Battery Banks For Sustainable Home Electricity

Step 1 in any sustainable home electrical system is setting up a battery back up system. Although the next step is coming up with a source of electricity, like solar, wind or radiant energy, I will cover just step 1 here.

Batteries can be wired in parallel to keep the original voltage of each battery. I think this is a better strategy because the batteries contribute to the bank individually - one bad battery doesn't negatively affect the rest of the batteries in the bank like they do in a series string.

Batteries can be wired in series to increase the battery bank voltage as in two 6-volt batteries wired in series (a series string) become a 12 volt battery bank or four 12-volt batteries wired in series become a 48 volt battery bank.

When wiring more than three batteries in parallel or when wiring more than three series strings in parallel, it is recommended that they have a common positive and a common negative wiring point.

This will allow all of the individual batteries to be charged and dis-charged the same. If your battery bank goes out of balance, meaning that there is a
large difference in voltage and state-of-charge of each battery,
the working capacity and life-span of the battery bank can be reduced significantly with parallel wiring versus series.

To start with one area of your home and build out an entire home system later, it is important to factor how homes are currently wired (US homes considered here). They generally use 15-20 amp wire and although you have a 220 volt connections to your circuit box, it is broken up into two 110 volt sets of circuits. Each circuit runs a different area of your home, and certain areas are probably running a single outlet, such as the area in your kitchen designed for a refrigerator.

Typical refrigerators are rated at 1500 watts on a 110 volt circuit. The equates to 15 amps (watts divided by volts equals amps). You may have noticed that most devices you buy and use are rated at a maximum of 1500 watts. This includes hot plates, small space heaters, etc. They are designed to rate this way to accommodate the 15 amp wiring of most homes. The old code in most places used to be 20 amps, but for the last 20+ years, 15 amp wiring is now the norm.

Batteries are not rated for 20 amps but instead for 20 hours which is called the "C20" discharge rate.  This means that an 80 AHr rated battery is supposed to be able to deliver 80/20 = 4 amps for a period of 20 hours.  If that current draw is exceeded, then the battery life will be seriously shortened and pulled down from say, 1000 charge/discharge cycles to maybe 100 cycles or less. - Patrick Kelly

One of the problems I had when I built my commercial music production studio was that when connect audio equipment to circuits that are on opposite sides of the 220 volt circuit board, the alternating current creates a noise in the lines that comes through in the recordings. Most music processing equipment uses very little energy, so the solution was simple, just run everything on one circuit.

Currently, my work station, which includes a music studio set up, runs at just 120 watts when maxed out. This is a little more than 1 amp on a 20 amp circuit (my home uses 20 amp wiring), so my studio/office uses little energy and touches just 5% of capacity.

I had purchased a small device that can measure Kwh (Kilowatt Hours) usage per device - you plug it in and then plug devices into it to see how much energy each device is using. I learned things like a hot plate, which is rated at 1500 watts only used 943 watts an hour (0.943 Kwh) and not the full 1.5 Kwh as it is rated. So, when I got around to measuring my office/studio equipment, here's what I found.

• Power Strip (6 watts)
• Mixer (5 watts)
• Computer (12-28 watts)
• 20" LCD Monitor (16 watts)
• Two 5" Studio Speakers (20 watts at 3/4 volume)
• Fan (30 watts)
• Florescent Lamp (15 watts)

That's about 120 watts at maximum, or about 75 watts when I am not using the fan and lamp (most of the time). Just one amp of power would cover this, so if I had a 12-hour battery back up system, it would have to supply 12 amps in those 12 hours, or 12 amp hours (AH). For a battery back up system, I would need at least 12 AH, or with Patrick's correction, around 50AH.

Most deep cycle batteries are rated in AH based on 20 amps of use - the current standard of measurement for batteries, but there are issues here. Batteries backup systems are not 100% efficient - some energy is lost from heat and chemical reactions when charging, discharging and inverting the power - DC to AC. Depending on the charger used, the loss can be from 5-50%. If you use 1000 watts from a battery, it might take 1050 or 1250 watts or more to fully recharge it. Also, to get AC power from a battery, which is DC (Direct Current), and inverter is needed, and these are very inefficient - the quality of inverters varies greatly.

A simple single-battery back up system is a good place to start and would consist of one deep cycle battery, an inverter and a charger. The system I am looking to test is 1500 watts (15 amps - the equivalent of one circuit), which will cost about $423 - you could do it for less, but I want a descent inverter and charger. The battery is rated at 125AH, so to be conservative, I am now estimating that I will get 15 true AH (25%+ loss).

Note: I have purchased battery back up systems in the past for $300-$400. They were generally good for about 5-6 hours with a similar amount of usage (80-120 Kwh) and usually dropped to half that within 12-18 months. 5-6 hours represents about a 95% loss rate on the system I have designed - I don't expect this level of poor performance, but we'll see!