Below 12V: USB, Powerpack and Battery Lighting

Making portable lamps has become a much more viable proposition, with modern battery technology storing a lot more power for a given size and weight, and LED lamps being a lot more energy efficient than filament bulbs.

The two things we need to look at initially are the supply power, and the bulb specifications. Then we can specify the wiring requirements, and anything else that may be needed in between the two, such as for voltage regulation.

Supply Power


The USB specification states that the voltage can be anywhere between 4.75 and 5.50V DC.

The wires in a USB cable have standard colours, and for power red is +ve and black is ground.

USB versions 1 and 2 provide a maximum current of 100mA (0.5W), or 150mA (0.75W) for USB 3. This is called the low power mode; high power modes with significantly higher power outputs are available, but these require signalling from the lamp to the USB power supply to switch them on, and they are not universally available. So, unless supplying the USB power supply, the safe option is to assume that there is just 0.5W available.


A powerpack is just a marketing term for a rechargeable battery which typically has multiple outputs, e.g. USB, 12V (often with a "cigarette lighter" socket), and 230 or 120V AC mains.
If the powerpack is part of your lamp, and it has a high power output on its USB ports, then you can provide the necessary signalling to switch on the high power output from the USB port.


There are a number of battery sizes and technologies available, the most common of which are given below.
Note that the voltage given is the maximum, when the battery is fully charged and not under any load. As the load increases, and the charge in the battery reduces, the voltage will reduce, e.g. a 1.5V battery may drop down to 1.0V.

SizeIEC nameANSI nameTechnologyNominal voltageTypical capacity (Wh)Rechargeable
AAALR0324AAlkaline1.51.3 - 1.8Some
AAAKR0324KNiCd1.250.38 - 0.63Yes
AAAHR0324HNiMH1.250.75 - 1.6Yes
AAZR6?NiZn1.60 - 1.652.97Yes
PP36KR6111604Ni-Cd7.2 / 8.40.864 / 1.008Yes
PP36HR617.2H5Ni-MH7.2 / 8.4 / 9.61.26-2.16 / 1.47-2.52 / 1.68-2.88Yes
PP3Varies-Lithium polymer7.43.848Yes
PP3Varies-Lithium ion7.44.588Yes
PP3--Lithium iron phosphate9.61.92 - 3.072Yes