I would appreciate help with a schematic and component list.
I would like to build a backup battery pack for a (i.e. Samsung Galaxy III) cell phone based on rechargeable AA batteries that will plug into the phones USB charger port to power the phone when the phones internal battery is low. I would also hope to recharge this external pack with a USB charger.
Is it even a valid concept? Assuming NiMh batteries at 1.2 V x 4 = 4.8V. Is that enough margin to drive the charger circuit for a phones 3.8 V battery? I think a USB chargers Nominal output is 5V, Is that enough margin to charge 4 NiMh? If not, Can they be charged as two cells each in parallel and discharges as 4 cells in series?
Thanks for any guidance.
Battery_pack rechargeable USB Cell_phone schematic
Yes this is a valid concept but there's more to it than simply connecting power, depending on what the power source is. In your implementation the charging voltage should conform to the USB 2.0 standard of 5 volts +/-0.25 volts and be able to supply 1.5 amps. The four NiMH batteries will need to go though a DC-DC converter to provide a USB legal voltage. The cell phone will be looking for a short between the USB D+ and D- lines to tell it that it's connected to a 1.5 amp charger and can draw current without negotiation. This Maxim ap note decribes this.
To get 5.0 volts from the NiMH pack a switching power supply can be used to boost the voltage. Or, use a six NiMH pack with an LDO (Low Dropout regulator) to regulate it to 5.0 volts.
To charge the four cell NiMH battery pack a constant current source having a voltage compliance of at least 5.6 volts will work. This can be as simple as a 6 volt source with a series limiting resistor. Or it can be as complex as a NiMH charging IC connected to a source of DC power.