Battery Venting for Small Systems

ASK THE EXPERTS: Battery Venting for Small Systems
Dan Fink • Buckville Energy Consulting
HP Online

I have a small 75 W solar-electric system that I use to charge three marine lead-acid batteries. I use it to power my ham radio station, minimal lighting, and a small inverter so we can watch TV during occasional power outages.

The battery’s capacity is about 350 Ah, and the maximum PV short-circuit current is about 5 A. I have a charge controller that prevents battery overcharging. There seems to be no battery gassing from this setup, so I have not worried about ventilation and have the batteries in my house. However, I am thinking seriously about increasing our solar capacity—not to the point of replacing our normal electricity usage, but to have enough charging and storage to run the motors in our pellet stove during a winter power outage or keep our freezer running during a summer outage.

Should I be worried about keeping flooded lead-acid batteries inside the house? Should I either provide a vented enclosure or put the batteries outside? Is there a rule for an acceptable ratio of charging current to Ah capacity for using batteries as I am now using them?

Albert S. Woodhull • Leyden, Massachusetts

Keeping your batteries indoors in their own enclosed, ventilated space is usually the best practice, since this protects them from potentially damaging temperature extremes. Most renewable energy installers recommend a sealed, vented battery enclosure, no matter how small the battery bank or what battery technology is used.

Article 480.9 of the National Electrical Code (NEC) states that provisions for ventilation must be made to prevent the accumulation of explosive gases, but the NEC doesn’t go into the specifics. Under the NEC, sealed battery technologies don’t require venting. American Boat and Yacht Council (ABYC) guideline 10.7.9 recommends a sealed, vented enclosure no matter what the battery type. Some local electrical codes even require power venting of the battery enclosure.

A properly designed and installed power system with a modern, three-stage charge controller keeps hydrogen gas emissions to a minimum (as with your present system), and battery technologies like sealed lead-acid don’t gas during normal operation. But what happens when the situation becomes abnormal? A poorly programmed or malfunctioning charge controller can cause any battery to gas, and even “sealed” batteries have internal valves to release the gas and prevent a case from rupturing.

Battery banks also pose other hazards—exposed high-amperage terminals and wiring; corrosive buildup on the terminals; thermal runaway (with certain battery technologies); and the danger of spilling acid electrolyte. A mishandled wrench that shorts out a battery can turn red hot in a moment, not to mention giving a dramatic sound-and-light show for the unfortunate person who dropped it. Battery banks should be securely isolated from anyone who doesn’t have any business with them.

So, that’s the logic behind always using a vented battery bank enclosure. Most recommendations call for a minimum 2-inch-diameter PVC pipe vent from the top of the box, and a hinged, slanted lid, so any hydrogen gas rises to the top and out the pipe. Hydrogen is so light that it will find its way out even with a flat lid, but the slant also prevents the homeowner from piling things on the battery box lid. That makes access for regular battery maintenance easy, and gives quick emergency access.

For a typical flooded lead-acid renewable energy battery, the maximum recommended charge rate is usually about a C/5 (battery capacity in amp-hours divided by 5), tapering down during the final charging stages. But there are so many different battery technologies and manufacturers that you should be sure to follow the manufacturer’s recommendations. Some modern charge controllers let you tell them the battery bank type, capacity, and recommended charge rates. Their circuitry then does the math for you, and sets up the controller automatically to keep your battery bank healthy.

Things aren’t always normal, and lots can go wrong. Keep your batteries accessible but secure, and check on them regularly.

Dan Fink • Buckville Energy Consulting