the speaker while connected to AC. Block Diagram The amp: http://www.ebay.com/itm/like/121201068010?lpid=82 The battery is one of the sticking points. I am considering something like this NiMH 18V 4200mAh High Capacity Battery Pack for the battery. Or maybe these in a holder UltraFire Protected 18650 3.7V 2400mAh Lithium Batteries. Six of these series provide about 25v due to the max charge of the individual battery being 4.2v. The Step-down buck convertor is simialr to this:DC-DC dual double output 7-16V Step-down BUCK convertor The 2nd sticking point is the charging circuit. Is there a PCB already built that I could use to charge the batteries and use the AC power while the battery is charging? Or can I use a switching power supply? Just looking for some advice or a point in the right direction. This doesn't need to run for 12 + hours. Just some thing to use around the house while, for example, folding clothes in the laundry room and watching netflix on the iPad and i need a little more volume than the iPad speaker. Thanks in advance.