OFFGRID SOLAR BATTERY STORAGE

Understanding Battery Capacity Ratings

Amp-hour capacity

Every deep cycle battery has a capacity which is measured in amp hours. Amp hours are a measure of current flow over time. An amp-hour figure is derived simply by multiplying current (amperes) by the amount of time the current flows (hours). This applies equally to the amount you take out of a battery (discharging) or the amount you put into it (charging). amp hours are frequently referred to by the abbreviations A-h or Ahr.

Discharge rate

Deep cycle batteries have their amp-hour rating expressed as "at the x-hour rate". This is an average rate of current flow that would take x number of hours to discharge the batteries. Common amp-hour figures are at the 6-hour rate, the 20-hour rate and the 100- hour rate. A battery is classified as having fewer amp-hours if is being discharged at a faster rate, such as the 6-hour rate. There is an inevitable amount of heat associated with the flow of current through a battery. The higher the amount of current, the greater the amount of heat generated. The heat is energy which is no longer available to the battery to power loads. Hence, at a higher discharge rate, the batteries effectively have fewer amp hours available. Generally the 20-hour rate is the most common one. If you are trying to gauge the battery size on a battery that is says it has different amp hours at different hour rates, use the 20-hour rate.

Battery Bank Sizing

Running time and size

The battery bank’s size determines the length of time the inverter can supply AC output power. The larger the bank, the longer the inverter can run and the longer the recharge time. Depth of discharge In general, the battery bank should be designed so the batteries do not discharge more than 50% of their capacity on a regular basis. Discharging up to 80% is acceptable on a limited basis, such as a prolonged utility outage. Totally discharging a battery can reduce its effective life or permanently damage it. Days of autonomy For off-grid, stand-alone applications, design a battery bank that can power the loads for three to five days without requiring recharging. This design calculation assumes a worst case scenario where there is no recharging taking place during these days of autonomy. Days of autonomy may vary depending upon the availability of other charging sources, the critical nature of the load and other factors. If the system is to be powered by renewable energy sources such as solar, wind, and micro-hydro, determine the appropriate number of days of autonomy by allowing for cloudy or calm weather as well as other seasonal variations in available energy. Important: Batteries must have a known amp-hour rating (Ahr or A-h) of some size in order to be appropriate for an inverter system.

Understanding Amp-hour Requirements

Amp hours To estimate the battery bank requirements, you must first calculate the amount of power you will draw from the batteries during your period of autonomy. This power draw is then translated into amp hours (Ah)—the unit of measure to express deep-cycle battery capacity.

Amp hours are calculated multiplying the current drawn by the load by the length of time it will operate. Watts-to-amps To calculate amps when the power consumption is expressed in watts, use the following equation: A = W/V (where W = watts and V = volts DC) For example: A 100 watt light bulb will draw approximately 8.33 amps DC (8.33 Adc = 100 watts/12 Vdc) If the light runs for three hours it will consume (8.33 Adc x 3) or 25 Ah of power. For these calculations, do not use the AC amp rating of a device. AC amps are measured on a different scale and will not give correct results. Time and power The length of time a load is operated will affect the power draw. In some cases, an appliance which draws a large wattage may not consume as many amp hours as a load drawing fewer watts but running for a longer period of time.

For Example: A circular saw draws 1500 watts or 125 amps DC. It takes 5 seconds to complete a cross cut. Twelve such cuts would take a minute and you would consume 125 Adc x 0.016* hour = 2 Ah (*1/60 = 0.016) Observation The circular saw, while it draws more power, consumed fewer amp hours of electricity because it ran for a short period of time.