I recently connected two car batteries to a UPS and plan to use the setup indoors. The space is fairly small and doesn't have dedicated ventilation. Is this generally safe, or should I be concerned about battery gases and other hazards? The batteries may need water refilled every few months, so they might be flooded lead-acid batteries rather than sealed units.
3 Answers
You should not operate ordinary car batteries in a completely enclosed, unventilated room. Flooded lead-acid batteries can release hydrogen while charging, and they can also produce acid mist. Hydrogen is highly flammable, so use a properly designed, ventilated battery enclosure or place the batteries somewhere with reliable fresh-air circulation. A small gap under a door is not something I’d consider adequate protection.
Refilling the electrolyte level means these are probably conventional flooded automotive batteries. They can vent hydrogen during charging, leak corrosive acid, and potentially spill if they are tipped over. Keep them upright in a suitable acid-resistant battery box, provide ventilation to the outdoors, and follow the UPS and battery manufacturer’s requirements. Moving the setup to a garage or other properly ventilated location would be safer than using it in a small closed room.
The UPS itself also needs airflow and may not be designed for automotive starting batteries. Check its charging voltage, maximum battery capacity, fuse requirements, cable sizing, and whether it supports the battery type. If the batteries become hot, swell, leak, smell strongly, or make unusual sounds, disconnect the system safely and stop using it.
If you have to add water periodically, that strongly suggests flooded batteries. Those need more care than sealed AGM or gel batteries, but even sealed lead-acid batteries have pressure vents and should not be treated as safe for a sealed closet.