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Tablets have gone mainstream a few years ago, and Li-Ion batteries have a lifetime of a few years only. This paves the way for a new need for us couch potatoes: to be able to use a tablet when the (Li-Ion) battery is no more reliable and has to be taken out (and no replacement is available).

In order to make my question the more general possible, since I believe many will benefit from advice on how to tackle this problem, I'll have to be overly wordy.

The problem:

  • a) A tablet (for which we assume we have no schematics) whose battery has ballooned to the point it popped open the chassis
  • b) no replacement battery available from the manufacturer or seller; other batteries of similar capacity are too big to fit
  • c) it appears it is not possible to run the tablet without a battery with the charger only, and leaving the ballooned battery in is a bit too risky.

The idea for a solution:

To use a suitable circuit that will simulate the battery presence so that the tablet can be run when attached to the charger (we assume that with the original battery fully discharged the tablet runs ok). The circuit might use a smaller battery and or a supercapacitor or some other magic gizmo to do what follows (note: only point I is strictly required):

  • I) MANDATORY - operate the tablet while connected to the charger with no battery inside. In this case a smaller battery or supercapacitor (or even capacitor) will only act as a buffer to protect the mobo (this should be easy), while some other circuit might be needed to fool the tablet in believing there actually is a battery and it is being charged (this might be tricky). Let's say that the main purpose here is to avoid a ka-boom.
    If necessary, a custom charger (or to be more precise, power supply) can be built form scratch (either with a 7805, LM317 or a DC-DC converter) - there are no problems in finding a schematic for that.
  • II) OPTIONAL - the smaller battery or supercapacitor should act as a temporary buffer to power the tablet for a handful of seconds to allow for a soft shut down in case the charger is accidentally disconnected.
  • III) OPTIONAL - the additional circuit might fool the tablet into believing there is a battery (as above), and that it has been charged to the point it is possible to update the OS (I have read, for example, that the Nexus modifed to run with directly with the charger won't update unless it detects a charged battery).

Pointer to reference needed:

To implement point I) we need to know

  1. What storage element to use: would a (flat, <9mm) supercapacitor do?
  2. Can we simply hook that up with a diode to the charger leads on one side and to where the battery leads went on the the mobo?
  3. Do we need a thermistor to fake an overheating protection?

To implement points II) and III) requires knowing:

  1. How do (most commonly used) Li-Ion batteries communicate with the motherboard
  2. Is it possible to create a custom circuit that fakes the communication for "battery is ok, and it is 100% charged", small enough to fit in a battery form factor?
  3. Is it possible to infer how the charging circuit works by looking and the PCB of motherboard and battery protection?

Answers to single points are welcome. Partial solutions can be used as starting point for tackling one's specific problem. If needed I can post pictures of my tablet circuits but that would defeat the purpose of generality.

(I see this question has been put on hold to be closed. I had a look at the guidelines mentioned below, and I do not think they apply to this case. This is not a "repair" question for a single device. This is a general question on how to create a new circuit that will make all those tablets with dead batteries work again. I am not claiming that doing so will revert climate change but a couple of polar bears, those might be saved)

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closed as off-topic by Nick Alexeev Sep 20 '15 at 22:58

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the repair of consumer electronics, appliances, or other devices must involve specific troubleshooting steps and demonstrate a good understanding of the underlying design of the device being repaired. See also: Is asking on how to fix a faulty circuit on topic?" – Nick Alexeev
If this question can be reworded to fit the rules in the help center, please edit the question.

  • 1
    \$\begingroup\$ Describe how many connectors your battery has. If the battery is isolated from the other circuitry, it may only have two leads. That would be the simplest case and you can just replace the battery with a supercap or tiny lithium ion battery. If it's 3-4 terminals, then you probably need to have a thermistor or just get a phone battery with the same 3-4 terminals. If there's more than that, you've likely got on-board SMBus communication protocol and you're far better off buying a new tablet. Every manufacturer is different in their use of SMBus. You don't want to reverse engineer that. \$\endgroup\$ – horta Dec 21 '14 at 15:23
  • \$\begingroup\$ I have two 3.7 V batteries hooked together via 2 wires (red and black). One of the batteries is attached to the board via three wires: red, black and white. The pad for the white wire is labeled LCD1 on the board. Inside the battery pack there is a small PCB (about 3 mm wide) with SMD components and pads labeled B+, B- (gee, I wonder what that could mean :-) ) and TPP. So, I guess this is case 2: battery (series?) with a thermistor. \$\endgroup\$ – Sredni Vashtar Dec 21 '14 at 16:12
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    \$\begingroup\$ I would try putting a cap or supercap in place of the batteries. If that doesn't work, then you would either need a thermistor or you need to fake out a thermistor. I'm not sure if it's connected in analog or in a digital manner. If in digital, you need to buy a new tablet. Usually, when a li-ion battery goes, the controlling pcb, does a permanent shutdown to it preventing it (or the pcb) from ever working again. If that's the case, then make sure you remove the battery side pcb. As I've mentioned before, if there's any sort of digital communication necessary, you're wasting your time. gl \$\endgroup\$ – horta Dec 21 '14 at 19:53
  • \$\begingroup\$ Thanks @horta, this is very helpful. I'd hate to brick the device: even if it's a low cost chinese tablet, I sorta got used to it. It appears this sort of hack is way more system-dependent than I had foreseen. FWIW, the battery side PCB has an 8 pin SMD chip with "8810 3V1H15" and another 5 pin one with "K135M". Google was not friendly about those. \$\endgroup\$ – Sredni Vashtar Dec 21 '14 at 21:04
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    \$\begingroup\$ Note that the transient current draw of a tablet is sometimes much over the amount the average micro usb charger is able to provide. I had my tablet do stuff, drawing from battery, while still connected to a 2A usb charger... \$\endgroup\$ – PlasmaHH Dec 22 '14 at 11:40
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Your solution may be as simple as modifying the charger output (installing a resistor to cause a sufficient voltage drop so that the charger output will be equal to the Li-Ion battery output and then connecting the modified charger output to the B+ & B- solder points or wiring on the tablet circuit board.

If you have a bench DC power supply, before you hack the charger, you could touch the leads of the bench power supply (properly pre-adjusted of course) to the B+ & B- pins to determine if that works. B+ and B- are on the board, hidden at the top of some removable phone batteries. You unpeel the sticker from the top where the wires or pins come from the battery, and then pull the black plastic sleeve. There you will find the tablet's/phone's B+ and B- connections. Just remove the battery and solder to those two pins

Good luck!

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