Apple’s new iPad isn’t always telling the truth when says it has 100% battery.

The gadget starts to say that it is “full” when it’s at 90% – a difference that can cost users 1.2 hours’ usage.

The reason, says ZDNet, is that the formula used to calculate when the device is “full” is slightly out – a safety mechanism used by Apple to prevent people overcharging their gadgets.

All iPhones, iPads and iPod Touches misreport whether they are full – albeit at a slightly higher level around 97%, ZDNet claims.

But with the new iPad’s enormous battery capacity – 1.7 times bigger than iPad 2 – the slight difference in the formula used to calculate when it is “full” makes a noticeable difference to the battery life.

Raymond Soneira of monitoring firm DisplayMate says that the difference is measurable on the new iPad.

The new iPad's enormous battery capacity, 1.7 times bigger than iPad 2, the slight difference in the formula used to calculate when it is “full” makes a noticeable difference to the battery life

The display gives a claimed battery life of 10.4 hours, whereas the real figure is 11.6 hours – and the difference can leave users thinking the device is fully charged when in fact it’s not.

Raymond Soneira also claims that users who attempt to charge new iPad while using it could be in for a surprise – it can take up to 20 hours.

The problem should be easily fixable via a software download – often used by Apple to fix battery glitches and other teething difficulties with gadgets.

His measurements lend weight to earlier complaints on Apple forums that the device has shorter battery life and painfully long charging times.

DisplayMate claims that the huge increase in power demands is due to the density of pixels on the new screen.

The LED backlight needs a huge amount of power just to shine through the high density of transistors in the panels.

“The power consumption of the LEDs is 2.5 times that of iPad 2, and the battery is 1.7 times larger – if you run your new iPad at full brightness, the battery run time is less because you only put in 70% more battery but you’re using 150% more power,” Raymond Soneira of Displaymate said in an interview with CNET.