Designing a portable usb charger - Page 1
Forum section: electronics / projects

author: ChrisGreece52
author_rank: Frequent Contributor
timestamp: February 19, 2015, 07:19:55 pm
content: Hello its been about 3 months since i got my portable usb charger from ebay (its a cheap product which i got for about 5 usd) but i dont think it is reliable at all.Its defects:Falsely advertised capacity :it said 2 Ah my phone's battery is 2,3 Ah and fully charged it charges it about 20-30 % with this figure i think its around 1,2 Ah or so (correct me if i am wrong it is just an assumption)Takes a lot time to charge: The IC (which was scratched clean) is only drawing about 0.5A from the usb port (Not on my pc !! it is connected to my powercube (which can provide up to 2A)http://www.allocacoc.com/product-pages/rewirable-family/powercube-rewirable-usb/Unreliable : I do not think that the design of the product is top quality!!! you can also verify that by the price tag (5 USD)So i started designing my own portable usb power supply.Using these batteries :http://www.ebay.com/itm/131324283630?_trksid=p2060778.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AITThe circuit will be attached bellow.The principal of the circuit is this.The batteries will be in 2 packs of 2 connected in parallel2 USB inputs / outputsThe 2 inputs will be connected via a switch in parallel so i can only charge one pack or both.then the batteries will be connected in parallel and the output will go to a DC to DC converter which i did not find yet because i would like a 2A output on both usb ports.The IC used to charge the batteries is the MCP7381 from MicrochipDatasheet :http://ww1.microchip.com/downloads/en/DeviceDoc/21893F.pdfAnd here are my questions :1) Can these Lithium Ion batteries handle the 4 A load?2) Can the IC i chose provide a safe charging for the batteries?3)Will the step up regulator be able to supply at least 5A (just to be safe)4) Will this parallel USB input method work ?5)What would be the most efficient DC to DC converter?Thank you and if there are any suggestions i would be very happy if you posted below thank you in advance.

author: HKJ
author_rank: Super Contributor
timestamp: February 19, 2015, 08:15:35 pm
content: Quote from: ChrisGreece52 on February 19, 2015, 07:19:55 pmUsual it is the battery capacity that is specified, not the output capacity.Quote from: ChrisGreece52 on February 19, 2015, 07:19:55 pmProbably not, and do not expect more than 1500mAh to 2000mAh from them.You can find lot of battery and other tests here:http://lygte-info.dk/

author: rx8pilot
author_rank: Super Contributor
timestamp: February 19, 2015, 11:06:33 pm
content: If the data pins are left open, most devices will limit themselves to 500ma or so regardless of what the supply can do. To get a tablet or high-end smart phone to fast charge will require the data pins to be connected to a voltage divider. Apple and Samsung have variations of the scheme so I have used a Maxim controller to ID Apple and Samsung products and they will charge about about 2A or so.

author: ChrisGreece52
author_rank: Frequent Contributor
timestamp: February 20, 2015, 01:34:31 am
content: Quote from: HKJ on February 19, 2015, 08:15:35 pm2 amps would work i will just use one USB output and ill be fine

author: ChrisGreece52
author_rank: Frequent Contributor
timestamp: February 20, 2015, 01:46:17 am
content: Quote from: rx8pilot on February 19, 2015, 11:06:33 pmI did not think of the data lines because i have an Xperia SP and it charges with the 5V and ground pins only.with a bit of googling i found this site that explains the resistor values neededhttp://www.obddiag.net/usb-power.htmlfor samsung products

author: HKJ
author_rank: Super Contributor
timestamp: February 20, 2015, 07:21:57 am
content: Quote from: ChrisGreece52 on February 20, 2015, 01:46:17 amMost advanced usb equipment checks the data pins to determine charge rate. The simplest is to short them, this says that the port can deliver 1A to 1.5A according to the usb standard.Apple, Samsung and some other are using their own coding, the most common is, of course, Apple with codings for 0.5A, 1A and 2.1A.

author: ChrisGreece52
author_rank: Frequent Contributor
timestamp: February 20, 2015, 11:46:11 am
content: For now i wont be needing the divider but i can add it later if i want ... what i really want to know is what dc to dc converter should i use and if the MCP7381 is a good choice.I am pretty confident about the circuit but i would like your opinion!