A good quality, low cost high capacity battery for the F.I.Z. Adaptor and the HU3.
In our tests, a fresh one powered a handset 2 for 16 hours! How? It's Lithium Ion technology, much more powerful than the original manufacturer's. You can rest assured that the one you are reaching for is hot, because ours have an instant reading power level indicator. Never embarrass yourself again with a dead battery. They are cheap enough to replace often, so you can always make sure that yours are tip-top.