Related product links are one-way in that if you create a link from product 'A' to product 'B', there will be no link from product 'B' to product 'A' (unless you create this separately of course). The logic is probably obvious – in the case above regarding batteries, you would not want to visit the battery page and see links to every single electronic item that links to this type of battery.

Single direction links gives you more control and choice than bi-directional links; you can always achieve bi-directional links by adding relationships on both items, if you wish.