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Issue #216 July 2008
Intelligent Energy Solutions
Living And Working Off The Grid
Part 1: Planning And Design
by George Martin
Start | Power Usage | Estimating Solar Radiation | Equipment | Solar Panels | Inverters | Charge Controller | Battery | Miscellaneous Items | System Cost | Sources & PDF
BATTERY
The battery is where power is stored for overnight operation and cloudy days. The best cost/performance battery for off-grid applications is still the lead-acid type. However, these batteries are not your typical car or golf cart battery. The chemistry is the same but the construction is significantly different. Each cell is removable and replaceable. In our battery, each cell measures 6² × 6² × 21² and holds 1.7 gallons of electrolyte. It certainly isn’t like your car battery; it’s more like a submarine battery.
The capacity of the battery needs to be sufficient to run the workshop for three days. And as a matter of efficiency, the larger the battery voltage, the less current flowing in and out of the battery for a given load. We ended up selecting the SO-6-85-17/48 battery from Solar-One. This is a 48-V battery with 845 Ah (20 h) and 32,448 usable watt-hours (20 h). It weighs 2,968 lb and has a 9,600-A short-circuit rating. Sunlight might give you a sunburn, perhaps even skin cancer in the long run, but this battery could kill you in a heartbeat. I cannot stress the importance of following all of the safety precautions when you’re around one of these creatures.
The dealer recommended that I get a larger battery, but I held back based on price alone. (Don’t tell my wife.) The cost of the battery was $7,000. Time will tell if that was a wise decision.
Let me add another factor I’m still trying to get a handle on: battery efficiency. This is the amount of power you put into a battery compared to the amount of power you get out of it. Normal lead acid batteries are charged up to the “bulk,” or “absorption,” levels (same meaning just different names). This is a slight overcharge that is good for battery life but wastes energy. So, I’m not sure how much of the energy produced by the panels will be available to be returned from the battery. This will be measured after the system is up and running.
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