For the time being this is going to be in Accessories, but eventually it might get moved to Development as I get even more data. It's borderline at the moment.
It's been known for a while that the Nexus 7 is finicky about power supplies. In addition to the usual requirements for a power supply that complies with the USB Battery Charging Standard (D+ and D- shorted by the supply), which makes it VERY hard to find good supplies as most use Apple's nonstandard convention - The Nexus 7 seems to be VERY finicky about supplies which drop their voltage under load. Samsung tablet supplies will charge the N7 faster than the stock supply.
The charger controller chipset in the N7 is a Summit SMB347, same as found in a number of Samsung tablets. No datasheet is available for this chipset, but we do know it has an Automatic Input Current Limiting (AICL) feature - If the power supply "browns out", it will reduce current demand until the supply voltage raises to above the trip threshold.
Previously, I only had the ability to measure battery input current using CurrentWidget, but the top of my Christmas wishlist was an adjustable bench power supply. It has adjustable voltage AND an adjustable current limit - so I can limit the current delivered to the N7 and determine what voltage it settles at. If the voltage kept bouncing between two values, I recorded the voltage as the halfway point. E.g. if it bounced between 5.1 and 5.2, I recorded it as 5.15.
A picture of my setup is at:
https://plus.google.com/u/0/101093310520661581786/posts/gkbHaKKDnj6
Components:
Sinometer HY3005D from Mastech (Adjustable bench supply, 0-30v 0-5A)
22 gauge solid wire (the best USB cables I've seen are only 24 gauge - lower gauge = thicker wire)
Sparkfun MicroUSB breakout with D+ and D- shorted
Here are the results (Bear with me as I try to figure out how to properly format a table here on XDA... The HTML tag doesn't work as expected...):
Below 1A, I think the tablet goes into a more severe AICL mode - it will drop to around 600-650 mA, the power supply will exit current limiting (back up to 5.1 volts), and current will not go up until the device is unplugged and replugged.
One observation here - If the voltage drops 0.2 volts, you lose 400 mA of charge current. Many previous efforts here put an ammeter in series with the power supply - many ammeters drop 0.1-0.2 volts under load! The ammeter used by those previous effort was having SIGNIFICANT effect on the results.
It's been known for a while that the Nexus 7 is finicky about power supplies. In addition to the usual requirements for a power supply that complies with the USB Battery Charging Standard (D+ and D- shorted by the supply), which makes it VERY hard to find good supplies as most use Apple's nonstandard convention - The Nexus 7 seems to be VERY finicky about supplies which drop their voltage under load. Samsung tablet supplies will charge the N7 faster than the stock supply.
The charger controller chipset in the N7 is a Summit SMB347, same as found in a number of Samsung tablets. No datasheet is available for this chipset, but we do know it has an Automatic Input Current Limiting (AICL) feature - If the power supply "browns out", it will reduce current demand until the supply voltage raises to above the trip threshold.
Previously, I only had the ability to measure battery input current using CurrentWidget, but the top of my Christmas wishlist was an adjustable bench power supply. It has adjustable voltage AND an adjustable current limit - so I can limit the current delivered to the N7 and determine what voltage it settles at. If the voltage kept bouncing between two values, I recorded the voltage as the halfway point. E.g. if it bounced between 5.1 and 5.2, I recorded it as 5.15.
A picture of my setup is at:
https://plus.google.com/u/0/101093310520661581786/posts/gkbHaKKDnj6
Components:
Sinometer HY3005D from Mastech (Adjustable bench supply, 0-30v 0-5A)
22 gauge solid wire (the best USB cables I've seen are only 24 gauge - lower gauge = thicker wire)
Sparkfun MicroUSB breakout with D+ and D- shorted
Here are the results (Bear with me as I try to figure out how to properly format a table here on XDA... The HTML tag doesn't work as expected...):
HTML:
<table border="1">
<tr><td>Amperage Limit</td><td>Voltage</td></tr>
<tr><td>1.85</td><td>5.15</td></tr>
<tr><td>1.80</td><td>5.0</td></tr>
<tr><td>1.70</td><td>4.9</td></tr>
<tr><td>1.60</td><td>4.9</td></tr>
<tr><td>1.50</td><td>4.8</td></tr>
<tr><td>1.40</td><td>4.8</td></tr>
<tr><td>1.30</td><td>4.7</td></tr>
<tr><td>1.20</td><td>4.7</td></tr>
<tr><td>1.10</td><td>4.6</td></tr>
<tr><td>1.00</td><td>4.55</td></tr>
</table>
Below 1A, I think the tablet goes into a more severe AICL mode - it will drop to around 600-650 mA, the power supply will exit current limiting (back up to 5.1 volts), and current will not go up until the device is unplugged and replugged.
One observation here - If the voltage drops 0.2 volts, you lose 400 mA of charge current. Many previous efforts here put an ammeter in series with the power supply - many ammeters drop 0.1-0.2 volts under load! The ammeter used by those previous effort was having SIGNIFICANT effect on the results.
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