Bad NXT Battery?

[johncdelacy]

Greetings,

I'm new to this forum, just having discovered it.

I have an old dark gray NXT battery (the one Lego calls the "AC" battery). It does not charge with the AC charger anymore. The output voltage is about 5.4V and doesn't change when the charger is plugged in. The green light glows but not the red light.

I have a suspicion I would like to confirm and find out if anyone else has encountered this problem. One of our AC chargers died. I emailed LegoEducation and they told me the AC charger has been discontinued but I can buy one that matches the specs and use that. The specs are 9 - 12V AC at > 700 mA, I believe.

So, I found a Triad supply that is listed as 12 V AC at 1000 mA. I tried it once and it seemed to charge the battery (i.e., red and green light glow for a while, then green light only. The battery seemed fully charged). Now this same battery is dead as detailed above. I measured the no-load voltage of this Triad unit and it reads 15.0 V. An old Lego charger, that is listed as being 10.8V AC at 700 mA measures 12.0 V no-load. Note: my inexpensive DVM is probably not a true RMS reading meter and I don't know what kind of wave form these power supplies put out.

Did I kill my battery with this "12 V AC" supply? Has anyone else experienced this kind of issue?

Thanks.

John

[mattallen37]

I'm afraid that you most likely did blow it with the higher voltage.

The output wave form would match that of the power source, so most likely a sine-wave.

[johncdelacy]

Matt,

Yes, it is a possible conclusion that using a power supply that is listed as "12V" but actually produces 15V with no load (that means not connected to the battery) could destroy a device that cannot tolerate more than 12 volts. So, what does it mean for Lego to specify a supply from 9 - 12V? Their "10.8V" supply actually puts out 12V with no load. Perhaps they should have said that 10.8V AC is the maximum allowable input voltage. Or some lower voltage, since other manufacturers might have different internal circuitry that could produce a higher no-load voltage?

As to the waveform, I don't think you can deduce the output waveform from the assumed sine wave at the input. The circuitry inside that black box can produce a lot of distortion.

John

[mattallen37]

Lego assumes you will be using only their power supplies. As always with an unregulated supply, the open circuit voltage will be significantly higher than the rated voltage.

That's true, you can't know for sure the wave form without connecting it to an O-Scope. I haven't done so for a lego brand AC transformer, but I have for another AC wall transformer, and it certainly was a sine wave. Why does the wave form matter to you?

[johncdelacy]

The waveform refers back to my comment about RMS readings with the DVM. A non-sine wave could give an erroneous reading if the meter isn't true RMS.

Lego told me they have discontinued the AC charger power supply and recommended I purchase one on the internet that adhered to the 9-12V spec. The transformer I bought is labeled 12V.

John

[mattallen37]

Oh, okay. You could take it up with lego's customer support. I don't know if they can help you, but they seem pretty good.