From wikipedia :

The nickel cadmium and nickel metal-hydride designs have efficiencies of around 66%.[43] However, modern lithium designs have almost negated this wastage as they can have efficiencies of around 99%.[44]

But are the latter batteries really available ? It may be more reasonable to assume a 66% yield instead. This is what I usually do. :) With the idea that it is possible to have a much better yield using "modern" design.