Difference between VBUS and Charge

[Aypac]

I'm thinking about creating a solar charger case. I created two different cases for the Samsung Galaxy Note 1 and one for the Samsung Galaxy 2. For that I basically took 8x0.5V solar cells (first case) and later 5V solar panels. Then I soldered to the battery pins (mobile phone side), connected it to a small PCB (with just two areas +/-) and connected to them with pogo pins. As "charge-regulator" I used a small led and a zener diode in parallel and a Schottky diode in series. Results were mixed, could never really determine how good the charging really was.

Now I'm thinking about using this controller: http://www.analog.com/en/products/power-management/switching-power-converters/switching-regulators/adp5090.html#product-overview as MPPT and charge controller with only 3x0.5V cells. Or I'll use the ATtiny85 and make my own controller. Anyway, for this I have a few questions (which I might answer myself, given the time to measure it): Is there any possibility to get power from the device? VBUS? Is it 5V or battery level? Is there any voltage level on the Charge-pin? What's the specs for that? Is it directly wired to the battery, so I should not charge with more than ~4.2V or shall I provide 5V at all times? Is a maximum voltage known?

[Aypac]

From the forum:

So, looking at the Palm touchstone, I measure 5.5v unloaded, and 5.3v when I load the charger.

Measuring the bus pin on the back of the Fairphone, I get 4v with the phone not charging, which makes me a bit nervous going over the standard 5.25v limit.

[Aypac]

Finally had time to do some measuring. All Bat-Voltages are taken by removing the bat from the phone.

Case a) -No Charger connected -Battery Voltage at 4.12V -Phone turned on

Charge is at 0V VBUS is at 3.90V

Case b) -Charger (Laptop) -Battery @ approx 4.1V -Phone is "off"

Charge pin @ 0V VBUS pin @ 3.99V

Case c) -Charger (Laptop) -Battery @ approx 4.1V -Phone is "off"

Charge pin @ 0V VBUS pin @ 3.94V

Case d) -Bat fully charged (says Phone) @ 4.31V -Almost no difference if FP is on/off and charger is dis-/connected

VBUS @ 4.12V

My personal understanding of this is, that there is a Schottky diode (or something similar) between VBUS and Bat+ => You can not charge via VBUS, you won't get higher Voltages than ~4.15V. Charge is always at 0V. With a conductivity test I get a reading of about 1.4V for the voltage drop between the Charge-pin and Bat+. This lets me believe, that there is some kind of regulator in between.

Surprisingly I got an almost perfect conductivity test between (GND)->(VBUS). This might be some kind of protection against wrong polarisation? But that would fry the PCB if enough power was connected, right? Nothing like that between (GND) and (Charge).

[dirkvl]

Wow, thanks for this! Will see if I can find some nice way to add this info!

[dirkvl]

Added it!

Also regarding your conclusion of the schottky diode: seems like this is just what you would see with a regular step-up circuit like this:

http://www.eleccircuit.com/wp-content/uploads/2007/09/lm2577-12-circuit.gif

Perhaps they do some voltage sensing and when there is a load and the voltage drops the step-up kicks in?

[Aypac]

Thanks!

I just took one more measurement today: If the phone is completely empty, the battery voltage is around 3.5V - which I find quite high - and VBUS is at 3.3V. Most charge controller for Lithium batteries stop at 2.5V... Phone is off, but difference is only 200mV, which is too little for any diode I know. ((Maybe a MOSFET?))

I don't believe there is any step-up. I just applied a 100Ohm resistance between VBUS und GND (around 40mA ~160mW), and the voltage drops.

I just noticed that there is no entry for External Port or something in the battery overview. I'd be interested what happens, if I'd apply a big 10Ohm resistance and discharged the battery mostly through it.

One of the days I will etch a small pcb with a small breadboard, so I can make some better measurements (eg charge behaviour etc).

BTW: I applied 5V to the CRG and GND for a very short time (few sec) and nothing broke. Could not see if the phone indicated charging...

[burakozhan]

but difference is only 200mV, which is too little for any diode I know.

If there is no load, the voltage drop across a diode is lower than the usual 0.7 Volts. With only a multimeter connected we can assume there is no load on the tested output.