ServiceNomad RV · Knowledge
RV Batteries Boiling or Venting
If you hear bubbling sounds from your RV house batteries, smell a rotten egg odor (hydrogen sulfide), or see liquid leaking from the battery caps, your batteries are being overcharged. This causes the electrolyte solution inside lead-acid batteries to boil, creating dangerous hydrogen gas, damaging the battery plates, and potentially causing an explosion. Understanding what causes overcharging helps you stop the damage before batteries are ruined or safety is compromised.
Normal vs. Dangerous Gassing
All lead-acid batteries 'gas' slightly during charging—this is the normal electrolysis process where water is broken down into hydrogen and oxygen. You might see tiny bubbles at the end of a charging cycle, and this is fine. However, vigorous boiling, large bubbles, or continuous gassing even after the battery should be fully charged indicates overcharging. Overcharging happens when the charging voltage is too high (above 14.8V for flooded lead-acid batteries, or above manufacturer specs for AGM/Gel batteries). The excessive voltage forces too much current into the battery, generating heat and causing the electrolyte to boil off. This depletes water levels, exposes battery plates to air, and causes permanent sulfation damage.
Converter Voltage Issues
The most common cause of battery boiling in RVs is a failed or misconfigured converter/charger. If the converter is stuck in bulk charge mode (14.4-14.8V) and never switches to float mode (13.2-13.6V), it will continuously overcharge the batteries. This often happens with older converters, converters with failed voltage sense wiring, or converters that have been subjected to voltage surges. A technician will measure voltage at the battery terminals while the converter is running. If voltage exceeds 14.8V for extended periods (more than 2-3 hours), the converter needs adjustment, repair, or replacement. Modern smart converters have multi-stage charging that prevents overcharging.
Solar Charge Controller Failures
If you have a solar panel system, a failed or misconfigured solar charge controller can overcharge batteries. PWM controllers without proper voltage limiting, or MPPT controllers with incorrect battery chemistry settings, will push too much voltage into the batteries on sunny days. This is especially dangerous because solar charging happens silently—you may not realize the batteries are boiling until you smell them. Check your solar controller display for voltage readings and charging mode. If batteries are at 100% state of charge but the controller is still showing high charge current and 14.5V+ voltage, the controller needs reconfiguration or replacement.
Old or Damaged Batteries
Sometimes batteries boil because they're old and internally damaged. When battery plates sulfate or short internally, they cannot accept a proper charge—the current goes into generating heat and gas instead of being stored as chemical energy. These batteries will boil even on proper charging voltage because their internal resistance has increased dramatically. If batteries are more than 5 years old, have been deeply discharged multiple times, or have visible plate damage (visible through the translucent case on some batteries), they may need replacement regardless of charging system fixes.
⚠️ Safety Notes
- Hydrogen gas from boiling batteries is extremely explosive—keep all sparks and flames away from the battery compartment.
- Never check batteries with a flashlight or lighter—use a LED flashlight only.
- If batteries are boiling, shut off all charging sources immediately (shore power, solar, engine alternator) and ventilate the battery compartment.
- Battery acid (sulfuric acid) is corrosive—wear gloves and eye protection when inspecting or servicing batteries.
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