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Step 5 Choice of the battery
The choice of the battery is crucial. The battery will determine the investment in solar power for 20-30%. But the battery will determine the resulting price for power for 40-50% due to the lower lifetime (zie hier). If the battery is chosen badly, this percentage can be higher (and the end-user’s worries also).
Determination of battery capacity needed
The battery capacity is given in Ah (Ampere-hour). A 12V battery of 50Ah can supply 1A during 50 hours at a voltage of 12V. In kWh, the capacity is 50A x 12V = 0.6 kWh. However: for the lifetime for the battery it is not wise to discharge him completely. Therefore, the basic principle is to utilise only half of the full capacity. In step 2 the minimal autonomy was determined. In the example of 3 days of autonomy with a daily power demand of 2.0 kWh, the battery capacity should be 6.0 x 2 = 12.0 kWh. In the case of a 12V system, this would imply to buy a battery of 1000 Ah; in case of a 24V system, a 500Ah battery is needed.
Choice of the battery type
Lead –sulphur acid batteries still are the best option for solar power systems. Still there is quite some to be chosen in type, quality and price. The table below gives an overview.
▼ Simple subdivision of batteries and appropriateness for solar power systems.
| Battery type | description | ||
|---|---|---|---|
| Wet batteries (batteries with caps, to top-up with distilled water) | Start-batteries | Designed for short and high currents, like starting an engine. Not appropriate for solar power systems | $ |
| Semi-traction batteries | Suitable for partial day-night-cycling like most of the solar power applications. | $$ | |
| Traction batteries | Designed for daily complete cycling, like fork lift trucks. Not for solar power systems. | $$$ | |
| Dry (maintenance free) batteries. Also known as VRLA = Valve Regulated Lead Acid | AGM batteries (Absorbed Glass Mat) | Are commonly used for UPS-systems (Uninterruptible Power Supply). Suited for short time power supply. They can be used for solar power systems. | $$$ |
| Gel batteries | Most expensive, but also most sustainable choice for solar power system. Really suited for partial cycled behaviour (day-night), can be discharged deeply, long lifetime. | $$$$ |
A readable, understandable and interesting technical treatise about batteries is found in chapter 2 of Reinout Vader’s booklet “Energy Unlimited”.
Battery configuration
It is possible, but not desirable, to connect more battery stings in series. The problem that can occur is that the batteries at their end of life will discharge in each other, in stead of supply power to the load. Therefore, our guideline is to connect maximal two strings in series.
▲ Batteries are preferable connected in series