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I am working on a project which the goal is using some android tablets to show pieces of video file to form a multiscreen. One electronic problem that I have faced is:

The tablets should be always on. Currently I am using their own wall adapter which is rated at 5V 2A but when the tablets screen is at 100% brightness their battery eventually depletes. I guess they are not designed for this kind of usage (Google nexus 7). The only solution I could find so far is to reduce the brightness to 30% but this kills the whole idea.

So do you suggest me to try another power source for example I have some LED drivers rated at 5V 4A. Is using this rating with the tablet is safe(considering USB cable and port, charging circuitry and battery itself)? Or the battery charging circuitry of the tablet is limited at specified 2A. I really need your opinion before I try to do something stupid!

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  • \$\begingroup\$ Are you able to open up the tablets and power the display backlights from a separate PSU? \$\endgroup\$ – RedGrittyBrick Dec 10 '13 at 10:09
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I'll assume a single LiIon or LiPo battery - nominally 3.6V, actually <= 4.2V and as low as about 3V. Some devices MAY use eg 2 cells and 7.2V and use a boost converter for charging, but this is rare.

If you are willing to access the tablet battery connections and to possibly use an external charger or even an external battery or power supply then you will be able to run it as 100% brightness continually, subject to cooling issues. See "The easy way" below.

If not, then:

There are two most likely internal circuit arrangements.
Also some variants, but the following two should cover most cases.

  • In one case - power input goes to device always and only via the battery, a higher current power supply will not help. This is because the internal charger is limited to 2A and the path to the device is via the battery so 2A is your limit.

  • In the other case - power supply goes to device proper and battery is charged from this point, a higher current supply may help. This is because the battery is "backfed at 2A max by the supply but there is also a direct path from external supply to device.

    In ether case it is probable that NO damage will be caused by a higher current supply - but this is not 100% certain.


The easy way

If you can disconnect the battery and feed the battery terminals with a 'virtual battery' with enough current capability then you should be able to operate the device continually at 100% brightness. Say for discussion that Imax is 3A. If you connect a power supply capable of providing >= 3A at anywhere from about 3.2V to 4.0 V to the battery terminals (~ 3.6V best voltage if selectable) then the device will see this as a battery and operate correctly. The supply should be able to provide any startup or other transient peaks (if any) and MUST NEVER provide voltages outside the usual battery range.

The external supply could be a battery plus a charger capable of sourcing >= 3A.

As long as you do not connect power to the device charging input it will see the externally connected power source as a discharging battery.

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The actual battery charging circuit is inside the tablet. People call the wall adapter "charger", but that's a bit of a misnomer. The actual charge circuit in the tablet accepts constant voltage DC, which is supplied by the wall adapter.

If you present the charge circuit with a constant voltage power supply capable of delivering more current, the charge circuit will not pull more current and charge faster than it does normally. It will just charge as usual.

in response to the comment: Here's an example to illustrate:

  1. Suppose, you have a charge circuit in the tablet. It's designed to charge the battery from a Supply #1, which is a constant voltage 5V 2A DC supply. Supply #1 can not deliver more than 2A. Charge circuit doesn't not use (or pull, or demand) more than 2A, by its design.
  2. Replace Supply #1 with Supply #2. The latter is a constant voltage 5V 3A supply. Notice that it can deliver more current. However, the charge circuit inside the tablet will not use more current and charge faster. It will still use only 2A max, because that's set in its design.

If you present the charge circuit with a constant current power supply, you are risking to fry (!) the charge circuit in the tablet.

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  • \$\begingroup\$ Thanks for answer. I have difficulties interpreting your sentence the charge circuit will not pull more current and charge faster than it does normally' I think its negative and it means I cant make the battery to keep charging while in heavy use? \$\endgroup\$ – Sean87 Dec 10 '13 at 2:08
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I'm not an electrical engineer, and no I didn't stay at a Holiday Inn Express last night, either.

The actual charging current may not be limited by your external charger or by circuitry in the tablet. There is more to this than a simple charging IC on the main board that controls charging.

It's software, too. Kernel, specifically. On Android devices you can flash different kernels to them which control things such as max battery charging rate. I'd do a Google search for "Nexus 7 fastcharge Kernel" (fastcharge is one word) and you'll see a few options that include that feature. Here is a link to one that includes the ability to fast charge.

http://rootzwiki.com/topic/37755-timurs-kernel-usb-rom/

You must make sure the devices are rooted with unlocked bootloaders, install ClockworkMod recovery and then use it to flash the file at the above link. Read all associated documentation. It will show you how to enable fast (higher current) charging while powered on.

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