Can some devices be powered directly from a wall outlet?

[theducks]

Originally Posted by burnafterreading

incorrect as far as your claim that ALL devices not using a battery when plugged in.

my Galaxy Nexus has a removable battery (a Li-ion), and when plugged in with a microUSB cord, it'll charge the battery. while running, removing the battery causes the phone to completely shut off, and the power flowing from the USB charging port will not keep it turned on. and this is just a smartphone, not a power-hungry tablet. this is dead-easy to actually check on a case-by-case basis on any device in which you can remove the battery while using the device.

there's (at least) 2 ways of designing a device to run on AC-plus-DC

1 - AC charges battery only. battery feeds power to device. separate circuits.
2 - AC feeds power to device and charging circuit. when AC is gone, internal circuit switches to draw power from battery.

#1 is how most portable devices behave. there's NO interruption of power as long as the battery is connected (and in many cases, the battery is not easily replaceable anyways, so it's a safe bet)

#2 is how most affordable UPS / power backup systems work. the connected devices (pc, tv, etc) are usually able to handle a short blip in power (like one missing cycle from the 60Hz line frequency)

Smart phones are still power hungry (just less ), which is why they usually ship with a power adapter with an output GREATER than 500mA (the Max permitted on a standard USB port).

Removing the battery just proves that the current limiter is working properly on your computer (or the internal sense circuit detects a real USB port and throttles the power to prevent USB port shut down).

Try your test with a 1A+ charger,

[davidfor]

Originally Posted by burnafterreading

incorrect as far as your claim that ALL devices not using a battery when plugged in.

my Galaxy Nexus has a removable battery (a Li-ion), and when plugged in with a microUSB cord, it'll charge the battery. while running, removing the battery causes the phone to completely shut off, and the power flowing from the USB charging port will not keep it turned on. and this is just a smartphone, not a power-hungry tablet. this is dead-easy to actually check on a case-by-case basis on any device in which you can remove the battery while using the device.

Sorry, all that proves is that an idiot designed the device.

there's (at least) 2 ways of designing a device to run on AC-plus-DC

1 - AC charges battery only. battery feeds power to device. separate circuits.

So, how does that work? There are wires connected to the battery terminals that charge it. There are other wires connected to the same terminals to run the device. As they are both connected to the same terminals on the battery, there is a direct connection from one to the other. How does the electricity know that it has to come down the charging wires in to the battery and not out the device powering wires?

And for a Li-Ion battery, that sounds like a nice quick way to kill the battery life and possibly turn it into a bomb.

2 - AC feeds power to device and charging circuit. when AC is gone, internal circuit switches to draw power from battery.

#1 is how most portable devices behave. there's NO interruption of power as long as the battery is connected (and in many cases, the battery is not easily replaceable anyways, so it's a safe bet)

It's not how all portable devices behave. It definitely isn't how any of the laptops I have used behave. And it isn't how my Kobo devices behave. I have opened them and run them with a cable connected and the battery disconnected.

[burnafterreading]

Originally Posted by theducks

Smart phones are still power hungry (just less ), which is why they usually ship with a power adapter with an output GREATER than 500mA (the Max permitted on a standard USB port).

Removing the battery just proves that the current limiter is working properly on your computer (or the internal sense circuit detects a real USB port and throttles the power to prevent USB port shut down).

Try your test with a 1A+ charger,

um, i tried it with a 2A charger, so there was no issue of current limiting as far as the charger was concerned. and with the phone having no SIM card (ergo no cell data) and the screen on 25%, it was by no means a power hog, not even close.

Originally Posted by davidfor

Sorry, all that proves is that an idiot designed the device.

/ and a bunch of other stuff /

you said ALL devices behave like that, and i proved you wrong, and now you're arguing that since your specific devices worked like that then ALL devices on the planet work like that?

it's cool - you made a blanket comment that was incorrect. accept it and carry on

[5thWiggle]

Removable LI batteries have a built in temperature sensor (a thermistor usually) included in the package. Ever notice how they have more than two terminals? One of the terminals (or two depending on the manufacturer) is for sensing the temperature and possible explosion if charged too fast. When you removed the battery, the phone's internal charging circuit shut down because it could no longer measure the temperature. If you knew what terminals are the sense terminals (different for each manufacturer) and what resistance the thermistor is a room temperature (usually around 10Kohms), you could connect a resistor to the correct terminals on your phone and run it from the wall charger or computer USB port without a battery. It has nothing to do with the charge only being able to come from the battery, or current limiting from the computer USB port.

Devices with non-removable batteries have this thermistor (or other temperature sensing) built into the device itself and usually glued onto or near the battery which is why the non-removable batteries only have two terminals.

[speakingtohe]

Batteries do what batteries do in my experience. I have a lot of battery powered devices and numerous chargers.

Everything works fine mostly, but a week ago 5 ereaders all flatlined on the same day despite 2 being charged within a week. Not all the same brand. No big deal, the batteries are all holding a charge now, but seems peculiar,
Helen

[davidfor]

Originally Posted by burnafterreading

you said ALL devices behave like that, and i proved you wrong, and now you're arguing that since your specific devices worked like that then ALL devices on the planet work like that?

it's cool - you made a blanket comment that was incorrect. accept it and carry on

I probably should, but...

Your example didn't prove that the device runs on the battery when plugged in. It simply demonstrated that the device will not run without the battery installed. That is a very different thing. There are reasons to do this. Personally, I go with bad design, but as 5thWiggle suggests, it might be a safety feature.

My "all" statement was based on my understanding of how Li-Ion batteries work. They can't be trickle charged. All, and I do mean all, Li-Ion battery chargers will charge the battery to full and then stop. They don't kick in again until the charge level drops below a certain point. That drop will be either because the device is disconnected and used on battery, or due to self discharge. For laptops, the recharge point is usually 95%, but I have one laptop that uses 97%. For phones it recharges when connected at 99%, but, I've never had a phone connected to power for long enough to see if it restarts charging after the battery has self discharged a bit.

Maybe I shouldn't have said all, but because of the above, and that I can't work out how to do it, I feel pretty safe in saying it.

[DuckieTigger]

@davidfor: I don't believe either that phones usually run off the battery if plugged in. One thing I do know though from experience is that, if needed, the phone will discharge the battery on top of using power from the charger. If you have a device that has fast charging capability then it is impossible to drain the battery while plugged into the fast charger.

[TechniSol]

Speakingtohe,

This is pure conjecture but, after consulting my brother who lives with his wife, my two lovely nieces and their female dog -and is a survivor of similar phenomena, that because of their close proximity under one roof your e-ink reader's cycles have likely sync'd... ;-) Look for my article in "UnScientific American".

[crich70]

I have a small android operating system run laptop that has to be plugged in when you turn it on otherwise once the battery is drained it won't turn back on until the battery is fully recharged. I think it was made over in China.

[Masutin]

Thanks a lot for all replies! I'm sorry if I misled anyone by fallible notions of electronics.
To recap, I was curious about a tablet or similar device, with a screen of any size or type, that can
run on AC power (and bypass the battery when plugged in) and
switch to the battery only when unplugged.
The majority claim that many devices can do this, however the discussion hasn't produced complete agreement yet.

[burnafterreading]

I guess the question would be: why would the battery be removed when on AC power??? And would the user need to insert the battery before removing from AC power? Seems needlessly convoluted...

[cromag]

Originally Posted by Masutin

Thanks a lot for all replies! I'm sorry if I misled anyone by fallible notions of electronics.
To recap, I was curious about a tablet or similar device, with a screen of any size or type, that can
run on AC power (and bypass the battery when plugged in) and
switch to the battery only when unplugged.
The majority claim that many devices can do this, however the discussion hasn't produced complete agreement yet.

The debates seem to be concerned with extreme scenarios.

I have run Android tablets until they have alerted me that the battery was too low and shut themselves down. When I plugged them in to charge I was able to turn them back on (while charging) and use them (while charging). It doesn't really bypass the battery, which is still charging, but I can use them.

I hope this is what you were asking about.

[davidfor]

Originally Posted by burnafterreading

I guess the question would be: why would the battery be removed when on AC power??? And would the user need to insert the battery before removing from AC power? Seems needlessly convoluted...

A lot of people recommend removing the battery if running a laptop on AC power for a while. The reasons given are to prevent overcharging and to keep the battery cool. The first is invalid but the latter has some sense. Heat makes Li-Ion batteries age faster which means the available capacity reduces faster. And if you store the battery correctly, the ageing is slowed and the overall life of the battery is extended. Personally, I don't see the point as I think the advantage of having the battery in the laptop far outweighs any extension to the life of the battery.

The same would apply to the other devices such as tablets, ereaders and phones. But, from what I have seen, most are designed in a way that removing the battery is difficult making this impractical. Plus, for most users, being able to disconnect without thinking about it is a big plus.

As to a use, I have seen a few business with a tablet of some sort used as a display or customer interface. In each case, a battery isn't needed as they are plugged in all the time. Yes, the battery is a UPS, but none of the systems they talked to were protected, so keeping the tablet running isn't needed. Removing the battery might make some sense for these installations.

But, the OP isn't interested in this or removing the battery. I think he is really wanting to know if there are devices that if plugged in for a longer time won't cause wear on the battery. Personally, I think the answer is all devices do this, but maybe some are designed differently.

[Masutin]

davidfor,
"if there are devices that if plugged in for a longer time won't cause wear on the battery. Personally, I think the answer is all devices do this, but maybe some are designed differently"
Yes, a fine way to put it. The issue is to avoid (unnecessary) use of the battery when the unit is on AC power. I was under the impression that out of portable devices only laptops and netbooks were designed with this in mind. The conclusion seems to be, most do it and the proof is in removing the battery if possible and trying it.

[TechniSol]

Unless a device is supplied with an adaptor incapable of powering it and charging at the same time, it is extremely unlikely that the battery will be used while the adaptor is providing power. Further, modern devices typically employ a combination charge controller chip and circuitry which independently controls battery charging and in some cases offers soft power control activated by a momentary contact pushbutton rather than a traditional on-off switch. The charge controller typically controls the connection between the external power jack and the battery for charging as well as the connections between both the battery and power jack and the device being controlled.

Back in ye olden days(say back when microcontrollers were mask programmed and you probably only had one in your family microwave or Beta VCR, maybe one in your car, and modems were king, cellphones lunch pail sized, e-ink not yet a gleam in a designer's eye and I in my youthful exuberance played "pin the tail on the dinosaur..." or at least Caves of Karkhan on an Apple ][+) a relatively cheap diode was inserted between the battery and the circuit, and the external power was connected after the diode before a traditional on-off switch which then connected to the circuit load. The polarity of the diode prevented the backflow of current to the battery so charging did not occur, and as long as the voltage of the power coming in exceeded the voltage of the battery minus the voltage drop of the diode no battery power was consumed. Nice cheap trick, but it did not allow for charging rechargeable batteries, however it was a handy way to build a device that could be powered by "dry cells" or primary non-rechargeables without switching or requiring users to remember to remove the batteries lest they be overcharged and do messy, icky, leaky things...