ASK THE EXPERTS: Battery Venting for Small Systems

Installing a battery box vent
Solar installer Jay Peltz works on a power vent for a new battery box.
Installing a battery box vent

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

Comments (7)

dcapps's picture

Does anyone have ideas about how to keep a battery bank at reasonable working temps during the winter? I'm off the grid in upstate NY, PV/inverter/batteries in (currently) unheated garage. We think the batteries were damaged some last winter when temps were below zero and panels got covered with snow so couldn't keep batteries fully charged. We're considering a small propane wall furnace to heat the battery enclosure but are loathe to take up precious space in the garage. Has anyone used the electric underfloor heating mats with back up generator power?

Graig Pearen's picture

The temperature isn't the real problem, it is the lack of charge that you need to worry about. I assume you have a flooded lead acid (FLA) battery.

Temperature: Build a box or room around the battery to reduce the volume that has to heated. Set the batteries on foam insulation, 2" minimum, more is better. When batteries sit on a cold surface, they tend to stratify. A vigorous (longer than usual) equalize is needed so the rising bubbles can remix the electrolyte. Use an RV furnace, they are small with thermostatic control. They require 12vdc for the controls so buy a dc/dc converter (ebay) to run it off your 24v or 48v main battery.

Charge: If the snow can not be cleared from the array, mount one or two solar modules on the south wall (so they are vertical) low enough that they aren't shaded by the roof overhang in the winter. Use a second small charge controller just for them or connect them to the battery with a fuse only. A small array won't overcharge your battery in the winter and the very gentle equalizing will in time correct the sulphation that has taken place.

It sounds like your battery is sulphated. I was able to recover a severely sulphated (useless) 24v 600ah FLA battery string by connecting 150w of PV directly to it and leaving it that way, with no load for a year. They were on a very gentle remedial equalize charge all day, every sunny day for a year and the battery came back to nearly full capacity. Water usage was minimal but you need to check it occasionally.

dcapps's picture

And here's another question: does anyone have experience with water repellent or hydrophobic coatings for PV panels? My panels are on a 45 degree slope (nice southern exposure) but seem to actually grip snow -- or at least I've been surprised how much the snow stays on the array. I can brush it off, but it doesn't slide off very well at all. Any ideas?

Graig Pearen's picture

This was a topic in Home Power Magazine a few years ago. The conclusion was that nothing works. If you can, set your array vertical for the winter. That's what we do at mountain top radio sites (and at my home).

dcapps's picture

Thanks very much indeed -- this is very useful. Our batteries are actually AGM -- and our fear is that they were damaged when they sat discharged and at 20below for about a week when we were gone (this is a second home). The system shut down appropriately, but the temp stayed below or close to zero for several days. Do you have any ideas about rehabilitating AGMs? They work OK during the summer, but don't see to be able to hold charge very well, even when they've been at float charge for a few hours during the day. Will look into the RV furnace -- great idea. My propane company has suggested an Empire DV210d direct vent wall furnace, but they'd like me to close off a full-height section of the garage for the battery compartment. I'd like to build a cabinet that functions as battery compartment and also work surface/bench -- maybe 40 inches high.

Ally Electric and Solar's picture

How many batteries will be necessary to operate an off grid solar system for a commercial yard space of 2500 sq ft? What equipment will I need to run electricity for a 200 sq ft office trailer on an completely off grid system? I welcome all advice.

Graig Pearen's picture

None. Neither the yard nor the office uses any energy. The occupants do. You have to ask yourself a differant set of questions

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