Electricity: The 411 on the battery bank

(12) T-105 6 volt batteries capable of 675 amps of stored electrical power

(12) T-105 6 volt batteries capable of 675 amps of stored electrical power

The Battery Bank, not a pretty picture, but most battery banks for an off grid home usually are not beautiful, just functional. This is our stored electrical power and how we power the home at night and during cloudy or low solar producing days. What you are seeing is (12) T-105, 6 volt batteries hooked in parallel and series. Each battery has 225 amps of stored power. Each row creates 24 volts x 225 amps = 5400 watts. To understand how much power that is, I could run one 60 watt incandescent light bulb for 90 hours, or I could bake a potato in a conventional electric oven for 1 hour.

The reason we bought the T-105 batteries is because we wanted a battery that would work well for us also we didn’t want to break our other bank of the piggy kind.  We also figured since we never had an off grid house before that to experiment and learn on an expensive battery bank could be costly, especially if we made mistakes in maintenance or neglect.  However, we have had these batteries for 4 years now and they still function as if in mint condition. I would consider replacing these batteries with the T-105 batteries again when the time comes.  If I decide I want more power to last longer in the winter months, I could easily modify the battery box for a 4th string, increasing my overall battery storage to 900 amps or 21,600 watts.

Calculating a fair estimate on how long a battery bank will last due to the home's use.

Calculating a fair estimate on how long a battery bank will last due to the home’s use.

Since 3 rows are here, we have 675 amps of stored power times 24 volts or    (675 x 24) = 16,200 watts of stored power. Amps x Volts = Watts. Now there is an important rule to follow. Never discharge your batteries to 0% of power. It creates a dead battery for one and also limits the life of the battery. Since these are deep cycle cell batteries, the manufacturer recommends not using them below 20% of charge. When discharging or using the energy down to 20%, the batteries life on average is capable of 700 recharges to full before the batteries begin to degrade significantly. So technically, we only have 80% x 16,200 watts = 12,960 watts of usable power.

This is where the game begins, having a home that functions like a conventional home, but uses the least amount of power in all its appliances to stretch the need to recharge through the cloudy days to the next sunny day or partial sunny day for a recharge. Because we designed our home to function efficiently, we do not worry about how much power we are using from February to late October each year, the solar panels keep the batteries powered up during these months. Unfortunately, from November to January, we have to rely on a backup generator for about 5 – 7 charges during these 3 months (Electricity: When the backup generator is called upon…).

Another way to maintain battery life is to create a comfy environment for them. I built the battery box in our utility room. The utility room is part of the main house and the temperature fluctuations aren’t as extreme as if stored outside. Extreme cold can greatly reduce battery efficiency, so to store them outside in an unheated environment isn’t the best option to maintain battery efficiency. I also placed a 2 inch foam board on the bottom of the box to prevent the cold concrete floor from cooling the batteries below room temperature. I placed a fan direct to outside in the box as well that when turned on opens the outside vent and blows out any off gassing hydrogen from the batteries without letting cold air in. Also, the battery box is directly below the inverter and charge controller, so the electricity doesn’t have to travel far. I also equalize them every month. In the winter I use the generator, but in the spring, summer, and fall months the solar panels are scheduled to equalize the batteries through the charge controller. After I equalize the batteries, I always make sure to add more distilled water to them.  I usually end up adding about a quart of distilled water to the battery bank of 12 batteries once a month.

Energy Efficiency Kit possibly at your local library.

Energy Efficiency Kit possibly at your local library.

If you are interested in finding out more about how much power you use on average in your home.  There is a unique program starting at our local library in which a patron can check out an “Energy Efficiency Kit”.  I have a feeling this program is offered in several states, if not nationwide, since I found the same kit on a library website in California.  It includes a meter and pamphlet that explains how to measure your electricity usage.  It’s not necessary to have, but it’s an easy way to see how much power certain appliances use.

Sorry about all the mathematics, but when I asked my high school math teacher when I would ever use math again, I had no clue what I was in for.


7 thoughts on “Electricity: The 411 on the battery bank

  1. I have no idea where they got them…but I’ve seen battery vent caps that have a small tube sticking up from the cap. A flexible hose attaches to this tube and the other end of the flexible hose attaches to a similar tube in a 2 or so inch diameter PVC pipe. The PVC pipe has elbows on it and those extend through the wall to the outside The PVC pipe is set at a 45 or so degree angle and screens are put over the ends of the pipe to keep the critters out. On the outside ends of the PVC pipe was another set of elbows that pointed downward. Some stainless steel screen to keep the “critters” out finished off the system.

    Heavier than air stuff flows outside via the bottom opening of the PVC pipe and lighter than air stuff flows outside via the upper opening.

    This was at a 2 way radio site atop a 3500 meter above sea level mountain. Outside winter temperatures got cold enough that the propane wouldn’t vaporize. and they didn’t want to have bring outside air in to vent the battery room.

    I wish I had pictures…..


    • The coldest I have ever been in -35F and that was in Nebraska. Since we lived in N. Idaho, I have only seen it drop to the -20F. During one of those nights, I miscalculated the heat output of the stove and heated the house to 85F. The wife and I were hanging out in shorts and a t-shirt with windows cracked to let in some of that -20F to cool down.

      Liked by 1 person

  2. Another way to extend the battery life is to reduce the depths of discharge. If you discharge to the maximum recommended depth of discharge of 80%, your battery will not last as long as say, an average depth of discharge of 50%.

    Liked by 1 person

    • Yes, I mention this on posts “Electricity: When the backup generator is necessary” and “Electricity: the 411 on the battery bank.” During the Spring, Summer, and early Fall, our panels keep our battery bank full most days. During these months our battery storage rarely drops below 90% capacity. During the months of November December and January it’s a different story. We go weeks without a full sunny day and run the generator around 35% capacity to bring the batteries up to full and equalize them. By being conservative in power usage, we have increased our battery life considerably. We should easily get 7 years, but I am optimistically hopeful for 10 because of our maintenance schedule has shown no drop in power levels yet after 4 years.

      Liked by 1 person

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