Size a wind turbine’s battery bank as daily energy use times the days of calm to ride through, then convert to amp-hours by dividing by the bank voltage: 2 kWh a day for 3 days is 6 kWh, which is 500 Ah at 12 V, 250 Ah at 24 V or 125 Ah at 48 V. That is usable energy, so the nominal capacity must be larger, because lead-acid should not be drained to empty; at a 50% usable fraction the same bank becomes 12 kWh.
Part of Advanced Wind Turbine
A battery bank is the stored energy that covers the hours and days when the wind is too weak to run the village’s loads.
Days Of Calm Times Daily Use
The carrier’s rule is to store 2 to 3 days of the community’s consumption, with a worked example: 2 kWh/day × 3 days = 6 kWh; at 12 V, 6,000 Wh ÷ 12 V = 500 Ah. Elsewhere in the same article it says every wind site has multi-day calms and a system needs battery storage for 3 to 5 days, a backup generator or a complementary source. The two figures disagree, and the corpus does not settle it; the safer reading is that 2 to 3 days is the least you should store and 3 to 5 is the target if calms are long at your site. Measure the longest calm in your wind record (Wind Resource Assessment).
| Days of storage | Usable energy at 2 kWh/day | 12 V | 24 V | 48 V |
|---|---|---|---|---|
| 2 | 4 kWh | 333 Ah | 167 Ah | 83 Ah |
| 3 | 6 kWh | 500 Ah | 250 Ah | 125 Ah |
| 5 | 10 kWh | 833 Ah | 417 Ah | 208 Ah |
Each entry is energy in Wh divided by voltage. The amp-hours shrink as the voltage rises, but the energy does not change, and neither does the cost of the cells.
Usable Is Less Than Nominal
The Charge Controller page treats 11.8 V as a depleted 12 V lead-acid battery. Running a bank flat shortens its life. The corpus gives no depth-of-discharge figure for storage design; a commonly quoted rule, which I recall and have not verified, is to use no more than about half of a lead-acid bank’s nominal capacity. At 50%, the 6 kWh usable store needs 12 kWh nominal: 1,000 Ah at 12 V or 250 Ah at 48 V. The carrier’s 500 Ah is the energy to be drawn over the calm, so a bank of exactly that nominal size would be taken to empty by it.
Deep-cycle marine or forklift batteries are preferred; the carrier gives car batteries 200 to 300 cycles against 1,000 or more for deep-cycle. If the bank is cycled deeply once every 3 days, 1,000 cycles last 1,000 × 3 ÷ 365 = 8.2 years, and 200 to 300 cycles last 1.6 to 2.5 years. This assumes one deep cycle per three days; shallower cycling lasts longer.
The Turbine’s Average Output Must Refill The Bank
A 2 kW turbine at a 5 m/s site averages 300-500 W, which is 7.2 to 12 kWh a day (Average Wind Speed & Power Curve shows a 3 m rotor averaging about 338 W). Against a 2 kWh/day load the average supply is several times the load, so the bank refills quickly after a calm if the wind returns. If the load is 8 kWh/day instead, the bank would never recover from a long calm. Check the supply against the load before the bank.
The bank can only absorb charge at a sensible rate. Charging Systems charges a bank at C/10, one tenth of its amp-hour rating per hour. For the 500 Ah bank that is 50 A, and at 14.4 V it is 14.4 × 50 = 720 W. A 2 kW turbine in strong wind makes 2,000 W, so 1,280 W goes to the dump load once the bank can accept no more (Dump Load & Overspeed Protection).
Method: Size And Assemble The Bank
Step 1: list the loads with their watts and hours per day, and add them for the daily energy (Inverter & Load Management). Step 2: choose the days of storage, 3 as a start. Step 3: usable energy = daily energy × days; nominal = usable ÷ the fraction you will allow to be used. Step 4: choose the voltage (24 or 48 V for long runs, per Electrical System) and divide to get amp-hours. Step 5: build the bank from identical batteries: in series to raise the voltage, in parallel to raise the capacity. Do not mix old and new, or different types, in one bank. Step 6: fuse the bank and ventilate it.
Wind banks cover calms; hydro banks cover peaks and shutdowns
Hydro makes steady power, so its bank covers surges and a 2 to 3 day shutdown for maintenance (Battery Charging vs Direct AC). Wind is intermittent and the bank is the only supply during a calm.
Common Mistakes
- Treating nominal capacity as usable — a bank drained to empty is damaged.
- Mixed batteries — the weakest cell sets the whole bank’s limit.
- A bank too small to take the turbine’s current — the controller sheds more power than it should.
Safety
Lead-acid banks: acid, hydrogen and heavy shorts
Charging gives off explosive hydrogen, so ventilate and keep sparks away. A bank of this size can deliver hundreds of amps into a short and melt metal. Fuse the bank, insulate the terminals, wear eye protection, and lift heavy cells with two people.
What’s Next
- Learn next: Inverter & Load Management for the loads on the bank
- Combine with: Lead-Acid Construction