At the final stages of charging, all lead-acid batteries begin dissolving some of their electrolyte into hydrogen and oxygen gases, generally kept contained by sealed gel cells and AGMs; under certain circumstances – for instance when consistently overcharging occurs – enough internal pressure can build to open pressure release valves and vent them outside; with wet cell batteries which require topping up from time to time however, their gases must always be released outside.
Hydrogen gas, although highly explosive, tends to rise and dissipate quickly as long as there is even minimal venting from battery boxes or compartments in which they reside. But if a pocket of hydrogen gas forms within deep cycle batteries or their surrounding compartment, any spark (for instance from starting up an electric motor brushed motor kicking on) may ignite it and result in powerful explosions – as happened at Fukushima nuclear power plants in Japan; internal short circuits within batteries also sometimes create sparks which ignite hydrogen gas within batteries before eventually blowing their case open; note this occurs with sealed as well as wet cell batteries!
Modern cruising boats contain far more electrical equipment than was ever present before, subjecting batteries to increased demands, with increasing hydrogen emission rates from powerful charging devices that exceed acceptable charging voltage limits and can damage them, including cheap automotive chargers such as that used on Thursday’s Child at the time of its explosion; as these may lead to overcharging leading to substantial hydrogen generation; even expensive chargers, with tight regulation can sometimes push batteries to their limit deliberately, leading to aggressive gassing – such as when equalising or conditioning wet cell batteries before use or equalisation/conditioning wet cells batteries in advance of use – potentially leaving vessels vulnerable.
Battery overcharging occurs when lithium ion batteries are overtaxed to the point that their internal heat production increases significantly, increasing their ability to accept charging current without its voltage increasing, effectively bypassing any voltage regulator on their charging device and resulting in excessive overcharging. Battery can experience what’s known as “thermal runaway”, whereby all available charging current is consumed, producing hydrogen, oxygen and heat which poses serious safety risk to users. One way to help avoid this situation is with temperature sensing at both ends – battery and charging device – of any potential issue. Temperature sensors don’t work when attached directly to a battery’s top surface as there’s an air gap within. Instead, sensors must either be secured directly onto its posts, or taped down about halfway. They’re an indispensable piece of any charging system designed for batteries subject to heavy usage – while automotive chargers often don’t include such features.
Even with rapid hydrogen formation, provided a battery compartment is vented from its top for release of hydrogen from inside, hydrogen will usually escape harmlessly into the boat’s air supply and out through venting tubes. But should any pocket form and any sort of spark occur within or without battery compartment walls resulting from any kind of electrical event, then consequences could be dire indeed.