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Battery Glossary – Deep Cycle Battery

Some types of secondary batteries (rechargeable) vary in design in order to satisfy different consumer needs such as:

  • Cranking batteries – designed to release large amounts of energy over a short period of seconds and the recharge. Their state of charge is not expected to go below 80% and if it does regularly this may cause internal damage.
  • Standard batteries – designed to release small amounts of energy now and then (remote controls, digital cameras, etc) or gradually (laptops).
  • Deep cycle batteries – designed to release small to medium amounts of energy over a long period of time to a low state of charge before recharging.

“Deep cycle” labelling usually refers to lead acid batteries – both flooded lead acid and sealed lead acid batteries – in order to differentiate them from cranking batteries of the same chemistry. Lithium ion batteries, for example, can deep cycle but they cannot provide high energy bursts needed for cranking applications and so no classifying terminology is needed – Lithium ion batteries are deep cycle only.

In lead acid cranking batteries the internal lead plates are thin in order to expose as much lead to the electrolyte as possible. However this make the plates weaker and more prone to buckle if the battery is discharged excessively on a regular basis. Buckling causes the plates to shed the active material they need to recharge and can cause plates to touch shorting out the battery completely.

Deep cycle lead acid batteries are made with thicker plates to provide greater strength against buckling when at low states of charge. Emergency Lighting batteries, Alarm System batteries and PLC backup batteries are deep cycle units because of the need for a steady flow of energy over a period of time should the normal mains supply be interrupted by events such as a power cut.

Their name derives from a ‘cycle’ (each time a battery is discharged and recharged) – and the depth of that cycle (what State of Charge or Depth of Discharge occurs) . While a cranking battery should not (ideally) discharge below 80% State of Charge the thick plates in deep cycle lead acid batteries are designed to discharge to a ‘deeper’ 50%+ State of Charge.

Although deep cycle lead acid batteries are better at discharging to lower States of Charge this does not make them immune to damage caused by the process. The deeper the cycle, the more damage that is done and the less cycles a unit will achieve in its life time

The numbers of cycles a products can achieve are referred to as a battery’s cycle life (See also Why don’t lead acid batteries last forever?)

The graph below demonstrates the results of testing a sample 6 volt 6 ah sealed lead acid battery. This shows that when the battery is regularly completely discharged to a 100% depth of discharge (D.O.D) – which could also be written as a 0% State of Charge (S.O.C) – it will last approximately 200 cycles. That’s 200 discharges and recharges before it would need replacing. Alternatively if the battery is only ever discharged to 30% D.O.D / 70% S.O.C it lasts for up to 1,100 cycles.

SLA battery Cycle Life versus Depth of Discharge
A sample SLA battery – Cycle Life versus Depth of Discharge shows that heavier discharges shorten the life of even deep cycle batteries.
As such, purchasing a deep cycle battery that appears excessive to your needs in terms of capacity can actually represent substantial savings due to massive increases in cycle life.

See Which deep cycle Ah battery for steps to identify the right deep cycle battery.

Deep cycling other battery chemistries such as lithium ion has no detrimental affect on their cycle lives, this is an attribute of lead acid units only which begs the question, “Why doesn’t everyone just use lithium?”

At the start of the millennium this was due to higher costs of production and lithium products being designed to power relatively small devices such as laptops or mobile phones. However great strides have been made in lithium development and they are now the battery of choice for electric cars. Increasing economies of scale in production may make them a more viable replacement for lead acid deep cycle products in the near future.

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