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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.

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closed as off-topic by Leon Heller, PeterJ, Ricardo, Daniel Grillo, JIm Dearden Jul 10 '15 at 17:30

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – PeterJ, Ricardo, Daniel Grillo, JIm Dearden
If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ I hope you recover, but my first question is, are you absolutely sure that you connected the outside pair correctly to the sweeper? Positive to Positive, Negative to Negative? \$\endgroup\$ – R Drast Jul 10 '15 at 11:29
  • \$\begingroup\$ @RDrast, to support your question: I've seen (car) SLA battery leads using reddish-brown for 0V and black with a subtle red mark at the end for +12V. That caused a big spark when going for a jump-start. \$\endgroup\$ – Chris H Jul 10 '15 at 13:24
  • \$\begingroup\$ R Drast Yes I am 100% certain and I also asked a co worker to check the polarity of all connections which only basically amounted to two connections as the sweeper batteries were hard wired and the jump batteries wired correctly in series \$\endgroup\$ – Pedro147 Jul 10 '15 at 16:55
  • \$\begingroup\$ I have had an automobile battery explode because internally plates had become loose from the connecting post. As heavier current passed through, a spark was produced that ignited the gases inside. This can be from loading or charging the battery. \$\endgroup\$ – Optionparty Jul 10 '15 at 19:35
  • \$\begingroup\$ Optionparty Yes I feel that this maybe a similar case here. Thank you for your input \$\endgroup\$ – Pedro147 Jul 11 '15 at 0:25
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schematic

simulate this circuit – Schematic created using CircuitLab

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.

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    \$\begingroup\$ As for why it exploded (and not just got warm or did nothing), this is because lead-acid batteries produce hydrogen gas while charging. Normally, this is vented to the atmosphere as it's created, and it harmlessly floats away. However, in a damaged, vent-hole plugged, or heavily-used battery, a lot of hydrogen can collect inside or near the battery. Any kind of ignition source (spark, excessive temperature, etc) can ignite the hydrogen gas. Here is a video demonstrating this. \$\endgroup\$ – rdtsc Jul 10 '15 at 12:36
  • \$\begingroup\$ The "jump" batteries are virtually brand new and have seen very little servive. The sweeper batteries are a few years old and I suspect that one of them was damaged causing an extremely high resistance and subsequent explosion on connection but this is pure conjecture on my behalf and that is why I am here asking for some expert advice. \$\endgroup\$ – Pedro147 Jul 10 '15 at 17:01

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