Smart phone batteries

[Brian Howie]

My not so smart phone had some, but not much water ingress and stopped working. Its death throes involved a lot of vibrating. I dismantled it and kept it on a radiator for a couple days in hope of the resurrection, but decided rigor mortis had set in. It was 5 years old anyway, so I got a new one.

Being a curious chap I lifted the battery connection and found the charging voltage was present, but the battery was o/c with not a hint of voltage. Is there something inside the battery that self protects it in the event of a catastrophe, or is something on the phone not enabling it. It might be worth trying a new battery and keep it as a spare. I can get one on e-bay for around a fiver.

Brian

[Theo]

There is usually an over/under voltage protection chip, such as a DW01:

There is typically also a thermistor on a third pin (separate to the chip)

You could try hooking up the battery terminals to a 3.7v bench supply and see what happens. It is possible it still needs the thermistor present though.

You could also try trickle charging the battery (to 3.8v at say 50mA) and see if you can bring the voltage up to the working range. It is possible its extended vibration has just flattened it below the low voltage cutoff.

Theo

[Martin Brown]

Yes.

They are designed to fail safe if there is a stress that might cause them to catch fire. A bit like a fuse but more complicated as it also catches situations where the thing might go critical but before it does.

It has to be quite clever since once the cell temperature goes beyond a certain point catastrophic failure is more or less inevitable.

I have seen iThing batteries that were 3x their nominal size prize apart the casing. I have a picture of the worst example I've seen somewhere.

Despite this things can and do go badly wrong from time to time even in aerospace where they are supposed to be using gold standard protection!

[Jock]

No.

No, it has just gone open circuit.

Yes, that will work fine.

[Jim Stewart ...]

cheapskate...tee hee

[Brian Howie]

Well I did buy a new one, but had a poke around on the old one out of interest.

Brian

[Brian Howie]

Thanks. I found a hack on utube for the same phone, where they bypassed the battery control board with a couple of thin wires from the real battery terminals to the logic board. The commentary was in a foreign language, but it had something to do with the thermistor. I exposed the real terminals and the voltage is about 2.9V. I suppose I could try a trickle charge as suggested. I'm very unkeen on the bypass solution as a permanent fix , but maybe to see if the phone is totally bricked.

Cheers

Brian

[The Natural Philosopher]

I had similar with a model plane pack. These have no protection. The charger thought it was one cell only due to the low voltage, so I temporarily used a nickel charger to get it up to a reasonable voltage, and finally charged it, but it had lost well over half its capacity and developed too high an internal resistance to do much mire than weakly spin the motor and get hot. My gut feeling these days is that if its too low to charge on a standard charger its dead enough to simply chuck.

[Adrian Caspersz]

On 11/04/2022 17:43, Brian Howie wrote: I

Remarking and reselling scams all over the place...

If it is a worthwhile phone, then do some research on provenance, or just accept that the replacement will not have the matching capacity of the original no matter what the advertising.

[The Natural Philosopher]

A fiver is the going rate for a small two cell Li-ion. Without protection anyway. Probably possible to re cell a pack...

[newshound]

Agreed.

Personally, with "third party" phone batteries being so cheap, the likelihood that the capacity is down, the time you would spend faffing around, and the risk of "unforseen consequences" I would just try fitting a replacement battery.

[Animal]

Grade C batteries are scams in the sense that they're used and not always the capacity they claim, but they make a perfectly practical repair option in most cases, and at a fiver or so they're very good deals. I usually would not bother getting a genuinely new one.

[Brian Howie]

The cells look OK, but I think the protection has protected. I'm guessing the thermal fuse on the internal board has been crowbarred. I might have a look at that.

Brian