dinsdag 27 september 2011

Magic Mouse battery management

If you have a wireless Mighty Mouse and you're tired of the scroll ball getting clogged with gunk, you may consider buying a new fancy Magic Mouse. In many aspects it will be a step forward, except one.

The battery management of the Mighty Mouse was very good: it could run on a single cell, which means that when it told you its batteries were empty they were both guaranteed to be empty. The mouse would always keep drawing power from whatever battery still had some juice left. This made it possible to run for weeks even on a set of old worn-out rechargeable cells.

The battery management of the new Magic Mouse is not so mighty, nor is it magical. In fact it is absent. The people at Softpedia have already discovered this but they did not explain the root cause of the problem. I do not believe the Magic mouse draws much more current than the Mighty Mouse. The problem is that it treats the two cells in series like any primitive battery-powered device. It expects a total voltage of more than about 2 Volts. This means that if one battery is still going strong at 1.2V and the other one is dead at 0.8V, the mouse will shut off. The Mighty Mouse would still be going at that point because it would ignore the empty battery and draw current from the other one.

Especially with Apple now selling their own NiMH batteries and charger, their decision to drop the advanced battery management is puzzling. If there is any type of battery that benefits enormously from the separate treatment it is rechargeable ones. Contrary to alkaline batteries, NiMH batteries will maintain a quite steady voltage until they're empty, at which point they exhibit a very quick voltage drop. If one battery has less capacity than the other, this leaves no margin for the higher-capacity battery to further discharge because the series voltage plummets below the threshold. With alkalines the steadily dropping voltage provides more spring-action between the two cells.

I tried using 2300mAh cells in a Magic Mouse and they lasted less than two weeks of non-intensive usage. The scenario was as described above: at the end of the run one cell was empty and the other one still had quite a lot of juice left.

If you have multiple pairs of rechargeable batteries and a means to measure their actual capacity (e.g. with an advanced charger), try to match the batteries in pairs of equal capacity. They will give you more mileage. This does not only apply to the Magic Mouse but to any device that connects its cells in series (i.e. practically every device).

P.S.: Anyone who had a wireless Mighty Mouse might remember reading the mysterious notice “3Vdc Agency approvals inside” on its underside. Whatever it meant (I never found any ‘approvals’ while disassembling the mouse), I think the 3Vdc Agency would not approve of the Magic Mouse.

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