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I've got a Sony MDR-RF4000 wireless headset, which is already 7 years old, so obviously its NiMH battery (Sony BP-HP550-11, basically a pair of AAA NiMH rechargeable batteries tied together) shows the signs of age.

The battery compartment has a note that explicitly says: "You cannot charge any rechargeable battery or dry-cell battery other than the supplied battery". And it doesn't lie. I'm using a pair of Eneloop batteries, which are also rechargeable and also NiMH. And they do provide power to the headset just fine. But the cradle station doesn't charge them, as described.

What is the technical reason behind that? How could Sony even achieve that, be that for an evil purpose or not? I'm using NiMH batteries in place of NiMH batteries after all, why do they refuse to charge? Are there many different kinds of NiMH batteries incompatible with their respective charging stations?

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  • $\begingroup$ Can you please elaborate on But the cradle station doesn't charge them, as described. $\endgroup$
    – Mahendra Gunawardena
    Sep 1, 2020 at 11:24
  • $\begingroup$ @MahendraGunawardena If I put a pair of rechargeable NiMH Eneloop batteries into the headset and put he headset onto its charging station, the station doesn‘t light up the red LED and doesn‘t charge the batteries. This doesn‘t happen if original branded batteries are in the headset. Maybe it has to do with them being enclosed in a plastic case, which triggers some switch? $\endgroup$
    – Megan Caithlyn
    Sep 1, 2020 at 14:15
  • $\begingroup$ Are the batteries you describe as "tied together" also electrically connected, making the bundle a higher voltage than a single battery or unconnected pair? $\endgroup$
    – fred_dot_u
    Sep 1, 2020 at 16:01
  • $\begingroup$ @EliaIliashenko, So you are placing loose Eneloop batteries in the head phone, where as original batteries are in a prepackaged pack. Correct. $\endgroup$
    – Mahendra Gunawardena
    Sep 1, 2020 at 16:18

3 Answers 3

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Looking at photos of the battery used in your headphones, I believe they use one of two ways to do this:

1- The cells are parallel in orientation, leading me to believe they are electrically parallel. The IC could be matching them somehow, or perhaps the voltage is slightly different, as the eneloops are low discharge cells.

the second, more likely scenario: 2- The little cradle could be triggering some sensor, allowing the headphones to know. Your description leads me to believe that you are inserting the eneloops directly, try placing them in the plastic case.

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Based on the information shared and available there mostly three more possibilities.

  • There is a make break switch that is triggered based on the battery packaging. Review the original battery package as well as the housing compartment for specific mechanical features associated with a trigger. Below are some battery packs that are similar to original.

enter image description here

enter image description here

  • The battery system might have something similar to a Reed Switch. In such situations the mechanical package would have a small magnet embedded, that is been read by a proximity sensor which triggers the charging system.

  • As complicated it is there is a real possibility that battery charger has intelligence to detect between a non-rechargeable primary cell battery and a rechargeable battery prior to enabling battery charging. In the Sony MRD-IF240RK has following statement provides some evidence. Check the Sony MDR-RF4000K manual for similar statements. If so the eneloop battery might not be satisfying the electrical specifications.

enter image description here

Technically there are designs similar to the one below that can detect a rechargeable vs non-rechargeable batteries.

enter image description here

How this done is based on the internal resistance of the battery? Below except should provide some insight the methodology.

The typical internal resistance for new high-capacity NiMH rechargeable AA batteries is between 30mΩ and 100mΩ, and for an alkaline battery it is usually between 200mΩ and 300mΩ (but as high as 700mΩ, depending on its charge status). Faulty rechargeable batteries have a much higher internal resistance. The DS2711/DS2712 chargers therefore calculate the internal resistance of batteries to be charged, using the measured battery voltages (VP1 and VP2) and the charging current that has been set.

Therefore there is high possibility that the Internal Resistance Eneloop NiMH battery is outside of the chargers technical specifications.

Additionally Stefanv.com in article Electronics Reviews has some interesting observations including the follow for Eneloop NiMH batteries

Lower internal resistance, meaning higher voltage reaching equipment that uses high currents

A final point to note. The combination of internal resistance and charge current causes change in temperature inside the battery. As you would imagine high temperature affects the battery adversely. Most modern charging IC have a temperature monitoring system as part of system safety.

Full and other related articles can found in the reference sections.

References:

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They designed it that way so people would not be able to charge other, maybe larger capacity, batteries and cause the device to overheat causing a hazardous situation like a fire.

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    $\begingroup$ How did they make a charger to identify the „correct“ battery though? $\endgroup$
    – Megan Caithlyn
    Sep 1, 2020 at 7:41
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    $\begingroup$ @EliaIliashenko While the question is the "why", and this answer answers it: fire hazard in theory, I also believe that also big corporate hostility is a reason. But I agree that having also the "how" would improve the answer a lot. I click "Looks OK", but I don't vote it up. $\endgroup$
    – peterh
    Sep 6, 2020 at 11:48
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    $\begingroup$ This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review $\endgroup$
    – Algo
    Sep 6, 2020 at 12:53
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    $\begingroup$ @Algo, the question asked "why" and that was answered. And, just to note, neither of the other answers give "why" they concentrate on "how" so they miss the question in the title. $\endgroup$
    – Solar Mike
    Sep 6, 2020 at 12:55

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