BT Stratus 1500 DECT Phone NiMh charging problem

[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.
The phones used to charge properly when new so I think something has probably drifted over time.

My theory is that the reference used by the A/D converter to monitor the battery voltage may have drifted
such that the processor now fails to detect the low battery condition and fails to re-enable the charging current.
I suspect the reference is built into the control chip itself.
If that is the case then the only thing I can think of is to replace the existing charging control system with something else.
It's easy enough to prevent the processor from disabling the charging current simply by removing a resistor.
Charging would then be permenantly enabled (at 30mA) whenever the handset is on the cradle.
That would not be good for battery life.
Cheap DC/DC converter modules (made in China) designed for NiMh charging are available on eBay.



According to the data, these modules monitor the battery voltage and reduce the charging current to a float charge.
I think it should be possible to fit one of these inside the cradle. Would that work?

[tom66]

How big are the original cells?  30mA seems very low for charging current.  They might be attempting to extend the lifespan of the cells but it seems a bit too low to me.  I would expect 100-150mA.  Perhaps it simply does not top up the cells enough to maintain normal operation, due to a drift in the current regulation circuit or something like that.  It might not monitor the voltage at all and just rely on run vs charging time (these things are built to a low price).

Drifting voltage reference is possible but... meh, seems unlikely as things go. 18 ohm series resistance implies maximum current of around 300mA, PSU might limit to 100-150mA, I would expect charging current would be right around that upper limit.  If cells are something like 600 or 800mAh then that is about right for optimum charge rate (C/4 to C/8) for longevity vs charging time. 

30mA continuously would probably be OK for a cell around C/20 charge rate if done continuously.

[brian_mk]

The batteries are AAA cells.
I have tried different capacities from 750mAh (as originally supplied with the phone)  to 1100mAH.
I used a -100mA - 0 - +100mA analogue center zero meter in series with the batteries to monitor the current.
From my measurements, the maximum current drawn from the battery is around 80mA.
This is at startup when both transmitter and receiver are enabled and communicating with the remote base station, the display backlight is on etc.
The current then drops to about 2mA when the handset enters standby mode with the display dimmed and only showing the clock.
In this state the handset can remain useable for several days without being recharged.
I measured the charging current into the battery at 30mA for both continuous and pulsed modes.

I believe the processor does measure the battery voltage rather than simply measuring the 'on' time:-
If I pull the batteries when the handset is in pulsed charging mode, it remains in pulsed mode when the batteries are reinserted.
If it relied on time measurements then I would expect it to revert to the initial continuous charge mode.

Also, according to the label, the wallwart power supply can only supply 150mA current max.

I would guess there is a potential divider between the battery and the A/D input. A drift in resistor values would upset the measurement.
There might be an electrolytic cap in the path somewhere to reduce noise.
As we all know, SMD electrolytics are notorious for leaking and degrading over time.

[tom66]

I would have thought that keeping the cells on 30mA continuous trickle current will be fine.  30/750 = 1/25C.  Normally, trickle chargers are 1/20C or lower.  You could lower the charging current to 20mA if you want to be more cautious.   Just force the charging circuit to run all the time.

Maybe the cells will only last a few years, but it sounds like the easiest solution compared to bodging in a custom NiMH charger.

Unlike Li-Ion cells there is no safety hazard from trickle charging NiMH it just tends to prematurely wear the cells.

[brian_mk]

The first attached photo (I only have an analogue 'scope) shows the battery current during pulsed charging.
I used a 10 Ohm current sense resistor in series with the batteries.
Positive voltage indicates charging current flowing into the battery.
Negative voltage indicates current flowing out of the battery.
The scale equates to roughly 10mA/div in the vertical direction, < 0.5s/div in the horizontal direction.
(NB The timebase on my 'scope only goes down to 0.5s/div (5s for a full sweep). To get a photo showing the complete charging waveform I needed a sweep of about 9s. I had to slow the timebase down using the uncalibrated adjustment pot. Hence the horizontal scale in the photo is not calibrated.)
0V is about half way up.
When the charging current is switched off, that leaves the standby current used to keep the real time clock and dimmed display running.

I have not figured out what the narrow pulses are. They are about 620mS apart.
When charging, the current drops to zero during each pulse.
The pulses continue during the charging off period: Something is being regularly switched on that draws more than the standby current.
They continue even after charging has been disabled as shown in the second photo.

Could it be that the transmitter and/or receiver are being enabled to communicate with the remote base station to check for incoming calls, messages on the answer machine etc?
Is the charging current switched off when measuring the battery voltage? I imagine the processor will be averaging a number of successive samples.

[brian_mk]

The phone has been left on the cradle for over 2 days in pulsed charging mode (red charge LED flashing).
By now I expected charging to be complete and the charging current switched off.
This morning as soon as I lifted and replaced the handset, charging stopped (red charge LED went off).
After pulling the batteries a couple of times and taking the handset on and off the cradle, pulsed charging was re-enabled (red LED flashing).
The batteries indicate 1.47V. (i.e. fully charged).

My second identical phone also went into continual reset + beeping mode on Saturday.
I had to lift and replace the handset to reset it. It displayed the 'low battery' warning message.
The warning message eventually went away and the battery voltage is now over 1.4V, so fully recharged.
I don't have a diagnostic charging LED fitted to that handset so I can't easily tell if the battery is currently still being charged.

I am baffled. I don't understand the wierd battery charging algorithm and am still unable to diagnose the problem.

[tom66]

Just bodge it so it charges the batteries continuously at 20-30mA.  It'll be fine for normal use cases.

Probably the case of someone trying to do something novel when a simpler solution works fine. 

[brian_mk]

I will probably end up doing just that unless I can find a root cause of the problem.

Just now I opened up the second handset and fitted a red diagnostic charging LED as I did on the first.
The batteries measured 1.35V so I suspect they were not being charged.
When I put the handset on the cradle, the red LED remained off.
I tried it a couple of times including pulling and refitting the batteries.
Then I fitted a different pair of cells that measured around 1.3V each.
That seemed to trigger the handset into continuous charge mode with the red LED on.
Then I reverted to the original 1.35V batteries.
Guess what? The handset remains in continuous charge mode with the LED on!

I am beginning to wonder if the processor is using non-volatile memory to store the battery voltage history and is using that to control the charging algorithm. The handset is fitted with an Atmel serial EEPROM chip.

[brian_mk]

The PCB.

So much of the functionality is built into the procesor and RF module.
There isn't really that much on the PCB to go wrong - unless it's some kind of firmware bug of course.
Nah - software never has bugs 

Points for anyone who can identify the processor and the A/D input port.

There is a 470uF electrolytic cap rated at only 4V.
It looks like it may be associated with a DC/DC convertor.
I have had issues with these in the past.
I desoldered the cap and checked it for DC leakage, ESR and capacitance.
It's a bit low by 10% (420uF) but otherwise ok.

The 8pin IC just above the processor is an Atmel serial EEPROM.
The label had been painted over but was easily removed with propanol.

Maybe it's an age thing but I find it annoying that so much electronic equipment can no longer be easily repaired.
Most people would give up, throw it in the nearest skip and either buy a new DECT phone or dump the VOIP based land line and use a smartphone instead.
I have to admit that VOIP is subject to scammers using spoofed numbers.

[brian_mk]

Out of interest I carried on investigating the charging issue for a while.

One of the things I found is that the animated 'charging' icon on the handset display bears no obvious correlation to the state of the charging algorithm.
The icon consists of an image representing a battery cell containg 3 rectangles in a row.

In pulsed charging mode, the icon sometimes appears static with all 3 rectangles displayed.
To most people that would suggest that the battery is fully charged. However that is not the case - it is still being trickle charged.

If you then pick up and replace the handset, the icon changes to an animated image showing a repeating sequence of 0, 1, 1+2, 1+2+3 rectangles updated at 1 second intervals.
This is the same in both continuous and pulsed charging modes.
After being left on the cradle for some time, the charging current is switched off completely by the processor.
However, the icon remains animated as described above.
To most people this indicates that charging is active when it is not.
Eventually the battery voltage drops to the point where the handset no longer works and the processor continually resets and beeps until the handset is lifted from the cradle.
This has to be a software error.

My workaround is to reset the charging algorithm whenever I notice the diagnostic LED indicates the charging current has been switched off for more than a few hours.
This requires the following steps:-
Lift the handset off the cradle, pull out one of the battery cells.
Replace the handset on the cradle for a second or so (leaving the battery removed).
lift the handset again, refit the battery and replace the handset on the cradle.

After this sequence, the handset enters continual charging mode.

I don't think these phones could ever have been properly tested before being sold.