We have an electric floor sweeper at work which uses two 12 volt deep cycle batteries in series to supply 24 volt. The floor sweeper was not starting up on switch on so I connected two 12 volt lead acid batteries in series to the floor sweeper in parallel as one would if jump starting a motor vehicle with a 24 volt power system. In hindsight this was not a good choice because upon connection one of the lead acid batteries violently explode lacerating my face with a piece of the plastic case but luckily no acid entered my eyes although extensive flushing of the eyes at work and later at the doctor was performed as a preventative measure. I am wondering what may have caused this explosive situation but my guess is that there was a fault in one of the floor sweeper batteries, possibly a dead short which lead to this explosion. What are peoples thoughts on this please and how may I have better tackled the diagnosis of this no run situation in the first place please.I realise that in hindsight I should have charged the floor sweeper batteries but they did not seem to be accepting a charge from the battery charger supplied with the unit.
You've described the above situation. Ri * 3 represents the internal resistances of all three non-shorted batteries. If the number I gave is correct, then 2.4kW will get dissipated somewhere when the switch is closed. If the sizes or constructions of the batteries are different then most of that power could be dissipated in a single battery. That could cause the electrolyte to boil and the battery to vent. I don't think it should explode but maybe using an external battery caused more gas production than the vents were capable of dealing with or maybe the vent was plugged, or it was a badly made SLA battery.
Of course reverse connection would be similar but much worse with 3x the voltage and 22kW of dissipation.