Storing Solar Energy with Rechargeable Batteries

Without a means of storing solar energy, you have to run your electric devices while the sun is shining.  That’s not much help if you want to turn on lights or do anything involving electricity after sunset.   In order to have solar energy available around the clock you need a means of storing it.  Rechargeable batteries are the solution if you are trying to go off-grid.

Storing the Sun’s Energy

Case in point:  Solar-Breeze is a solar powered robotic pool cleaner.  It motors around the surface of your pool removing the debris before it has a chance to sink and decompose.  This significantly reduces the need to run your electric pool pump and filtration system, thereby reducing your home’s carbon footprint.

Since most storms happen in the mid to late afternoon, debris is being dumped into your pool just as the sun is diving below the horizon.  The hours of operation of this pool robot are extended with a rechargeable battery.  The unit runs on straight solar during the day while recharging a Lithium-Ion battery.  When the sun isn’t out, Solar-Breeze switches to battery power.

A little about how batteries work:

Batteries are in so many of the appliances and electronics we use that we pretty much take them for granted, but do we remember how they work?  Time for a refresher.

Essential components of a battery are:

–          Anode

–          Cathode

–          Electrolyte

When a circuit is created by connecting the anode and cathode terminals, the Anode has an oxidation reaction with the Electrolyte.  Ions from the Electrolyte combine with the Anode and the reaction causes the Anode to release Electrons.  The Cathode combines with the Electrolyte in the opposite way and absorbs Electrons.  This flow of Electrons from the Anode to the Cathode is Electricity.  A battery turns Chemical Energy into Electrical Energy providing mobile power for your device.

Non-rechargeable batteries are called Primary Cells.  In your everyday alkaline battery, the anode is a zinc powder, the cathode is a manganese dioxide mixture, and the electrolyte is potassium hydroxide, an alkali (hence the name, alkaline).  When the chemical reaction between the anode and the electrolyte exhausts all available ions, the battery ceases to produce electricity.  Your flashlight dims and goes out.  The battery is dead.  Not only is it waste, it is hazardous waste because of the heavy metals and acids involved.

A Rechargeable Battery, or Secondary Cell, functions in the same way as a Primary Cell.  The difference is that the chemical reactions also happen in reverse during the recharge cycle.  The rechargeable battery that most people are familiar with is a Lead-acid car battery.  It starts your car and runs your radio and A/C.  The operation of the car (specifically through the alternator) recharges the battery for the next time you need to start the engine.

A car battery is a pretty heavy object and only portable in a car.  Lighter and more portable rechargeable batteries include:  Nickel-cadmium (NiCad),  Nickel-metal-Hydrate (NiMH), and Lithium-ion (LIB).  The first suffers from the “Memory Effect”.  If the battery is not fully charged and then fully discharged every cycle, its capacity to hold a charge diminishes over time.  NiMH suffers less from the Memory Effect but requires a more stable source of energy than the sun can provide on cloudy days.  It isn’t as good a match with solar energy.

Lithium-Ion batteries turned out to be the perfect match for a solar pool cleaner.  They recharge well from a less stable source of energy (solar) and don’t need to be fully discharged in order to recharge fully.  This is important because some battery energy is required to operate the robot’s running lights at night.  The firmware shuts the paddlewheels down when the battery charge reaches 7.2 volts so the battery doesn’t fully discharge.

Not all Lithium-Ion batteries have the same composition, power output, or cycle life.  The battery in the Solar-Breeze has a life of at least 1,000 charge cycles.  We recommend not subjecting it to freezing temperatures and the firmware instructs the battery to not recharge when temperatures reach 130 F.

A pair of solar panels and a rechargeable Lithium-Ion battery work hand in hand to power this cool robotic pool cleaner.  Neither could do it alone. This moves the swimming pool one huge step towards being off the grid.

Solar-Breeze