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NiMH vs Alkaline Batteries (yes, yet another Bluetooth Mouse post)

Wednesday 23 June, 2004, 12:03 AM

A few people have kindly emailed me in response to my ongoing Bluetooth Mouse rambling to make some suggestions about why Microsoft recommend not using rechargeable batteries.

NiMH AA cells have, I'm told, significantly different power output characteristics from Alkaline batteries. They are rated at a lower voltage - nominally 1.2V rather than 1.5V. However, NiMH cells tend to give out 1.2V for almost all of the discharge cycle, dropping off only towards the end, while Alkaline cells tail off gradually throughout their whole life. So in practice, you only get 1.5V from a fresh alkaline cell.

This suggests a couple of reasons why the instructions might tell you not to use rechargeable batteries. The obvious one would be that the mouse might not be able to run on 2.4V (two NiMH batteries). However, empirical evidence suggests otherwise... Another possibility is that the battery charge level circuitry in the mouse will have a hard time predicting when NiMH batteries will go flat because of their sudden end-of-charge drop off as compared to the gradual death of an alkaline cell. That's plausible, but the battery level indicator is already only marginally better than useless, so I'm not too concerned about that one.

Unsurprisingly, the NiMH batteries are still going strong after all of 1 day's use... You'll doubtless be unsurprised to learn that I'll keep you updated as and when more mind-bogglingly exciting mouse battery news breaks.


Astonishing Developments on the Bluetooth Mouse Front

Wednesday 23 June, 2004, 04:00 PM

I honestly thought I was going to go a bit longer without posting this time, so I'm really sorry for yet another entry in this saga so soon after the last one (here, and then previously here here and here). It's really not my intention to turn this blog into The Daily Bluetooth Mouse Report. But while the title of this particular entry is arguably hyperbole, I just had to report this latest news.

As you may recall, I put fresh NiMH rechargeable batteries into my Microsoft Bluetooth mouse all of 2 days ago. (They're 1500mAh batteries by the way, so they're not the best NiMH ones you can get nowadays. I've had them for a while, but this set worked perfectly well last time I used them in my camera, so as far as I know, they're not defunct.) Today, I got my first warning dialog indicating that the batteries are running low! That's pretty poor compared to the three weeks I got from the alkaline cells supplied with the unit.

What I'm hoping is that this is a false alarm caused by the fact that, as mentioned previously, the nominal output of a NiMH battery is only 1.2V vs the 1.5V initial output of an alkaline cell. So I'm guessing that maybe the mouse is just assuming I'm using alkalines, and is interpretting this 1.2V level as being indicative of a nearly-dead battery.

Unfortunately, I don't see a way of telling it not to give me this early warning, and just to give me the 'critical' warning.

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