Batteries store excess energy created by Solar PV system to be used at time when there is no Sun energy. A battery is much like a piggy bank. If you take out money but do not replenish it, you will have an empty piggy bank in no time. The battery's capacity for holding energy is rated in amp-hours: 1 amp delivered for 1 hour = One Ampere-Hour. Battery capacity is listed in amp hours at a given voltage, e.g. 220 amp-hours at 6 volts. Manufacturer's typically rate storage batteries at a 20-hour rate. For example, 220 amp-hour battery will deliver 11 amps for 20 hrs. This rating is designed to compare different batteries to the same standard and is not to be taken as a performance guarantee.
Batteries are electro-chemical devices sensitive to climate, charge/discharge cycle history, temperature, location and usage patterns. Batteries can discharge rapidly and yield more current than the charging source can produce by itself. For every 1.0 amp-hour you use from your battery, you will need to pump about 1.25 amp-hours back in to return the battery to the same charge state as before.
Lead Acid Batteries
Different chemicals can be combined to make batteries. Some combinations are low cost but low power also, others can store huge power at huge prices. Lead-acid batteries offer the best balance of capacity per invested money and it's a common battery used in stand-alone power systems.
The lead-acid battery cell consists of positive and negative lead plates of different composition suspended in a sulfuric acid solution called electrolyte. When cells discharge, sulfur molecules from the electrolyte bond with the lead plates and releases electrons. When the cell recharges, excess electrons go back to the electrolyte. A battery develops voltage from this chemical reaction. Electricity is the flow of electrons.
In a typical lead-acid battery, the voltage is approximately 2 volts per cell regardless of cell size. Electricity flows from the battery as soon as there is a circuit between the positive and negative terminals. This happens when any load (appliance) that needs electricity is connected to the battery.
Batteries are rated according to their cycles. Batteries can have shallow cycles between 10% to 15% of the battery's total capacity, or deep cycles up to 50% to 80%. Shallow-cycle batteries, as those for starting a car, are designed to deliver several hundred amperes for a few seconds, then the alternator takes over and the battery is quickly recharged. Deep-cycle batteries or the other hand, deliver a few amperes for hundreds of hours between charges.
Deep-cycle batteries are capable of many repeated deep cycles and are best suited for PV power systems. Deep-cycle batteries withstand having a majority of their capacity used before being recharged and survive hundreds and even thousands of 80% cycles. It is recommended to use 50% as the normal maximum discharge and leave 30% for emergencies. The less deeply you cycle your battery, they longer it will last.
watt = amps x volt
watt-Hour= One watt delivered for one hour (a measure of energy consumption)
battery's amp-hour X Voltage = watt-hours
Appliance's Watt-hours divided by battery Voltage = Amp-hours
Example: A 60-watt light bulb for one hour uses 60 watt-hours. If a 12-volt battery is running the light it will consume 5 amp-hours (60 watt-hours divided by 12 volts equals 5 amp-hours)
Battery Bank Sizing worksheet
|1||Enter your daily amp-hour requirement
(Use our Load Sizing Worksheet)
|2||Enter the maximum number of consecutive cloudy weather days expected in your area, or the number of days of autonomy you would like your system to support
(Note: Ideally, a battery bank should be sized to be able to store power for 5 days of autonomy during cloudy weather. If the battery bank is smaller than 3 day capacity, it is going to cycle deeply on a regular basis and the battery will have a shorter life. System size, individual needs and expectations will determine the best battery size for your system.)
|3||Multiply the amp-hour requirement by the number of days. This is the amount of amp-hours your system will need to store.||AH ____________|
|4||Enter the depth of discharge for the battery you have chosen. This provides a safety factor so that you can avoid over-draining your battery bank.
(Example: If the discharge limit is 20%, use 0.2.) This number should not exceed 0.8.
|5||Divide line 3 by line 4||AH ____________|
|6||Select the multiplier below that corresponds to the average winter time ambient temperature your battery bank will experience.
Note: Batteries are sensitive to temperature extremes, and you cannot take as much energy out of a cold battery as a warm one. Use the chart on the Battery-Sizing Worksheet to correct for temperature effects. Although you can get more than rated capacity from a hot battery, operation at hot temperatures will shorten battery life. Try to keep your batteries near room temperature. Charge controllers can be purchased with a temperature compensation option to optimize the charging cycle at various temperatures and lengthen your battery life.
Ambient Temperature Multiplier
80ºF > 26.7ºC > 1.00
70ºF > 21.2ºC > 1.04
60ºF > 15.6ºC > 1.11
50ºF > 10.0ºC > 1.19
40ºF > 04.4ºC > 1.30
30ºF > -1.1ºC > 1.40
20ºF > -6.7ºC > 1.59
|7||Multiply line 5 by line 6. This calculation ensures that your battery bank will have enough capacity to overcome cold weather effects. This number represents the total battery capacity you will need.||AH ____________|
|8||Enter the amp-hour rating for the battery you have chosen
(use the 20 or 24 hour rate from the battery manufacturer)
|9||Divide the total battery capacity by the battery amp-hour rating and round off to the next highest number. This is the number of batteries wired in parallel required.|
|10||Divide the nominal system voltage (12V, 24V or 48V) by the battery voltage and round off to the next highest number. This is the number of batteries wired in series.|
|11||Multiply line 9 by line 10. This is the total number of batteries required.|
- Batteries require periodic maintenance.
- Sealed battery should be checked to make sure connections are tight and there is no indication of overcharging. For flooded batteries, the electrolyte level should be maintained well above the plates and the voltage and specific gravity of the cells should be checked for consistent values.
- Wide variations between readings may indicate cell problems.
- The specific gravity of the cells should be checked with a hydrometer particularly before the onset of winter.
- In cold environments, the electrolyte in lead-acid batteries may freeze. The freezing temperature is a function of a battery state of charge. When a battery is completely discharged, the electrolyte becomes water and the battery may freeze.