And here is the full press release by Nagra, as a reaction on that video:

Audio Technology Switzerland is keen to receive constructive and well documented criticism concerning its NAGRA products. However, amateur videos potentially putting customer safety at risk is strictly contrary to the company policy.
The technology developed and used in our batteries is excellent for energy storage for any environmental situation where a portable self-contained application is required. It is however a very sensitive technology, that needs to be handled extremely carefully. If not, lithium batteries can heat up and in extreme cases catch fire or even explode.
It is for these reasons that Audio Technology Switzerland takes extreme care to use certified (incl. CE & RoHS...) battery packs, in order to be able to guarantee the safe use of its products. This allows also our customers to travel with full security as this certification is valid for airline safety.
It would appear that the creator of this video is ill informed of various different possibilities. The battery pack was specifically developed so that if the customer needs to repair his machine, he can simply replace the defective single battery cell, or if he wishes to upgrade his machine, he can replace the single battery cell by a multiple battery pack, and the capacity is increased without the need to purchase the full battery pack. However, such operations should be undertaken by properly trained personnel, and full testing of the unit is necessary in order to avoid any safety risks or eventual damage to the recorder.
Audio Technology Switzerland - NAGRA develops and manufactures extremely reliable and high quality professional products, individually hand-made in Switzerland. This means long lasting products for the customers. The prices cannot be based on a single element of a particular device but reflect the precision, quality and service we strive to achieve.
NAGRA is against programmed obsolescence of its products, providing electronic equipment used and maintained for decades.
Nagra Team