Recovering my Sony PRS-T1 using an old mobile phone battery

Books are great, I mean “real” books are great.

However when you experience several relocations and you  want to keep on reading, then most likely your bookshelf will become virtual.

This is more or less my personal experience, since 2011 I am a rather happy owner of a Sony PRS-T1 eBook reader. Everything was almost fine until last june when I forgot it on the dashboard of my car, I was out for a sunny and warm weekend, the reader is black, my car’s dashboard is black too… result: the eBook wasn’t no more recharging. It was most likely due to battery failure.

Opening the PRS-T1

There’s no screws on the reader, to open it I found some videos around, like this one:

As usual the battery is connected by three wires: Vdd, GND and the termal control pin. Guessing the pin function was quite trivial, Red for Vdd, Black for GND and White for the Temperature sensor.

Measuring the voltage yield something like 2.3V, confirming my suspect that the battery was broken.

The first idea was to buy a new eBook reader, but unfortunately Sony seems to have discontinued the production and the models available on the market are not so appealing.

I decided to remove the broken battery and try to fit an old mobile phone battery into the reader.

Removing the old battery

Once the reader is open it’s just a matter to disconnect the wires, there’s no connector hence some basic skills with welding is required. The few things to care about are to work with some caution, having a good solder tip and using a relative high temperature in order to desold the ground wire.

Selecting the battery and finding the pinout

I have several batteries of old Nokia and Motorola phones, I decided to use a Motorola BT50.

The Motorola batteries have 4 pins, and often there’s no indication of the pinout.

I found all the info on CPBK, then I welded the 3 wires with the help of some capton tape to protect the other pins and avoid the risk of short circuits that can be dangerous on Li-Ion batteries.

Connection and first issues

After the “new” battery was connected, the eBook restarted,

however after a while I discovered that charging was ineffective.

When connecting the charger I got a message saying that the reader was overheating and charging was suspended.

The problem is that the thermal resistor of these batteries are typically made with an NTC thermistor and each model bring its own value. The battery was not overheating, but the Thermal control of the eBook was fooled by a different value of the NTC.

Cheating the temperature sensor

In order to force a valid temperature readout, I tried to add a fixed offset on the NTC, basically I added 60KOhm in series (I’m not sure about the value, some time passed and I haven’t take note).

I admit that this is a quite rough solution, in this way I am almost disabling the Thermal control and there’s no protection against overheating. I have to trust the charging circuit designed by Sony and of course the battery.
I think this is a reasonable risk for a low power device made by a premium company.

After the modification the device was finally charging! Yooooo!

Happy end and conclusions

The device was actually recharged and now is properly working, it sustained 4 recharges since the modification, so I am rather satisfied.

I really hope that it will last for many years, since in my opinion it’s still one of the best eBook reader ever.