Week 2--Problems occur What we have encountered As for the discussion last week, we decided to put two battery packs in a series, seeking to provide us with an approximate 20W charging power. Figure 1 However, we have discovered that this way the 5V charger would not be able to charge the battery packs since in parallel, it is 7.2V>5V. Jim has put up a thought that possibly we are able to charge two battery packs separately and use a toggle switch on the wire that connects two battery packs. However, this is not realistic for use as the user would need to switch it on each time we charge the phone. As mentioned in previous blogs, there exist two types of charger for an iPhone, one is able to provide a 5W charging power and the other is able to output a 20W charging power. We later compromised and decided to come up with another amplifier circuit with an input around 3.6/3.7V.