the combustion engine drives a generator only for charging the storage batteries (series hybrid). In all cases, a charge regulator is needed. This ensures that the batteries are charged at their maximum safe rate when power is available, without overheating or internal damage, and that they are not overcharged when nearing full charge. Battery bank There have been significant technical advances in battery technology in recent years, and more are to be expected in the future. Lead–acid batteries were still the most viable option until the advent of larger, lithium-ion batteries mass-produced for electric cars from approximately 2012 onwards. Deep-cycle, 'traction' batteries are the obvious choice. They are heavy and bulky, but not much more so than the diesel engine, tanks and fittings that they may replace. They need to be securely mounted, low down and centrally situated in the boat. It is essential that they cannot move around under any circumstances. Care must be taken that there is no risk of the strong acid being spilled in the event of a capsize as this could be very dangerous. Venting of explosive hydrogen and oxygen gases is also necessary. Typical lead-acid batteries must be kept topped-up with distilled water. Valve-regulated lead-acid batteries (VRLA), usually known as sealed lead-acid, gel, or AGM batteries, minimize the risk of spillage, and gases are only vented when the batteries are overcharged. These batteries require minimal maintenance, as they cannot and usually do not need to be refilled with water. Nickel metal hydride, lithium-ion and other battery types are becoming available, but are still expensive. These are the kind of batteries currently common in rechargeable hand tools like drills and screwdrivers, but they are relatively new to this environment. They require different charge controllers to those that suit lead-acid types. Lithium-ion in this case usually means lithium iron phosphate batteries, which although are heavier than other lithium-ion, is safer for marine application. They are expensive but in applications which need reliability and ruggedness like ferries which run most of the day (10–12 hours/day) this is the best option. It has a much longer life - 5 to 7 years life-cycle. Fuel cells or flow batteries may provide significant advantages in years to come. Today (2017) however they are still expensive and require specialist equipment and knowledge. The size of the battery bank determines the range of the boat under electric power. The speed at which