Having spent some time away from the office and having had the opportunity to speak to a few customers, we have found that depending on how the HyperMac battery is used it is possible to eke out some extra time of useful power.

Essentially the best results are to be had by, fairly obviously, starting with a fully charged MacBook and HyperMac battery. Then whenever running on battery power, always have the HyperMac battery connected to the MacBook. This way the load is spread between both the internal MacBook battery and the HyperMac, gently discharging both.

The alternative of running the MacBook until the internal battery is flat and then connecting the HyperMac has its plusses and minuses. What seems to happen is that fairly rapidly the HyperMac battery charges up the MacBook battery to a point where the available power is shared between the two batteries according to their relative size. A bit like sharing a pint between two glasses.

However, whilst the Macbook will now run on its own internal power for a while, which could be useful in many situations, in discharging the HyperMac and charging the internal battery, both get noticeably warm. Whilst this warming up is perfectly normal and is exactly the same as charging from mains power, it does mean that energy is being wasted as heat. The energy lost as heat is no longer available to power the MacBook and so there is less usable power from the combined batteries.

I have not tried to make any measurements of the difference, except to say that subjectively there seems to be noticeably longer useful life by making sure that the HyperMac and MacBook are working together from the start. Maybe as much as 10% but different usage conditions make it hard to say - but certainly enough for me to want to post the findings here.