I want to replace an old 300 mAh battery in my Chinese tablet with a new 3000 mAh one. The old battery has three wires: red, black and white. The white wire connects to a pad on tablet's mainboard labeled "T", which I have been told is used to connect to the temperature measuring thermistor built into the battery. However, the problem is that my new battery has only two wires: black and red. I connected the red and black wires of the new battery to plus and minus pads on the board and left the "T" pad unconnected. The tablet works just fine with the new battery, but the problem is that I can not charge the new battery. I connected the charger for one hour but the battery charge level does not increase, not even by a percent. I think this is because the "T" pad is not connected to battery. What can I do?
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\$\begingroup\$ Note This is the next stage of the series after this question from yesterday \$\endgroup\$– Russell McMahon ♦Commented Feb 2, 2015 at 9:12
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\$\begingroup\$ How do you know that the charge level did not increase? What are you using to measure charge level? What charger are you using? What is the charge level of the battery at present? How did you "introduce" the battery to the system? If you are using the original charger it will take 24 hours + to fully charge. Even when the level is low you will only get 10% charge in 1 hour. So your measuring system may be missing this. If you charge it for say 12 hours you certainly should see a change. The battery may need to be "in-sync" with the tablet's charge controller to work properly. \$\endgroup\$– Russell McMahon ♦Commented Feb 2, 2015 at 9:17
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\$\begingroup\$ If you answer the above questions we can probably help you. Failure to answer makes a good answer unlikely. \$\endgroup\$– Russell McMahon ♦Commented Feb 2, 2015 at 9:18
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\$\begingroup\$ The chrge level now is only 1% and still 1% after 3 hours charging with the orginal charger of the tablet, and the tablet is turnning off few seconds after discconnecting charger , "The battery may need to be "in-sync" with the tablet's charge controller to work properly" what that means? and how to do that ? thank you vey much" \$\endgroup\$– Marah AliCommented Feb 2, 2015 at 11:18
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\$\begingroup\$ Have you followed any of other suggestions (Michael's below or elsewhere) re what to do with the extra lead? If you leave it open or connect it wrongly the charger and tablet will think that the battery is faulty and will not charge it. \$\endgroup\$– Russell McMahon ♦Commented Feb 2, 2015 at 11:23
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2 Answers
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I know this is an old topic, but I thought I would throw this in so that those who stumble on this topic through Google have a solution. I had the same problem. I purchased a two wire battery, installed the battery, then found that the tablet ran fine but would not charge. To solve this, I soldered the white wire to the black wire. Seems to be charging now.
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There is a convention that the "T" wire is connected to ground via a 10kOhm NTC, as mentioned for example on this thread on AnandTech. Replacing it with a 10k fixed resistor should enabling charging, but on your own risk. The sensor is used to prevent charging when the battery is too cold or too hot (possibly caused by environment temperature) to avoid damaging the battery. Also it can be used as an emergency cutoff if the battery gets too hot during charging. A fixed resistor will disable these safty functions.
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\$\begingroup\$ Where is the risk ?! I think The battery comes with "protection circuit " which supposed to protect it from overcharging , overdischarging , and other dangers.. am I right ? \$\endgroup\$ Commented Feb 2, 2015 at 14:50
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\$\begingroup\$ The protection circuit inside the battery is typically meant as second layer of security if the regular charge controller failed. It's limits for overcharging and overdischarging are commonly chosen in a way that they make sure the battery doesn't burst or catch fire, but they still allow significant shortening of the battery life. Furthermore, integrated protection circuits might not have temperature sensing at all. \$\endgroup\$ Commented Feb 2, 2015 at 14:54
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\$\begingroup\$ What about the tablets and / or batteries which come with only +/- poles are these unsafe ? \$\endgroup\$ Commented Feb 3, 2015 at 7:33
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\$\begingroup\$ Tablets and / or batteries only use two wires are indeed less safe (I wouldn't necessarily call it unsafe) than a three-wire system, if they don't compensate the missing thermal sensor in the battery (for example by an on-board thermal sensor, mounted near the battery). A system completely without thermal sensing can not avoid applying charging current to the battery while the temperature is outside the specified range (typically 0°C..40°C). You have to decide yourself whether this is an acceptable risk. \$\endgroup\$ Commented Feb 3, 2015 at 20:39
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\$\begingroup\$ I used a 10k fixed resistor with this new 2 wires battery .. now I can charge this battery .. but I'm concerned about this "risk" .. I charged this battery to 90% -it took about three hours - with tablet open And touched the battery, which remained totally cool Does this mean it's safe ? dose the "risk" include both charge and discharge .. thank you very much \$\endgroup\$ Commented Feb 4, 2015 at 7:04