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If you're not charging, then there shouldn't be much danger of explosion as hydrogen is released as a product of electrolysis.
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I wasn't charging the battery when I blowed'erUPP. I was running a grinder on the header, shooting sparks past the vent tube & she said"Kerblammy!" . Sounded about like a 22 shot. Cracked my battery & caused me a lot of tropuble cleaning & treating the acid exposure with baking soda & water.
the caps were off too so any hydrogen should've floated off to neverland to meet mj.
1/4" is not much, I wouldn't worry (much)
If anal, use straw technique and get over it.

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I wouldnt sweat 2 cells slightly overfilled.