[Epic 4G General] dwallersv:
I don't believe Current Widget on the Epic.

A rough "back-of-the-envelope" calculation gives the following as the approximate current draw for an awake Epic with the screen on:

Assume full charge lasts about 6 hours under these conditions.

1500mAH / 6H == 250mA

Which, seems reasonable when awake, powering the screen.

When asleep, and Juice Defender doing everything within reason to make the device take tiny sips of power, I can get up to 20 hours. This yields,

150mAH / 20H == 70mA

Of course, lots of peeps don't do everything possible to reduce power drawn during sleep, so for a lot of people sleep current will be higher.

So, how to interpret these rough calculations?

First, the charging logic in the phone distinguishes between USB connections and charger connections, and adjusts it's current draw accordingly (much less on USB). It detects the type of connection by sensing shorted data pins on the USB port -- if USB+ and USB- are shorted together, it sees an AC charger. This is the universal indicator of such as adopted by the cellphone industry, and may even have been made an official standard at this point.

96mA is almost certainly going to be insufficient to charge the phone while it is on and awake, with an active display. This more or less matches my experience -- my phone doesn't charge meaningfull if I'm using it while plugged in to my PC (I encounter this situation often while doing development for the platform)

How does LiION charging work? Pretty simple:

LiION is considered fully charged at a 4.235V and a current draw (by the battery in a charging circuit) of less than C/100, where C is the max discharge current rated for the battery. Placing a higher voltage at the battery terminals by a charger is dangerous, and can cause the battery to explode and catch fire violently. For safety reasons, virtually everyone considers 4.2V @ I<C/100 to be full capacity.

Charging is two-phase: First, constant current, then constant voltage. The charging circuit will control the voltage presented to the battery to maintain a constant current to the battery, recommended as C/10. I don't know the specs of our Epic batteries, but presumably they are capable of 650mA charging current, which implies a C of 6.5A -- far more than the phone would ever draw. LiION batteries are generally considered as "empty" at a voltage of 3.6V under no load.

Voltage phase: Once the max voltage is reached under the constant current phase, the charging circuit switches to constant voltage, holding at 4.2V, and allowing the current to gradually decrease as the battery "tops off". Once the "full" threshold is reached (nominally C/100, but that's generic -- exactly what the cut-off is for a particular device is up to the manufacturer, and varies greatly depending on design constraints/requirements, but doesn't ever exceed the safe limits for a particular battery design).

So, here's how your phone charges the battery: It first determines what sort of connection it has, AC or USB, and then sets the current limit for phase one on that basis. Roughly about half AC for USB.

The phone then controls the charging circuit. AC or USB are "dumb" power sources: The present a +5V constant voltage supply to whatever their rated current limit is. So, as long as the device doesn't exceed that limit, it will always see 5V coming in.

The charging circuit will then control the voltage presented at the battery to produce a constant current, based on what sort of connection it sees. Let's say this is an AC charger, and the phone circuit then draws 650mA under these conditions. If your battery is at 3.8V, the charger will feed it a sufficient voltage necessary to cause a draw of 650mA. Since LiION batteries have a very low internal resistance, this may be 3.81V.

As the 650mA flows into the battery and energy is stored, the voltage slowly rises. The charging circuit will maintain the necessary differential between the unloaded battery voltage level, and that presented to the battery, to keep a constant 650mA flowing in.

When the input voltage reaches 4.2V, the charging circuit switches to maintaining that constant voltage, and allows the current to vary. I (current) will then slowly drop off, asymptotically, toward zero. However, it will never reach zero due to internal leakage in the battery. So, when the charge current crosses over whatever threshold the designers have spec'd (nominally C/100), the battery is considered fully charged, the light on the Epic turns blue, and the charging circuit switches to a low, trickle current to maintain full charge (remember that internal leakage).

JUICE PLOTTER

The most accurate way to determine charging characteristics (current draw, voltage, etc.), short of using a DVM for direct measurement, is to keep statistics on the battery voltage over time, and plot it. The only tool I know of to do this is Juice Plotter, and its free. Disclaimer: I have nothing to do with JP, other than being a satisfied user.

Knowing the voltage limits for your phone that represent 0% and 100% charge, and the capacity of your battery, you can interpret the slope of the plots in JP to determine current draw pretty accurately under any conditions of use/charge.

For the Epic, the values are 0%=3.6V, 100%=4.18V, capacity is about 1300mAH (rated 1500, but we never get all of that -- like so many things, battery ratings are inflated, based on a full discharge at 3.2-3.3V; it is damaging to discharge a LiION cell below 3.2V, so no one uses that extra little bit of capacity below 3.5-3.6V -- maintaining a safety margin).

This translates into 1300mAH / 100% == 13mAH/%

So, if in JP the charge drops from 75% to 50% over 8 hours, the average current draw during that period is

(75-50)% * 1.3mAH/% / 8H ~ 40mA

So... If you really want to figure this stuff out, stop wasting time on things like Windows USB hub current figures (which I wouldn't give a spit in believing as accurate), you need to either measure directly with a DVM, or use some proxy method, as I outlined above with JP, on truly accurate data. Internally reported battery voltage, and time, are both know to be very accurate in today's phones.