Looking for a good resistor

March 21, 2018 12:53 PM

What resistor value do I need to make a dummy load to check the battery charging circuit for my Sennheiser TR220 wireless headphones? A voltage of 9VDC exsists between the charging contacts on the base station with no load. The headphones have 2 new eneloop AAA batteries. I do not know if they are wired series or parallel. Everything else functions properly. Another question: If the headphones have to be replaced, could a bluetooth wireless dongle using the native W10 software match the Sennheiser base station in sound quality?
posted by Raybun to Technology (8 answers total)

Charging protocols are often more complex than you can trick with a dummy resistor. A good charger (even a cheap one) will limit current and watch the voltage to detect a small anomaly between charge pulses that indicates that the batteries are full.

If the headphones came with NiCd or NiMH (I forget which Eneloop is) then the charger can probably charge them. Otherwise, your safest bet is to charge them on the Eneloop charger and put them in when charged.

If I wanted to check a charging circuit as I think you're proposing, I'd figure out a way to monitor current through the batteries, and the voltage across them, during charging. One way would be to put a dummy battery in place of one of the real batteries (you'll have to put the replaced battery in the circuit in the correct place, too) and put my meter at the dummy battery. But that's a lot of work for questionable results -- you need pretty accurate measurements, over several full charge/discharge cycles, to figure out what's going on, and you won't know what charging strategy the headphones are supposed to be using -- you'll just be able to guess whether what they are doing is good enough to charge the batteries.
posted by spacewrench at 2:26 PM on March 21, 2018 [1 favorite]

Eneloops are NiMH. They're wired in series. Googling it apparently NiMh batteries run about 170-175 mOhms of internal resistance so really a 1 ohm resistor (pretty small) should be sufficient. But why not just try to measure voltage when the real device is in the charger? I'm not sure what problem you're trying to solve here with a dummy load.
posted by GuyZero at 2:32 PM on March 21, 2018

Response by poster: GuyZero and spacewrench.

Thanks for the suggestion of measuring voltage when the headphones are in the charging cradle. I want to have as much information available when I speak with a Sennheiser tech tomorrow.
Charging the batteries in an external charger is an option. The Sennheiser goes through a set of freshly charged batteries in about three hours and this is my backup. Sennheiser will not do repairs on this unit. A replacement base (if available) is about $200. I want to avoid buying a replacement.
posted by Raybun at 4:51 PM on March 21, 2018

Just a note that the quoted ~200 mOhm resistance is almost certainly the nominal output impedance when the batteries are discharging. The load they present while charging is going to look very different.

If the charging station is a simple source (meaning any charging circuitry is in the headphones, which is pretty common) then you could test it by finding its max rated output current @ 9VDC and choosing a resistor to draw that. If the output voltage is still 9 at that load then it's probably a working supply. As noted above, if the charging circuit is in the charging station then all bets are off and you may need a load that looks like a battery to test it.
posted by range at 4:49 AM on March 22, 2018

Putting 9v across a 1 ohm resistor is going to get you 9 amps of current (theoretically). That doesn't sound right for this situation since the batteries certainly aren't charging at 9A. As range mentions look at the charger specs and do the math V/I = R.
posted by achrise at 6:57 AM on March 22, 2018

Oh geez, yes, that's obviously wrong, sorry. Like a 50 ohm resistor would draw 180mA which is more like what normal battery charging would draw. Still the 9v is the open voltage, it may be lower under load. Again, I think you're better of measuring the real headphones and not a dummy load.

Your issue is the batteries die too quickly?
posted by GuyZero at 9:22 AM on March 22, 2018

Response by poster: GuyZero

The batteries would not charge at all.

I did check voltage across the charging horns with and without the headphones in the cradle.
Without the phones in the cradle, 9.03VDC, with the phones in the cradle and part of the circuit, 9.1VDC.

The problem has been retired without being solved. Sennheiser took them back and gave me an equivalent wireless headphone set at a substantial discount.

I am learning stuff from this discussion.

Thank you to all.
posted by Raybun at 11:09 AM on March 22, 2018

As already mentioned, charging is likely more complex than what you can mimic with a resistor.

However if you did want to replace a load with a resistor, you have provided insufficient information. You need to know the [on load] voltage and current to work out an appropriate load.
posted by HiroProtagonist at 7:02 PM on March 22, 2018 [1 favorite]