Rick's Free Auto Repair Advice

Car Battery Vent Tube

Car Battery Vent Tube — Why batteries have vent tubes

All lead acid batteries create hydrogen gas when they’re being charged. Older flooded lead acid batteries (non-maintenance free) used a vented cap above each cell to vent hydrogen and water vapor from each cell during charging. That why those older batteries required occasionally topping off with distilled water.

Maintenance free batteries, on the other hand don’t freely vent charging gases and water out of the battery. First, they have a different plate construction that doesn’t create quite as much hydrogen during charging. Then the case is designed to trap the gas and water inside the battery so it can return to the electrolyte. However, if internal pressure gets too high, the battery will vent to the outside to prevent case rupture.

Absorbed Glass Mat (AGM), like maintenance free batteries are designed to lower the amount of hydrogen off gassing during charging and to return the gas and water vapor to the electrolyte. And, like a traditional maintenance-free battery, they’re designed to vent in cases of high internal pressure. AGM batteries are built with an interval pressure release valve. Due to this feature, AGM and gell cell batteries are referred to as valve regulated lead acid batteries (VRLA).

But why the battery vent tubes?

When a car battery is installed under the hood, engineers don’t worry about escaping hydrogen gas because there’s plenty of airflow under the hood. But when a maintenance free or AGM battery is mounted in the truck, under the rear seats or in any enclosed area, the battery must be connected to a vent tube to move the escaping hydrogen gas out of the enclosed space.

Vented battery designs

Vented batteries are designed with two vent ports; one on each end of the battery. The battery is delivered with one vent plugged and the other open. This design allows the installer to mount the battery in the proper orientation and use either side for venting simply by removing the plug and placing the plug on the opposite side.

How to install a vented battery

Situate the battery in the battery tray with the posts in the proper orientation to connect to the battery cables.

Secure the battery hold down brackets/devices. Tighten until the battery doesn’t move, but do not overtighten. You only need enough torque to prevent battery movement. Over torquing the hold-down devices can crack the battery case.

Determine which vent port to use for the existing vent tube. If the port is plugged, twist the plug as you pull it out. Move the plug to the open port.

battery vent tube

Move the red plug to the unused vent port

Replace vent tube

If the battery vent tube is damage or missing or the elbow is broken, you can purchase a replacement at any auto parts store.

Battery vent tube kit

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Posted on by Rick Muscoplat


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