Thursday, June 20, 2013

Power Bank

I love free samples. Texas Instruments, Maxim IC, Microchip, and most other semiconductor manufacturers will send you quite a few parts for free, almost no questions asked as long as you are a student or engineer.

I discovered a new device by TI, the bq24195, that is able to efficiently charge a single cell Lithium ion or polymer battery, and also acts as a boost controller to boost the voltage to 5 volts in order to provide power to cell phones or tablets. It has lots of interesting features for controlling and monitoring the battery, and an I2C interface for control by a microcontroller.

I placed a sample order for a few of these devices as well as some TPS2511 USB port power switches from Texas Instruments, and am now building my own power bank/backup battery. It will be able to charge from any dc source between 5 and 20 volts, at up to 4.5 amps for quick charge times. The battery is a 1s2p Lithium polymer pack rated at 10,000 mAH, which should recharge my phone at least a couple times.

Many commercial backup batteries are severely over-rated as far as battery capacity goes, and some unscrupulous manufacturers even use recycled batteries. Good quality devices exist but can be expensive, plus I like the challenge of building my own to my own specifications. Currently I have the housing for the device made, and the circuit designed and laid out. The housing is made of walnut, and will feature two USB output ports, one micro USB charge port, and one dc input for higher voltage charging. The output ports use TI's TPS2511 device to allow my power bank to recharge almost any device. The microcontroller will be able to measure current exiting or entering the battery and will integrate over time to calculate the remaining capacity of the battery. Estimated charge state will be displayed on some LEDs on the outside of the case.

 The case is made from a solid piece of mahogany That I cut a pocket into. The top and bottom will probably be made of copperclad so I can build the display circuit directly onto the underside of the panel, allowing the display to shine through the PCB substrate. I've done this before, and it produces a good effect.

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