What are the best batteries for Stand Alone Power Systems?
by Tony Clark
(Perth Western Aistralia)
Flooded Lead - Acid batteries have been around for ever. They have a life span of about 10 years in a Solar Power System. Over discharging of these batteries will significantly reduce battery life.
Sulphation of plates occurs if battery is left for a long time at less than fully charged state
Gel (Lead Acid) batteries are somewhat newer technology, they have lower maintenance costs, still suffer from the same problems. In addition, they do not like being over charged, as they cannot be topped up for any electrolyte lost during the gassing phase of charging.
AGM Lead Acid batteries are lighter, cheaper and even less suitable than GEL or flooded Lead - Acid batteries with even less electrolyte present in the cells. Any loss of electrolyte due to overcharging has a significant impact on battery capacity and life.
Lead - Acid batteries of any variety are usually made up in banks (each cell having the same capacity ). To increase the battery capacity, additional banks can be added in parallel with the first bank. Cells are connected in series first, then in parallel second.
Flooded Nickel Iron batteries use caustic electrolytes.
- High cost
- Suitable for deep cycling
- High self discharge rate
- Long cycle life
- Tolerant of overcharge, excessive discharge, short circuit.
Flooded Nickel Cadmium batteries use caustic electrolytes
- High cost
- Long cycle life
- Low cell voltage - need more for given battery voltage)
- Toxic metal (Cadmium) and caustic electrolyte mean that end of life costs are high.
Lithium Iron Phosphate batteries (LiFePO4). These are new technology which also means that they are much more expensive than Lead - Acid batteries of the same capacity.
They do have significant advantages:
- 3v per cell, so you need fewer cells in your battery,
- Much more stable voltage during charge / discharge.
- Batteries can deliver over 80% of rated capacity without reducing battery life.
- Greater number of charge/discharge cycles (2000 - 7000 cycles)
- no toxic heavy metals, no corrosive electrolyte.
- Inherently safer than Lead-Acid - do not produce Hydrogen gas during charging,
- Half the weight of Lead-Acid battery of same capacity.
- Extremely low self-discharge rate.
- Can be used in extremely high temperature applications (up to 60°C)
When the battery life and usable capacity is taken into consideration, the lifetime cost difference becomes significantly less.
They do have disadvantages:
- Overcharging them causes loss of battery life.
- Over Discharging can cause failure of cells in the battery.
- Individual cell voltages should be monitored to ensure they never exceed their rated maximum voltage, or be discharged below their rated minimum voltage. At the upper extreme, the charging voltage must be limited or the battery disconnected from the charger. At the lower extreme, the battery must be disconnected.
The cost of LiFePO4 batteries is falling due to increased use in electric vehicles.
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