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Re-purposing Cel-phone Li-ION batteries? Answered

Excuse me Please:

I have a rather advanced Q on the re-purposing of Li-ION batteries.
the tiny battery within the SanDisk Sansa, I found to have a deplorable duration.
So in disassembling the Sansa, I discovered it had a seemingly standard 3 pin Li-ION battery (red, green, black).
Since a celphone of mine recently died, which I retained it and its battery, I decided to wire the albeit, larger, battery into the Sansa.
I was gratified in the the Sansa worked as expected, but worked for literal contiguous DAYS at a charge!!
(seems I had forgotten to turn it off in coming from the gym, to return to it the next day and it was still playing!)

Now, my conundrum:
due to the change in cellular tower technologies, and being forced to change to the newer "G" phone, I have an abundance of batteries, to which I continue to collect for myself and friends Sansa players with rewired batteries.

In that the Motorola BQ-50 battery has four(4) pins, which are suggested as the following:

pin 1 is battery +
pin 2 is a communication line back to the phone
pin 3 is a temp line back to the phone
pin 4 is gnd

pins 2 and 3 are used to tell the phone the charge condition of the battery and when the charge cycle is complete.

would anyone here have the deeper electronics knowledge to advise me which pin of 2 or 3 can be omitted and still allow the battery to be charged via the USB like the original 3 pin battery?

Thank you kindly for your time and knowledge.



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5 years ago

Charging lithium batteries is very dangerous. Over charged or over discharged they can burst into flames.


The extra pins allow the charging circuit which was designed for the battery the phone originally had to monitor the battery state and keep it safe.

You can draw your own conclusions from the above.


Answer 5 years ago

You can probably use a Li-ion charging module such as this one:


and there would be no need to use the other two pins. In a phone the charger module is in the phone and needs to talk to the battery to use the little "charge meter" on the phone.

When repurposing the battery, you'd be using the external module instead.

However, do be careful and check your charging current and voltage.


Answer 5 years ago

As I am one whom chooses not to live in fear, I await an answer to my original question.

I would want to avoid multiple devices for charging/use. However, the the use of dedicated *charging* and *use* environments is still open.


Answer 5 years ago

As I said:

The extra pins allow the charging circuit which was designed for the
battery the phone originally had to monitor the battery state and keep
it safe.