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Renewable Energy
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RENEWABLE ENERGY SYSTEMS & COMPONENTS
How
to Size a Deep Cycle Battery Bank Deep Cycle Battery bank sizing can be one of the more complex and important calculations in your system design. If the battery bank is oversized, you risk not being able to keep it fully charged; if the battery bank is sized too small, you won't be able to run your intended loads for as long as you'd planned. Many renewable energy (RE) systems incorporate batteries. Deep cycle batteries can be used in all types of systems including:
The energy stored in the deep cycle batteries can then be used directly to power DC loads or it can be inverted to power loads. The batteries recommended for RE systems are deep cycle batteries. To ensure you have enough reserve capacity to provide the electricity you need (without running additional generators), invest the time to size your deep cycle battery bank properly. Because of the various conditions affecting battery bank sizing, this process may be one of the more challenging calculations you'll have to do when planning your RE system. Before tackling the calculations, start by identifying a few key pieces of information:
Electrical UsageThe first thing you'll need to know is the amount of energy you'll be consuming per day. It's worth the time to do a careful evaluation of exactly what loads (appliances, electronics, etc.) you plan to use and for what lengths of time. Keep track of this information on a loads list; you'll refer to this list often for sizing other components as well. Your final tally should be expressed in Watt-hours (Wh) per day. If you know the kilowatt hours (kWh) per day just multiply that number by 1,000 to determine the Watt-hours per day. (Example: 1.2 kWh = 1,200 Wh)
Days of Autonomy
If you conclude that you need more then five days of battery backup, you may want to explore multiple sources of electricity generation or backup generator options (like a fossil-fueled generator). If your primary electricity source is wind power, determine the number of days when there is little or no wind. This information can be found in the data you've collected using your data-logging anemometer. Hydroelectric turbine systems are unique because they usually operate continuously, and therefore do not require extensive storage. If you're sizing a battery bank to be used in conjunction with an on-demand fuel-powered generator, the number of days of backup will represent the number of days you wish to go without using your generator. Depth of Discharge
Battery
Life based on Depth of Discharge
It's recommended that you never discharge a deep cycle battery below 50% of its capacity ; however, many battery manufacturers recommend even shallower DoDs. For off-grid applications, a 25% DoD will extend battery life significantly. On the other hand, if you're only using the batteries occasionally, as a backup system, you can factor in a DoD of 50% or perhaps more. Temperature
System Voltage
Calculations
Return to menuSelecting Deep Cycle Batteries to Meet the Amp-hour capacity
When batteries are cabled together in series, the voltage is additive. For example, you can put two 12V, 100 Ah batteries in series for a 24V bank. The capacity of that bank would still be 100 Ah. When batteries are connected in parallel, the voltage remains constant and the Ah capacity is additive. In our example with the 12V, 100 Ah batteries, connecting them in parallel would result in a 12V system with a capacity of 200 Ah. The deep cycle batteries you select must meet both your system voltage requirements AND the Ah capacity you calculated. In our example of the 48V system, we calculated that we needed 1,040 Ah to produce 6,000 Wh per day with 3 days of storage. More than one configuration of batteries can meet this need. For example, you could have four 12V batteries in series, each with a capacity of 1,040 Ah or more. Or you could use eight 12V batteries wired in two parallel strings where each battery had a 520 Ah capacity. Or you could use twelve 2V batteries in series, again with appropriate Ah capacities. In any given case, there may be multiple solutions. Your choices will be limited by battery availability and budget. Building the bank: Amps, then VoltsTo
build your bank, try first to select a deep cycle battery
that is rated close to the Ah capacity you calculated in
Step 5 above. Ignore voltage for a moment. If you can't
find one that's very close, look for one that has
a capacity either one-half or one-third your needed Ah figure.
These fractions represent the number of series strings of
such batteries you would need, in parallel, to complete
your bank (1/2 = 2 strings, 1/3 = 3 strings). Once you find
a candidate battery, divide your system voltage by the battery's
voltage. This will give you the number of such batteries
you would need in each series string. Total # batteries in bank = (# series strings) X (# batteries per string)
References: 1 A great article explaining the rationale behind this paralleled string limit is available in Home Power Magazine issue 114: "Top Ten Battery Blunders" Components:
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