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6 Posts

04-07-2012

Prague, CZ

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Charging via USB analyzed

2012-04-07, 17:10 PM

I have TPT for 10 days now.

I have usual iisue with USB charging. When used like standard smartphone = put it to charger overnight. Not usable anyway. Charge rate about 5% per hour, which means 20 hours per full charge???

I even consulted it with local technical support. I was adviced to buy docking station with external power adapter .....

 

So ... I took regulated power supply, made a USB socket connected to it and started to measure :-)

.. and look what I found

 

When device is switched on, usb power oscilates sligthly under 1000 mA. It is NOT using all available power of 2 Amps of stock power adapter!!!!!

 

But ... when I power the device off using power button. The current goes down to 300mA for about 5seconds and then raises to full 2 A!

 

Conclusion? It  seems like control issue of the charging ciurcits. What if we can enable full 2A power even when turned on? SW issue?

Is anyone to push THIS into official bug list?

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14 Posts

01-18-2012

Germany

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  • Message 2 of 6

Betreff: Charging via USB analyzed

2012-04-07, 20:54 PM
My guess is that this is a protection feature. When connected to a PC instead of the power adapter, it would most likely prove critical for the PC's USB circuitry if the TPT were to draw the full 2A.
2A exceeds the USB specification by far. When switched off it is not very likely that the TPT is connected to a PC and more likely that it is hooked to its power adapter, so it draws the full power.
I did not measure it but when switched on, the TPT charges quicker in the cradle than on a PC.
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6 Posts

04-07-2012

Prague, CZ

28 Signins

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  • Posts: 6
  • Registered: ‎04-07-2012
  • Location: Prague, CZ
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  • Message 3 of 6

Betreff: Charging via USB analyzed

2012-04-07, 21:12 PM
You're wrong. The system detects AC power when data in/out wires are shortened - like is the standard for AC adapters. You have to modify USB wiring to enable AC power mode. In such case there cannot be computer on the other side.
When data lines go to USB data line, the power is limited to 500mA. Exactly like you expect to protect the PC port.

The difference in USB wiring can be clearly seen as power is reported either as "USB power" or "AC power".
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468 Posts

01-30-2012

United States of America

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  • Message 4 of 6

Re: Charging via USB analyzed

2012-04-08, 0:46 AM

wrote:

When device is switched on, usb power oscilates sligthly under 1000 mA. It is NOT using all available power of 2 Amps of stock power adapter!!!!!

 

But ... when I power the device off using power button. The current goes down to 300mA for about 5seconds and then raises to full 2 A!

 

Conclusion? It  seems like control issue of the charging ciurcits. What if we can enable full 2A power even when turned on? SW issue?

Is anyone to push THIS into official bug list?


Good catch. Here is what I got after discharging the TPT battery to 50% and using a watt meter on the high side (120VAC in the US) of the various chargers I have here;

 

 

TPT State            USB Charger          Dock Charger           Fast Charger

Off                               12.1W                        22.7W                       19.5W         

On, no apps                6.2W                          9.9W                       22.9W

On, standby                 3.1W                          6.3W                       21.5W

 

You're on to something, dock is behaving in a similar way. Only the fast charger is behaving as expected. Note that charger power starts to fall off when battery charge levels exceed 70%. Any tests should be made with a battery charge level below 60% IMO.

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6 Posts

04-07-2012

Prague, CZ

28 Signins

119 Page Views

  • Posts: 6
  • Registered: ‎04-07-2012
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  • Message 5 of 6

Re: Charging via USB analyzed

2012-04-08, 8:27 AM
short offtopic: What is "Fast charger"? No mention about it in catalogue here in Czech rep.
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468 Posts

01-30-2012

United States of America

600 Signins

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  • Message 6 of 6

Re: Charging via USB analyzed

2012-04-08, 14:08 PM

wrote:
short offtopic: What is "Fast charger"? No mention about it in catalogue here in Czech rep.

Fast charger is basically a charger with circuitry that emulates only the charging aspect of the dock and uses cable and connector to plug into the dock connector on the TPT. Uses the same or similar regulated 20V power supply the dock uses. It's potentially more versatile/portable than the dock and doesn't block the HDMI port. That's all I've been using for over a month now.

 

http://forums.lenovo.com/t5/ThinkPad-slate-tablets/High-voltage-charging-WITHOUT-dock/td-p/669989

 

http://rootzwiki.com/topic/8523-reverse-engineering-the-dock-charger/

 

http://forums.lenovo.com/t5/ThinkPad-slate-tablets/Fast-charger-issue/m-p/713979#M12879

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