Hydrogen and air can be a very explosive mixture and especially towards the
end of a charging cycle when significant quantities of hydrogen can be given
off by the bank of batteries. To reduce the risk of a build-up of gas, leading to
an explosion, batteries must be properly stowed and adequately ventilated. It's
recommended that your battery stowage and ventilation arrangements comply
with the I.E.E. Regulations for the Electrical and Electronic Equipment of Ships,
where relevant. [4.2]

You might think that the weight of a big bank of batteries would keep them in a secure position. Not so according to reports gathered from accidents. Batteries can shift around as a result of the boat tipping in a lock or at a mooring when the water level drops. It shouldn't be surprising either that unsecured batteries on your boat can move when negotiating a roundabout on the back of a trailer! [4.2]