Wireless headphones Thomson WHP3001BJ doesn’t charge

I got these headphones from second hand and out of warranty, but it seems the model is still manufactured (year 2020). It's old good analog technology - simple, reliable and budget friendly. It will probably not satisfy you if you are Hi-Fi purist, but for occasional TV watching or for parents it‘s quite OK. It comes with a base which can be used as a stand and it should charge the two AAA batteries in the headphones. But unfortunately in my case the charging didn’t work.

At first I checked the contacts on the base and on the headphones - both seemed OK. Then I measured voltage on the charging pins of the base, it was also OK. So I connected variable resistor to the charging pins to verify the charging procedure. It seems it’s simple constant voltage charging (I didn’t expect anything else in this price range). It can do max cca. 100 mA charging current and the charging LED indicator goes out when the charging current lowers bellow cca. 25 mA. So it didn’t stop the charging and the batteries are charged all the time when connected to the base. This means two things: the charging of fully discharged batteries can take pretty long (10 - 20 hours or it may be even not possible to charge old batteries at all) and the life of the batteries will be probably slightly reduced by the low current which flows into the batteries all the time. This is interesting observation, but it didn’t help me with my problem.

Next, I disassembled the headphones. The power supply PCB is pretty simple, just the power switch, one diode and some condenser filters. By checking the PCB the problem was clear. Despite of being so simple, the power supply wiring seemed a bit over-engineered. There are second contacts for the battery GND (the "-" contact) that are offset from the battery end. These contacts are not apparent without disassembling the headphones. After a bit of thinking I realized that it’s probably ugly protection from insertion of non-rechargeable batteries. It seems you need specially modified rechargeable batteries with protracted GND strip. If you insert unmodified non-rechargeable batteries (1.5 V), the second GND pins will not connect and it will not charge. Also the diode will lover the voltage by cca. 0.6 V, so the resulting voltage will be 2 x 1.5 V - 0.6 V= 2.4 V, which is the same voltage as if powered by the re-chargeable NiMH batteries (1.2 V), i.e. 2 x 1.2 V = 2.4 V. The problem is that if you insert unmodified rechargeable batteries it will not charge them and the voltage will be lowered by cca. 0.6 V which will shorten the operational time. Also with the modified batteries for the charging to begin the headphones has to be switched off.

To be 100% sure, I tried to google the original manual on the internet. Luckily it is still there (year 2020), but there is written nothing about the special batteries. There is just a small picture of the batteries where the GND strips are visible. Fortunately, after more research it seems they added the problem to the FAQ section on their web pages - lessons learned: google first then fix problems :). They describe the reason behind it as: "To prevent a user of the wireless headphones from inadvertently inserting non rechargeable standard batteries and from trying to charge them". And they also describe how to modify the batteries: "Cut off the heatshrinking sleeve which covers the rechargeable battery on the side of the negative terminal at a length of about one centimetre and remove it." Personally, I can’t imagine myself trying to insert non-rechargeable batteries there - fortunately I am still sane enough :). It’s also quite unhandy for me to modify the rechargeable batteries each time they need to be replaced, so I simply modified the PCB - I added two white wires shorting the additional GND contacts and added label to the battery compartment stating "do not insert non-rechargeable batteries" - just in case my sanity would worsen during time :). If I would have any need to use non-rechargeable batteries in the future (which is unlikely), I will just cut the two added wires and everything will work as before.