USB Power from Mobile Phone and Case 1

[dvzant]

Hello All, I am hoping for some help and/or direction with a project. I trying to create a product that incorporates an existing mobile phone accessory. I am starting with a phone case that holds a secondary external battery supply. Is the existing battery in the phone working simultaneously with the secondary battery in the case or do you deplete the existing phone battery and then charge it with the secondary battery?
I would like the accessory to draw power from the phone's battery, not the secondary battery. The accessory should draw power from the same USB that the external battery is plugged into.

I am a Mechanical Engineer and in the past I've stayed away from electronics as much as possible. I know I will need to do a fair amount of research for this idea and I'm hoping someone on here can at least point me in the right direction. Any advice or suggestions are greatly appreciated. Thanks, Dave.

 

[VEBill]

Accessory phone cases that contain batteries typically connect to the very same phone socket (Micro USB, or Apple 30 pin or 'Lightning') as would an AC "Adapter". So you can derive that the external battery will tend to recharge the phone's internal battery. The internal battery would probably remain at "100%" as the external battery is discharged.

Exactly what happens when the external battery drops very low in voltage probably depends on the power management or DC-DC converter IC(s) used.

 

[VEBill]

I'm not sure how much current (if any) can be drawn from any given phone's USB (or equivalent) socket.

There was a recent news article about a TV Tuner dongle that would attach to smart phones. The news article (not the most reliable source) indicated that the dongle designers had to include a power cable because the phones' socket wouldn't supply enough power.

This is probably going to be phone specific.

I believe that there's phantom power on the headphone socket. I've seen several audio I/O accessories that contain preamps that obviously need power.

 

[IRstuff]

Hypothetically, a compliant USB connector should supply 500mA. Supposedly, one can query the USB interface to verify its current capacity.

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[dvzant]

I bought a battery case last night and hooked it up to the phone. This case doesn't keep the internal battery at 100%. The phone (and battery) operates normally and when the internal battery is low you activate the case to charge the internal battery.

In regards to the proposed accessory... I can run the accessory using the external battery. However, I am hoping to run it off the phone's battery so that I can use the phone's battery meter and not need a separate meter for the external battery.

I have read in several places that the phone usb does not provide any power so I can only assume that those tuner dongles have a battery. I've also read that the audio jack provides little power and may not be enough for the accessory. I'm still researching but it looks like I might need to run the accessory from the external battery.

I would still like to get the internal battery and the external battery to stay at the same level which would require the battery to charge the phone continuously?... I'm not sure how realistic that is, I may need to have a separate meter for the external battery and make it a stand alone accessory. I'm going to play with the settings first to see if I can make external battery to continually charge the phone battery. If that fails I'll take it apart and see if I can bypass anything. As I said before, I'm a novice when it comes to electrical. I can make sense of the settings and play with that a bit. I'm a little overwhelmed when it comes to modifying circuit boards.

Any more helpful hints?

 

[waross]

Have you checked your phone? I just popped the back off mine and the battery is 3.7 Volts. The USB standard is 5 Volts. USB devices are designated as Host and Device. Only a Host or a charger has 5 Volts available. Just buy another meter for the new battery case.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[VEBill]

Nominal 3.7 volts is perfectly normal for Li-ion cells. Single cell is extremely common for gadgets and phones.

I thought that many gadgets have DC-DC converters inside, sometimes for providing 5Vdc outputs.

I assume that it depends on the specific phone under investigation. OP hasn't yet spilled the beans about which phone he's investigating (e.g. Apple, Android, other...). Most phones have a ton of technical data available on the 'net; if not from the OEM then from all the geeks tearing them apart.

 

[dvzant]

The 3.7V cell will be enough and 500mA should be fine. I understand that I can get 500mA from a laptop or desktop USB but I can't find anything that indicates I can get 500mA from phone. Many people are stating that the phone USB will not provide any power at all. I will check a couple phones to see if I can detect any power output.

The accessory will be universal in concept but will probably need to be phone specific in application as it will need to be integrated into a case. I will most like start with the trusty iPhone as it's the most popular phone.

 

[bradrs]

I don't know if this is helpful, but a few years ago I worked on a piggyback battery idea for a handheld game system. If you just plugged the battery/charger into the adapter, it would constantly top off the game battery, not very efficiently. But it worked so that it would completely drain the external battery, then the system would go to the internal.

The charging current on the game system was proportional to the battery state. So if you measured that current, you could get an idea for the battery state. I haven't watched a phone charge, so it may or may not be similar. And even then, you'd probably need to spec a charging current vs. battery voltage that might vary between phones, batteries, etc.

Pulsing the current in could allow you to try to keep them at similar states of discharge.

 

[jimkirk]

You can get 100 mA from any STANDARD USB port. To get 500 you need to go through a process of enumeration. This is some back and forth communication that requires a micro controller or equivalent.

http://www.maximintegrated.com/app-notes/index.mvp/id/4803

has a heap of information on battery charging and USB.

 

[dvzant]

I am now using the battery on the phone case as I cannot get any power from the phone battery. I would like to connect the accessory to the external battery but also connect it to the phone itself. The connection to the phone is not for power, it is to monitor the accessory. I would create an app that would monitor the accessory and maybe even control it.
I know that I will eventually hire an electrical engineer to create a custom pcb for my design but I would like to be able to hack the current pcb to prove out the design first. How hard or easy (or possible) is it to hack a pcb?