Batteries
Solar batteries store DC energy generated from your solar system for later use when you need it. The use of deep cycle batteries is more common in off-grid solar systems, but they can also be used in some grid connected solar power systems that require battery backup.
Batteries used in solar systems are specially designed with deep-cycle cells which are much less susceptible to degradation due to cycling. Deep cycle batteries are best for applications where the batteries are regularly discharged.
The three types of batteries that are most commonly used in solar electric systems:
All batteries capacities are shown in amp hours (Ah) at the 20 hour rate. Battery capacity is calculated as Watt Hours = Volts * Amp Hours.
For AGM and Lead Acid: longest life discharge battery 50% or less of capacity before recharging.
For Li: recommended discharge battery 80% or less of capacity before recharging.
Batteries used in solar systems are specially designed with deep-cycle cells which are much less susceptible to degradation due to cycling. Deep cycle batteries are best for applications where the batteries are regularly discharged.
The three types of batteries that are most commonly used in solar electric systems:
- Flooded Lead Acid
- Absorbed Glass Mat Sealed Lead Acid (AGM)
- Lithium (Li)
All batteries capacities are shown in amp hours (Ah) at the 20 hour rate. Battery capacity is calculated as Watt Hours = Volts * Amp Hours.
For AGM and Lead Acid: longest life discharge battery 50% or less of capacity before recharging.
For Li: recommended discharge battery 80% or less of capacity before recharging.
Battery Bank Sizing for an Off-Grid System
The sun does not shine at night and many days will be overcast with little sun. During those times the batteries need to make up for the shortfall from our solar panels and provide the energy needed. Standard off-grid sizing calls for 3 days of autonomy, meaning if no energy is coming in from solar panels or other sources, and at the end of those 3 days the batteries will be down to 50% State-Of-Charge (SOC).
Standard sizing is a reasonable balance between (expensive) batteries and the frequency that we need to make up for shortfalls with a generator. There will still be times in winter when a battery bank sized like this will fall short, so a generator is generally still needed to bridge those gaps.
Next we need to figure out what size our battery bank needs to be. As an example, with an energy need of 2.9 kWh per day. To get to the total energy needed in battery storage we need to multiple that by the number of days (of energy storage) and divide by one minus the battery SOC we are willing to go down to (keeping in mind that standard sizing calls for 3 days and an SOC of 50% for flooded lead acid/AMG or SOC of 80% for Lithium):
Battery bank size (kWh) = Daily energy use (kWh) x Number of days of autonomy / (1 – SOC)
Battery bank size = 2.9 x 3 / (1 – 0.5) = 17.4 kWh
This number, 17.4 kWh, is the amount of energy our battery bank needs to hold in total, when fully charged. For batteries it is more convenient to work with Amp-hours. Amp-hours (Ah) is relative to the battery bank Voltage.
For this case we assume a 48 Volt battery bank:
Amp-hours = 1000 x Energy storage (kWh) / Battery Voltage (Volt)
Amp-hours = 1000 x 17.4 / 48 = 362.5 Ah at 48 Volt
When we connect single batteries in series (positive to negative) we increase the Voltage of the battery bank, while the Amp-hours stays the same. Likewise, when we connect batteries in parallel the Voltage stays the same, while the Amp-hours doubles.
For example, we could build a battery bank out of (8) Surrette S-480 flooded batteries, each 6 Volt 390 Ah. Connecting them in series makes for a total battery bank of 390 Ah at 48V.
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