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5V 2A USB charging

I am having some fun charging the Lenovo Tablet 2...

It charges well from the 2-pin European charger which came with it (which I jam into a UK socket, using a screwdriver in the earth terminal to push down the shutters ) but it won't quite charge from a genuine Apple "10W" USB charger even when in the standby state.

I know "Apple's standard" is to use a resistor of some specific value (I actually measure 52.3k on the above one) connected across the two data terminals of the 4-pin USB socket of their chargers, to identify a charger which is capable of 2A. If the Ipad (etc) does not find this resistor it will then do the usual thing of asking for the normal max USB power of 500mA and (if the USB controller refuses) it will ask for the next one down which from memory is 50mA.

But for some reason the Lenovo tablet doesn't work with this Ipad charger.

Is there more than one spec out there?

On the Lenovo charger, the two data terminals are shorted (< 0.5 ohm).

Administrator
Shoreham EGKA, United Kingdom

The current Apple USB charger is 12W so they changed the "spec" once again. Soon you will be able to use it to charge the Boeing Dreamliner batteries...

Shorting the data terminals shows to the device that it is not a USB adapter but an AC adapter. The Apple "spec" is different, it shows to the device that it's a USB adapter supporting more power. Look at the power symbol on the Windows start bar, it should show a different mode when the Thinkpad USB charger or some other (e.g. your PC) is connected.

Yes, there are quite a few mentions of the Tablet 2 requiring D+ and D- to be shorted to suck up 2 A instead of 500 mA.

Apple apparently uses a voltage divider instead. Quoting Wikipedia:

Before the battery charging specification was defined, there was no standardized way for the portable device to inquire how much current was available. For example, Apple's iPod and iPhone chargers indicate the available current by voltages on the D- and D+ lines. When D+ = D- = 2 V, the device may pull up to 500 mA. When D+ = 2.0 V and D- = 2.8 V, the device may pull up to 1 A of current. When D+ = 2.8 V and D- = 2.0 V, the device may pull up to 2 A of current.

LKBU (near Prague), Czech Republic

Many thanks - that explains a lot. The Ipad2 charger is not much good then for charging other 2A devices.

This also means that the 2A PSU I have just bought on amazon is likely to need the two pins shorting - unless Lenovo's "standard" is used by others.

Administrator
Shoreham EGKA, United Kingdom

I have just modified a power unit which I have for ages had in the plane, with a USB output cable. It has several outputs, the main one being 19V 4A for the LS800 tablet.

For the USB, it has a DC-DC converter outputting 5.0V 2A.

I wired up the USB exactly the same as the Lenovo-supplied power unit i.e. the two data pins shorted to each other but otherwise not going anywhere.

It doesn't work... the tablet says "plugged in, not charging".

Then I measured the Lenovo power unit output. It is 5.49V! It says "5.2V" on the label.

Quite obviously, the Lenovo tablet needs more than 5V at its terminals.

It's a real con to sell a product which ostensibly charges via USB (micro USB) but it will never actually charge from any USB port that is in-spec i.e. 5V +/- 5%.

Administrator
Shoreham EGKA, United Kingdom

Does the tablet charge when connected to the USB port of a PC?

Not according to my tests. Same result - plugged in, not charging.

I think it needs 5.2V-5.5V before it will draw enough current to actually charge.

Also, from 5V, it doesn't charge even when switched off, which is very bad. Due to the stupidity of Lenovo (trying to be "minimalist" like Apple) there is no "charge" indicator so the only way to tell if the unit is charging (or indeed has any power connected) is to turn it on after a while and see if the battery % has gone up.

Administrator
Shoreham EGKA, United Kingdom

Indeed, it seems to require its AC adapter. I found this matrix on the Lenovo forums.

Yes; I saw that table.

It is disingenuous as to the extent to which the "charge slowly" is really "charge so slowly it is barely perceptible"

It is also very hard to find 5.2V USB power units. I bought one from Amazon but it is 5V...

The upper limit on USB is 5.25V so maybe a 5.25V unit, with thick cable, is the way to go if one wants to be able to charge other stuff from it.

Administrator
Shoreham EGKA, United Kingdom

my experience with Galaxy Tab - it´s not only about the right power/volts/ampers, I do suspect there are some crossed wires inside the original 30-pin cable. If you simply plug this 30pin cable into a normal USB port on your PC, the tablet is not charging. But once you plug a convertor (like this one - http://dx.com/p/usb-charging-adapter-for-samsung-galaxy-tab-black-151322) into the same USB and the 30pin table into this converted, your tablet gets charged. Some ASUS 10 inches are doing the same. So lenove might be the same. The latest of Samsung Tab 3 should have microUSB, so hopefully it´s going to be harmonised for future

LKKU, LKTB
18 Posts
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