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Storage batteries are the heart of all off-grid wind/PV or inverter electrical systems. Their function is to balance the outgoing electrical requirements with the incoming power supply. They offer a reliable source of electricity which can be used when solar or wind power is not available. Batteries are able to provide short term power output many times higher than the charging source output. In diesel generator systems they allow power to be available 24 hours/day but mean that the generator need only run for short periods to charge the battery.
They are also used for grid connected systems to allow surplus energy to be stored to enable greater 'self-consumption' of generated energy or to provide back-up power.
Lead Acid Batteries:
Lead acid batteries are the most common type of energy storage.
All lead acid batteries are based on the same lead/sulphuric acid chemical reaction, however, they have evolved into different types - each designed for a specific need.
Smaller batteries look like big car batteries and contain several cells in tough plastic cases to give 6V or 12V with capacities up to ~ 200Ah. They are either flooded cell types: with a vent for gassing and topping-up with distilled water; or sealed/ gel types: with an immobilised electrolyte.
Larger battery banks are usually made up of flooded 2V cells ranging in capacity of ~100Ah to several thousand. Sealed types are also available. Some flooded cell batteries using tubular positive plates instead of flat plates have longer life.
Lithium Ion Batteries:
These have several advantages over conventional lead acid batteries:
- Higher energy density: more energy with less size or weight
- Higher charge & discharge currents possible so shorter charging times and bigger loads possible.
- Longer battery life (up to six times)
- Higher efficiency between discharging & charging.
- Higher continuous power available.
- More useable capacity - typically 90% + of the battery capacity is available for use.
This is how much electrical energy the battery can deliver and is measured in ampere hours (Ah) when uniformly discharged over a given period of time. eg. a 120Ah @ 100hr rate battery will be fully discharged in 100 hours by a 1.2 A current.
If discharged at a higher rate (by a higher current) then battery capacity is reduced considerably. The maximum charge/discharge current should be less than 10% of battery Ah capacity.
- Lower temperatures significantly reduce battery capacity (figures are at 25 deg C).
- The older a battery becomes, the lower will be the capacity that can be obtained from it.
The capacity of the battery bank needed depends upon
- amount of storage required
- types of charging source
- maximum charge & discharge rates
- temperature of operation.
The one requiring largest capacity will dictate battery size.
This is measured in number of discharge/charge cycles rather than years. The more deeply the battery is discharged the lower the number of cycles it will last for.
The percentage depth of discharge (DOD) of a battery means how much of the available power in the battery is used before recharge. Although deep cycle batteries can be discharged by 80% of their rated capacity (80% DOD); designing for less than 50% gives much longer battery life.
Overcharging a battery raises the temperature of the electrolyte, causing excessive gassing, loss of distilled water and eventually damage to the plates. Consequently, use of a suitable charge regulator is necessary with any battery charging system to limit charging current as the battery voltage rises.
Excessive discharge of a battery can also lead to permanent damage. If a battery is close to its fully discharged state it should be recharged immediately (eg. by using a generator & charger) - or if that is not possible all loads switched off until the battery can be charged.
A system voltmeter is recommended and low voltage alarms and load disconnection devices are available.
Lithium Ion batteries require very precise regulation normally provided by a Battery Management System (BMS) with communication between the battery and whatever it is fitted to.
Vented lead acid batteries need to be regularly boost charged and allowed to ‘gas’ freely for an hour or so. This equalises individual cell voltages within the battery and helps avoid electrolyte stratification.
No battery is 100% efficient. Energy is lost in storage, charging and discharging. With new cells efficiencies of ~ 90% can be expected, however this decreases with age, sulphation and stratification.
To maximise efficiency, batteries should be kept at room temperature, and sized correctly for their purpose, both to minimise self discharge, and to prevent them being charged and discharged too rapidly.
Sulphation is caused by a battery being left in a discharged state for a period. Stratification is caused by low cycling allowing the battery electrolyte to settle into layers of different densities. It can be prevented by regular equalisation charges.
The battery bank should be installed, preferably on its own, in a weather & frost protected, well ventilated shed or other enclosed area. Ideal temperature is ~ 20 ºC and should not be more than 43 ºC.
- For optimum performance, batteries should all be of the same brand, age and amp-hour capacity within a battery bank.
- Proper battery connections should be used, designed for high currents & long life. Connections should be tight and covered with petroleum jelly to prevent corrosion.
- Batteries produce explosive hydrogen gases during charging, so avoid sparks or flames. They contain corrosive chemicals.
- Utmost care must be taken whilst working with batteries.
Battery State Of Charge
This is how much of the battery capacity is available (how full it is). The most convenient way of determining this is by measuring the battery voltage. Ideally for a battery that has been at rest for three hours (ie. neither being charged or discharged).
A typical system will be usually charging or discharging (often at the same time) so there is a high degree of voltage variation throughout the day. This makes it difficult to rely on justvoltage reading to give an accurate gauge as to state of charge.
State of charge can also be measured using a battery hydrometer, 1.25-1.28 fully charged, 1.14-1.16 discharged. Care must be taken not to introduce contaminants into the battery cells. With tall cells, since the electrolyte can only be sampled above the battery plates, readings can be misleading due to stratification in the electrolyte. Specific gravity varies with temperature, higher with lower temperature, and lower the warmer the battery.
Measurement should only be done with an accurate hydrometer. The normal automotive type is not suitable. A hydrometer cannot be used with sealed or gell-cell batteries.
The best method is use of an Amp hour meter (See Amp-hour meters). These record net usage from a battery, accounting for battery efficiency - acting just like a car fuel gauge.