BT Stratus 1500 DECT phone NiMh battery charging issues

[brian_mk]

I have two BT Stratus 1500 DECT phones, now several years old. They contain a pair of AAA NiMh cells.
Both now suffer from a charging problem in which charging is not re-enabled when the handset is left in standby on the cradle and the battery voltage drops.
Eventually there is insufficient voltage to power the phone which then enters a continual reset loop.
This is accompanied by a continuous alarm beep.
It is extremely annoying when this wakes me in the middle of the night.

After carrying out some investigations, this is what I have discovered:-
The so-called blue 'charging' led on the cradle does not actually indicate that the batteries are being charged.
It simply indicates that the handset is on the cradle. The LED remains on even when no current is flowing into the battery.
The current flow into the battery can be switched on and off by the processor.

Using a microscope, I managed to draw out part of the schematic for the charging circuitry in both the cradle and handset.
Based upon that I added a red diagnostic led to indicate when the processor enables charging current to the battery.
Intially when the battery voltage is low, charging current is continuous (measured at around 30mA).
After some time charging switches to a pulsed mode (roughly 6 sec on, 3 sec off).
Eventually charging is disabled and the red diagnostic led is switched off.
The blue cradle charging led remains on.
I left the system in this state for a few days, regularly picking up the handset to measure the battery voltage.
The voltage continued to fall with no sign of charging being re-enabled.
Eventually when the voltage dropped to about 1.17V, the handset began to continually reset itself.
Picking up and re-seating the handset allowed it to start up with charging enabled as indicated by the red led.

The same problem exists on both handset/cradle combinations.
I have tried several different NiMh batteries with different capacities including Panasonic, Eneloop, Lloytron, Palocell, Unitech.
Other users have reported similar problems. Hence I think it must be a design issue.

For the full story and the results of further investigations, please refer to the following forum:-

https://www.eevblog.com/forum/chat/bt-stratus-1500-dect-phone-nimh-charging-problem/

[brian_mk]

No response.

No real surprise there.

As a retired electronic engineer, my advice to anyone contemplating buying a BT phone outsourced to China is 'DON'T'.

They are cheap rubbish and not fully tested.

Buy Panasonic.

[rbz5416]

Not sure what response you were expecting to a 15 year old phone starting to fail?

You might also want to check where Panasonic phones are actually made...

[brian_mk]

I want to  find out why the phone is failing.

The Voyager spacecraft is still working after 47 years in extreme environmental conditions.

Why is a relatively simple charging system failing after  only 15 years?

This seems to be how things work these days. Just throw it away and buy another one.

I don't really care which country things are made in as long as they are properly tested and last.

I used to work for Marconi. They managed to build equipment that lasted for decades.

Does BT think it is acceptable to sell a product that stops working after just a few years?

[licquorice]

I have absolutely no idea what response you are expecting from a customer to customer self help forum.

Marconi also used to build rubbish btw.

[brian_mk]

Well thanks for the helpful response.