The Battery System for the Experimental House 0

A battery bank of 2V cells

A battery bank of 2V cells

An off-grid renewable power system needs a way to store power for overnight and for calm and cloudy days.  This is done with a large battery bank, designed to store power for several days. 

The most common type of battery used for residential applications is the flooded lead-acid deep-cycle battery.  The battery consists of a positive plate of lead dioxide, which is the active material, and a high surface area lead negative plate. They are immersed in an electrolyte of sulphuric acid solution. Batteries designed for solar power systems have thicker plates than automotive batteries so that they can be operated at a deeper Depth Of Discharge (DOD), typically 50%.

Battery capacity is rated in “AH” – Amp hours. One amp hour is the ability to run something at one amp for one hour. If you multiply the AH by the system voltage, this will give you the kWh capacity of the battery. The capacity is not the same for all conditions, depending on the temperature, the discharge rate and the end voltage to which it is discharged. The batteries are usually rated at the 20 hour rate and the 100 hour rate. The slower discharge rate yields a higher capacity. In solar applications, the battery discharge rate is usually quite slow, so the 100 hour rate more closely approximates the actual capacity of the battery.

Batteries should be fully charged fairly frequently and should not be discharged for extended periods or a condition called sulphation will occur and a higher voltage equalization charge will have to be applied to the battery to reverse the sulphation of the plates.

Banks of batteries are generally sized to provide for the household power needs for approximately three days with no charging, if the charging system is purely solar, or for five days if the system is primarily a wind charging system.

“Make hay while the sun shines” really applies here. If you use the power directly while the sun is shining, the power is not stored in the battery first so you eliminate battery losses.

Large 2V cells

Large 2V cells

Most residential renewable power systems use a 24 volt battery bank and charging system because efficiencies are better than for the customary 12 volt systems, and because larger inverters are available at this system voltage. Other options include 36 volt and 48 volt system. The 24 volt system was chosen because it is the most common and therefore has the most complete selection of compatible components and accessories. Since large capacity deep cycle batteries are generally available in 2 volt cells, this system will require a bank of twelve cells.

We chose the KS-21 battery cells because they give us a reserve capacity (Runtime) of 3.4 days if there is no wind or sunshine.

The Charge Controller

Charge controllers are essential to protect the batteries from overcharging. They block reverse current that would discharge the batteries into the solar panels at night, but their main function is to prevent battery overcharge. If more charge is applied to a battery that is already fully charged it will separate the hydrogen and oxygen and “boil” off the gas. This can degrade the battery, cause overheating, and can stress the loads.

Some controllers regulate the charge to the battery by simply switching the current totally on or totally off – ON/OFF Control. Others reduce the flow of current gradually. This is called pulse width modulation (PWM) and holds the voltage more constant. A more sophisticated method that has been introduced in the last few years is Maximum Power Point Tracking (MPPT). This device “tracks” the maximum power point of the panel.

A renewable energy system must include a charge controller that is rated for about 1.25 times the charging current that is specified by the manufacturers of the solar panel array and the wind generator.  Specifications are based on test conditions at a particular altitude and temperature so actual current delivered by the units can be higher depending on the local climate conditions and the amount of diffuse radiation. Therefore the charge controller must be able to handle more than the rated current.

The Whisper H80 wind generator includes a charge controller for the wind generator and a PV array.  It is designed with a resistive dump load and a metering section that provides information on the charging, battery and DC load currents.  Although more sophisticated MPPT controllers are available for the solar array,  it was decided to use only the controller supplied with the wind generator for this experiment.

 

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