Hi Lin and Mike,
The leisure battery on my Renault based van as supplied by Hobby was a 90 ah Exide AGM. I recently fitted an additional leisure battery under the drivers seat and the closest current match from Exide was a type EK950 AGM Stop – Start. Exactly the same dimensions and as it’s AGM no need for a vent. Incidentally the original leisure battery fitted in that little box by the Truma boiler does not have a vent hence an AGM type. Others may say that a normal lead acid being cheaper, is fine but they do give off hydrogen gasses which are explosive and as the battery box is unsealed, inside the van and next to the heater. Not a risk I was willing to take.