Sealed PbA can vent when you overcharge them. When sealed PbA vent it is bad news, as there is no way to replace the vented hydrogen gas; it is important that this gas is preserved for the battery's recombination process. Floodeds vent, and because of it, you must periodically water them. There is no way to water sealed batteries.
I wanted to add that they can vent if over-discharged too. In a series string it is possible to reverse a single weak cell on discharge. It starts charging in backwards with the discharge current, and has little tolerance for that. It will quickly see high reverse voltages and start venting hydrogen.
They can also blow steam from discharging into a short circuit or near short circuit.
A forum community dedicated to DIY electric car owners and enthusiasts. Come join the discussion about electric vehicle conversions, builds, performance, modifications, classifieds, troubleshooting, maintenance, and more!