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I've recently ordered a LEGO® 8878 Rechargeable Battery Box:

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I was wondering if I really need to get the (rather expensive) LEGO® Power Functions Transformer or if I can substitute a (cheaper) generic one?

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If a generic one will do, what specification does it need? Obviously it needs the right plug and rating for my local AC mains socket, and that I need 10V DC, but what about the size / polarity of the DC plug?

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By the way, you don't need one transformer per battery. –  Joubarc Dec 9 '11 at 17:49
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up vote 6 down vote accepted

Yes, you can use another transformer, even if it's not recommended by LEGO and you do so at your own risk (which should be minimal if you pay attention to what you do).

The plug polarity is + in the center and - on the outside; it should be mentioned on the battery and the real transformer (I wish I could find the one I have to check), and the size is a standard (but small) one. (Note that there is a protection in place should you reverse the polarity, but it won't work). Usually if you get a transformer with multiple plug adapters, it will be one of them. Note that is must be DC, this may seems obvious but since older LEGO transformers used AC it's better to make sure that point is understood. (and of course that's also why polarity is important).

The trick is to find one which outputs 10v DC, as 9v will not be enough to trigger the charge circuitry (the threshold is 9.5v). Apparently, you can charge it at 12v without too much trouble, but it will be slightly slower.

Edit: just go read RailBricks magazine issue 7 which has an extremely detailed article on the topic on pages 46 to 55 (yes, a full 10 pages).

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From where one can buy such charger/transformer? I would be obliged if you can provide a URL. Thanks. –  Farhan Dec 20 '13 at 20:07
    
Well, as I said, the trick is to find one which outputs 10v DC, which isn't so common. See the kind of thing I have in mind on amazon, for example. Notice it doesn't do 10v indeed, so you'd have to use 12v - and the price may be cheaper than the LEGO one, but since you only need one most of the time, maybe it's safer to just buy the LEGO one and be sure you won't have any problems at all. –  Joubarc Dec 21 '13 at 8:41
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