Lithium batteries

[DavidA]

Hi all,

A dark cloud has descended upon my Hisense Sero 8 tablet.

The battery is in a bad state and soon to depart this world. It won't charge past 16%.

It's a couple of year old, so not unexpected.

Problem is, getting a new one. Or at least one that fits in the case.

Then I thought, does it need to fit in the case ?

So, my question is...

Can I make up a bank of LiPo cells (to 3.7 Volt, as the original) and connect this via three thin wires through three not quite so thin holes to the connections on the tablet board ?

There are three connection to the board from the battery. red, black and yellow.

The voltages measured at the board are, with the tablet turned off and not charging,

red - black       =  4 volt
black  - yellow  =  0 volt
yellow - red      =  4 volt.

I don't see why this won't work.  But I am not sure about the yellow connection.

Anyone done this before ?

Dave.

[PK]

You'll only need one cell in that pack. 3.7 is the often quoted nominal voltage of a LiPo cell, fully charged they are 4.2V and 3V is getting near flat.
Not sure about the 3rd wire though, might be a cell voltage sense wire....
PK

[Joules]

The third wire is most likely a thermistor to monitor cell temperature during charging.  I would suggest you match the cell current capacity as closely as possible as the charge circuit will be designed for that capacity cell.  Also check its C rating.

I would also consider the price of replacing the tablet small in comparison to a Lipo fire if you get it wrong.

[DavidA]

 Considering the fire risk.

I don't think there is any great risk of this as I would be simply replacing one battery with another one. I would keep the Amp Hour capacity close to the original 4000 MAh.  I can stick an Amp meter in circuit to monitor things.  The difference being the new one would be external to the case. A two wire connection, not counting the yellow wire.
I have read that the tablet will run without the yellow wire connected, but won't charge. And some where else that connecting the wire to one of the other two brings normal operation. I'll have to look further into that.

I have a strong reluctance when it comes to changing perfectly good equipment when all that is wrong is the battery.
In fact, I suspect that I could run the tablet from my DC lab supply (set at 3.7 Volt) as long as I don't plug in the charger.  I may try that as a later test. DC is DC, doesn't really matter where it comes from.

At the moment I am waiting for an USB charge meter to come so I can see just what is happening with the original battery.

I'll keep you informed of any progress.

Dave.

[eskoilola]

Running the tablet from lab supply is a really good idea. At least it is way better than risking a hazard with wrong type/size/capacity batteries.

Also it might not be that bad idea to build a power supply - not a charger - for this tablet.

It is my assumption that the power feed to the tablet itself is fairly simple.
The voltage on the other hand needs to be quite accurate as the tablet might not start with a "dead" battery.
Voltage regulation is really easy these days.
Voltage regulation with low power loss - that's art - but a tablet might not be that bad on power consumption so that is not an issue.

[DavidA]

I had another look at this tablet.

And I had actually made a mistake.

There are, as I mentioned, three connections from the battery to the PC board.

So I clipped them off close to the PC board (leaving enough for any future 'modifications) and connected the red and the black to a parallel pair of cells I recovered from one of my old lap top batteries.

So I have, when fully charged, 4.2 Volt at some as unknown Amp Hour capacity. I'm doing a complete discharge/recharge on a similar 2 cell pack that will hopefully give me the capacity.

Anyway, it works nicely. I haven't let it get down to minimum voltage yet (3.3 Volt) but it looks promising and certainlt runs for longer than the faulty original.

At the top I mentioned I made a mistake.

This effects the recharging.  At the moment I have to disconnect the  cells and put them on charge. No big deal as I have spare cells and I fit them with plug and sockets to make it an easy job.

But closer examination of the original (flat pack) battery I removed, complete with it's charge controller (Where all three wires from the PC board terminate) shows that there are only two connections from the charge controller to the actual battery. A Pos' and a Neg'.

So all I have to do is disconnect the faulty cell(s) from the charge controller at these points, then connect my new cells to it at the same points.

I can then re-connect the three charge controller wires to the PC board and I believe I can charge as normal via the mini USB connector. I have a 'charge Doctor' volt/current monitor that shows what the battery is drawing when charging.

Dave.