Gassing 



Hydrogen and oxygen gasses are given off 

 from lead-acid batteries during both charg- 

 ing and discharging, though the greatest 

 quantity emanates during charging. The 

 problems brought about by gassing depend 

 upon the packaging and location of the bat- 

 tery. If the battery is carried within the 

 pressure hull the problem concerns the di- 

 rect effects on occupants due to: 1) Inhala- 

 tion of hydrogen, 2) explosion of hydrogen or 

 3) inhalation of chlorine gas if the electrolyte 

 spills and comes in contact with seawater. 

 Silver-zinc cells also evolve hydrogen and, 

 according to Work (14), a "hot" short (where 

 the reaction is sufficiently vigorous to boil 

 the electrolyte) can vaporize mercury, which 

 is used in small amounts in these cells, to 

 present another potential hazard to the oc- 

 cupants. Batteries located externally and 

 pressure compensated by oil avoid the direct 

 safety hazards of poisonous gasses, but in- 

 troduce other factors bearing on short cir- 

 cuits and ballasting control. The effects of 

 gassing on pressure-compensated batteries 



will be discussed in the section on Protection 

 and Pressure Compensation. 



An atmosphere containing as little as 4 

 percent hydrogen (69 in.^ hydrogen/ft^ air) is 

 explosive in the presence of a spark or flame. 

 Generally, concentrations greater than 2 

 percent are avoided for operational safety. 

 Hydrogen is generated in greatest amounts 

 during charging, and for this reason, the 

 charging area should be well ventilated. 

 Being lighter than air, the hydrogen rises 

 and may become trapped in any dome-like 

 space above the battery. Even when idle, a 

 lead-acid battery liberates small amounts of 

 hydrogen, and if in a small confined space, a 

 dangerous accumulation can be reached 

 within a few hours. Another area of hydro- 

 gen accumulation can be the void space 

 above the electrolyte within the battery. (Ac- 

 cording to the Hydro-Catylator Corporation, 

 1 amp-hr of overcharging, under normal con- 

 ditions, produces 25.5 in.^ of hydrogen per 

 cell, and when idle, it can be assumed that 

 local action will be equivalent to an over- 

 charge flow of 0.1 amp for each 100 amp-hr of 

 cell capacity.) 



Fig 7.6 Hydrocaps on SEA OTTER s lead-acid balleries. 



324 



