Hello Jeff,

Thanks for all the great information you provide BHM readers. I’m planning to install a small PV system using the instructions in your article in issue #116. This will be a very small system and used primarily for back-up during a black out. I have a Uni-Solar US-32 panel and Sunsaver 10 amp controller. I’m installing the panel on a carport roof, (I live in Massachusetts), running the wires to connect to one or two 12-volt deep-cycle marine batteries in my attached garage. I’ll be using the 2-pole fused disconnect and correct wire size for the panel and ground wires you recommend.

I have three questions.

1. What can I use as a bracket to mount the panel to the roof? I’ve seen small RV racks for sale and wondered if there is an alternative mounting bracket that can be purchased at a hardware store.

2. The wiring diagram in your article shows the load wires going to two 12 volt lights, hooked-up in parallel. I plan on wiring one light for the garage and continuing the wires about 30 feet to be run to the inside of the house. Those wires will be hooked up to a wall plate with a 12-volt cigarette plug receptacle mounted on the wall. I want to use the plug for a 12-volt TV or any other 12-volt appliance/charger that works using the auto style plug. What do you think of this arrangement and do you have any suggestions or recommendations.

3. Do I need to vent the batteries outside if they are in an un-insulated garage? I plan on putting the batteries in an insulated box for the winter and wondered if I can drill a hole into the top of the box and let the batteries vent into the garage.

Thanks for the help.

Ernie Smith

Ernie,

I can give you some general answers, but since we would not know everything about a specific installation, we cannot be too specific.

Although most solar dealers offer an “approved” angle “foot” to attach any solar module to a roof, I have taken 1-1/2 X 1-1/2 aluminum angle and installed the leg standing up to the side of the solar module and the flat leg bolted into the roof framing. Keep in mind that if you do not use the pre-drilled mounting holes on the back of the module you will void the warranty, so you might try a combination of two short lengths of angle back to back to make a “Z” shape. This will give you a way to utilize the bolt holes on the back of the module.

As long as your loads are 12 volt DC and wired in parallel, they do not all need to be at the same location. However, I would increase the wire size to the more distant load as there is 10 times the voltage drop at 12 volts DC that there is at 120 volts AC.

Finally, a few RV or golf cart batteries out in a garage will not give off enough gas to cause a problem due to the large space, but don’t locate them next to a gas fired hot water heater. Hydrogen gas is only explosive when highly concentrated like in a small sealed up closet or battery box. If you do place the batteries in an insulated box, you will need a vent at the top. For larger battery systems we use a 1-1/2″ PVC pipe vented outside, with screening to prevent insects from entering. The pipe needs to slope uphill since gas rises, but you will need something to keep out the rain.

Good luck,

Jeff Yago

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Jeff,

I have been reading your articles on solar power and have decided to build a small battery room in the basement, as opposed to the battery box. However I am not certain about something and would like your advice. I will put the batteries on a treated plywood on upright 2×4’s floor in this 4′ x 7′ floor area room x 8′ ceiling height.

I am planning to run a vent from the wood furnace into the room to keep it at optimal temperature for battery performance. However I am not certain about how much venting to the outside is needed and won’t the vent to the outside also cause all the heat in the room to be lost?

Is there a one way valve of some kind that will allow fumes out but no cold air in? The furnace vent will provide fresh air as I am not going to have a cold air return from this room back to the furnace because of the battery fumes getting into the house air. Currently there are 8 batteries but there may be more batteries added in the future.

Your insight and advice in this matter will be greatly appreciated and hopefully will help other readers as well.

Sincerely,

John Mc Andrews

John:

We do not get into detail design advice for a specific application on this web site for obvious reasons, but I will pass along the following general guidelines for building a battery room. Under no condition would I duct supply air into a battery room from a furnace, wood stove, air conditioner, or anything else that moves air. Any room, no matter how well it is sealed, is not air tight, and if you duct or blow air into any room it will find its way out, and not always out the path you intended.

I have seen a sealed battery room with a small exterior pipe to vent battery gases to the outside actually “suck” air back into the room located in the basement due to wind pressure acting on the home’s exterior wall surfaces, and this backward air flow forced hydrogen gas back into the basement. You want to always EXHAUST air at the highest point in a battery room and let the makeup air come from a louver located near the floor, under the door crack, or anywhere else it can find a path, but the room should be under a negative pressure (air being forced OUT of the room to the outside) in reference with the rest of the house and basement so any hydrogen gas will be exhausted to the outside and not forced back into other rooms or up through the ceiling to the floors above.

We usually use a battery-powered in-line exhaust fan installed inside a 3″ PVC pipe. These are made for battery gases and the brushless fan they use will not spark and cause a gas explosion. I would expect almost any basement under a house to not get too cold or too hot and would not expect any heating will be required. However, if you want to heat a battery room the only safe method that I am aware of is a very expensive explosion-proof electric radiant heater, or baseboard radiation around the walls that is heated by a hot water heating boiler or domestic hot water tank.

We always build a battery shelf made from pressure treated 2 x 4’s on edge with 1/2 x 4 spacers between each to allow for good draining and air flow. We place these on 8″ concrete blocks spaced every 3 or 4 feet to provide good air flow and to keep the battery bottoms from touching the much colder concrete slab floor. A floor drain is also helpful as it can get a little wet when you wash down the batteries with water and baking soda. Finally, we always add a good ABC type fire extinguisher, a vapor-proof glass “jelly jar” type light fixture, and a locking door having a warning sign.

Good Luck and don’t light a match when you are in your battery room if you can’t see!

Jeff Yago

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