This system will introduce an additional weight of 7 grams (about 6 grams for the battery with the switch and 1 gram for the 2 LEDs and their wiring) .
All the parts will be arranged considering/improving the balance of the wheel to limit inertial forces for the smoothest ride and for the best lightening and visibility effect.
Step 1: Balance the Wheel
The rim has often reinforcements in its junction area and a hole for the inner tube stem on the other side. The weight of this stem is most of the time not enough to compensate what is on the other side so the wheel is not perfectly balanced.
For this reason I prefer to use inner tubes with long stems even with low profile wheels to have a better compensation.
Before installing the battery near the hub check the current balance of the wheel to see where its additional weight could give some benefit.
Put the wheel on a stand, otherwise turn the bicycle upside down, disengage the chain from the sprockets so it can freely rotate with the minimum effort required. The tyre should be inflated at the operating pressure.
Spin the wheel a little an let it swing until it stops. The bottom of the wheel shows where the heavier point is, so the battery should be installed on the opposite side of the hub and close to it (see picture). The closer to the hub the less inertial contribution it will give to the wheel without requiring any strong fixing.
Step 2: Installing the Battery Holder With Switch
You may need to find out the one that fits your wheel the best. In my wheel there are 36 spokes 4x crossed so the battery perfectly fits inside the triangle formed by two crossed spokes and the hub. With straight spokes wheels the installation between two spokes is even easier .
If necessary cut a little bit the plastic rim of the holder to fit the hub flange. Then mark where to drill buttonholes for the tie wraps so you can attach them to their appropriate spoke.
Step 3: Install the LED Holders on the Spokes
With rear wheels I prefer to install both LEDs on the left-hand side to avoid oil drops and dregs from the chain and sprockets, and also because bicycling on the right-hand side of the street (except UK, Australia and others) all the drivers coming from the back would also sit on the left-hand side of their cars or trucks so they can see LED lights directly without any shadow from my wheel.
Even not the maximum the visibility is still good for the vehicles coming from the back and right-hand side (e.g. at junctions or roundabout).
Mark the position of the two LEDs on diametrically opposed spikes at 110 mm from the outer side of the rim (mine is a 28" or 622 mm diameter). In this way the lights are not shadowed by the tyre but they still draw a big circle.
Wrap the duct tape on the spoke first and then when after a turn insert the tie in the next wrap or two.
Step 4: Fix the Wires on Each Spoke
Then wrap the corresponding wires to the battery poles together, solder each junction to give more strength, insulate with tape and link to the spokes to avoid any tearing. Some duct tape can be used to fix the wires on the hub flange edge.
Since I generally put reflective tape on the rim (except where the brake pads works) of my wheels, LED lights also generate a secondary circle.