Tony M Dec 10, 2018:

@ Asker From your customer's explanation, it seem this EN is very flawed, as the explanation makes nearly as little sense as the original.
As it stands, it remains ambiguous; as I see it, 2 interpretations are possible here:

1) the nitrogen capacity (i.e. how much it can hold)
2) the nitrogen charge holding ability (i.e. how well it can hold the nitrogen charge without its leaking out)

'nitrogen charge' just means 'the intended quantity of nitrogen with which it is to be filled'
Mareille has come up with yet another possible interpretation — a 'holding charge' — but that seems to me at odds with your customer's explanation; unless they are indeed talking about filling the system with nitrogen for 'holding'purposes, while awaiting its being filled with its final intended gas. This is a common enough technique, though doesn't seem to correspond to your customer's explanation.

I don't think it is possible to translate this term without further disambiguation — unless the surrounding text yields further clues — for example, a figure expressed with units.