Low-Power Car/Bike USB Charger

Posted on Feb 4, 2014

Looking for an efficient USB charger that can operate from a 12V car battery This unit functions at up to 89% efficiency and can charge USB devices at currents up to 525mA. Best of all, it won`t flatten the battery if it`s left permanently connected, as long as you remember to unplug the USB device. There are lots of USB chargers on the market bu

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t this device has two stand-out features: high efficiency and low standby current. In fact, its standby current is just 160 A, a figure that`s well below the self-discharge current of most lead-acid batteries. This means that you can leave the device permanently connected and it will not cause that battery to go flat (or at least, not much faster than it would of its own accord). Why is this useful Well, in September 2009`s Ask SILICON CHIP section, D. E. of Ainslie, ACT asked if it was possible to connect a 12V-to-5V USB charger directly to the battery on a motorbike. His reason for wanting to do this is that doing anything else might void the warranty. Our reply was that it is possible but that it would need to have a quiescent current (IQ) of less than 1mA to avoid draining the battery between uses. While USB car chargers are cheap and plentiful, finding one with a low enough quiescent current for permanent battery attachment is difficult. Even those marketed as low idle power devices don`t specify how much current they draw on standby. We tested a regular charger and found that it consumed 13mA with no load. Like many others, it has an integrated power LED and that would contribute significantly to the standby current consumption. However, since the cigarette lighter socket is only powered when the engine is running, there is no real reason for the designers of these car supplies to keep the quiescent current low....

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