Dummies Guide – Understand Deep Cycle Batteries

To the average non technical person, understanding how deep cycle batteries store energy is mostly a mystery and you just accept that they “do”.  Much of the information on the subject can be quite intimidating and complex requiring the reader to have an understanding of chemistry and electricity.  What follows here, is my humble attempt to provide a simple, yet accurate understanding of how lead acid deep cycle batteries operate.  These are by far, the most common storage batteries used in off grid power systems.

Batteries and Cells

Typically a battery is made up of several cells wired together so that the voltage of each cell is added to the others.  Normal lead acid battery construction provides 3 or 6 cells in a battery that provides 6V or 12V respectively.   OK, that’s how the cells are arranged, what makes one of these 2V cells store electricity?

Cell Construction

Each cell is made up of 3 general components.  They are the lead plates, a porous separator between the plates, and the liquid electrolyte that fills the cell.  The normal voltage for the cell is determined by the materials selected for plates and electrolyte.  For the lead acid cell construction this is about 2.15V for each charged cell.

Plates

The lead plates have have a surface area and a lead thickness deposited on the plates.  Lead is the active component that is normally deposited on the surface of and inert plate.

The greater the over all surface area created by the plates within a cell, the greater the discharge current possible from the cell.  The surface area can be increased with a more porous  or rough surfaced lead, or by paralleling more plates together in the cell.

The thickness of the lead on the plates helps determine how long the cell can deliver a steady current.  That’s why we like thicker lead on the plates for deep cycle batteries.  Thin lead, and high surface area are normal for car batteries that are not discharged deeply but need high current for a short time (starting).

Electrolyte

The electrolyte in the lead acid battery is mostly diluted sulfuric acid.  It is very dangerous and must be treated with extreme care.

The Chemical Reaction

So “how” does the lead acid battery store electricity?  In layman’s terms, the battery has the strongest acid in the electrolyte when the battery is charged.  The process of drawing current out of the battery, simply weakens the strength of the electrolyte acid, until the discharged battery electrolyte is mostly water!  Charging up the battery again, reverses the chemical reaction and makes the strong acid electrolyte again in the charged battery

 

 

Lead Acid Cell Construction (courtesy: Eurobat)

 

 

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With this understanding, it is easy to see why keeping discharged batteries outside may lead to damage as they freeze and expand 10% like ice!  Always keep unused batteries charged.  Discharged batteries will freeze closer to -32F compared to charged ones that are good below 0F.

Well hopefully you’ve found this “idot’s guide” helpful in understanding a little better how these work horses of the off grid power system work.  If you are interested in more details, there is a ton available from many Internet sources.

Did you find this helpful in your understanding?  Do you have questions or clarifications to add?  Tell us below in the comments!

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