The battery in a Nissan Leaf is either a 24kWh, 30kWh, or 40kWh Lithium-Ion pack depending on model and specifications. The 24kWh and 30kWh battery packs are made up of 48 individual battery modules, it is not a single sealed unit. As with similar battery packs in hybrid cars, like the Toyota Prius, in the unlikely event of a problem occurring it is most commonly in a single module. They say that over this life the battery may reduce its capacity by up to 20%. Be assured however these batteries are very very reliable and it is extremely rare that they give any problems. The failure rate is less than one in ten thousand, significantly better than a petrol car!