Deep Cycle Batteries

Deep Cycle vs Car Battery

In contrast to car or starter batteries, deep cycle batteries are designed to be regularly deeply discharged and are therefore ideal for off-grid solar power systems.

Starter batteries deliver short yet high current burst for cranking the engine while discharging only a small part of their capacity. If deep cycle batteries are used as a starting battery, they would need to be quite large.

Deep Cycle Batteries and their capacity

Lead acid batteries vary in capacity according to how quickly they are discharged. You often see a specification like C20, C100 etc. This relates to their capacity in amp hours at the 20 and 100 hour discharge rates.

Australian standards specify that stationary batteries (eg deep cycle solar batteries) should show the 120 hour rate (C120). Traction batteries (for electric fork-lifts, golf buggies etc) are usually specified at the 100 hour rate (C100). Traction batteries are sometimes called 'starved electrolyte' since they do not have a large reservoir of acid. The difference in say their C20 and C100 hour rates is not as large as you would typically find in a stationary battery with a large reservoir of electrolyte.

Parallel Connection of Deep Cycle Batteries

Series and Parallel

eg for 12 Volt 100 amp batteries
2 batteries in Series = 24volt 100 amp/hr
2 batteries in Parallel = 12 volt 200 amp/hr

Smaller capacity batteries are sometimes cheaper (per amp-hour) than larger batteries. I believe this is because the smaller batteries are more mass produced. This cheaper cost sometimes leads customers to place several in parallel to achieve the desired higher capacity rather than purchasing larger capacity batteries.

Ray Prowse, who used to inspect failed systems for the Solar Energy Industries Association of Australia, commented that in every case of system failure, there were batteries in parallel. While this may not be the core reason for the failure, it is an indication of poor practice. Noel from Solar Tasmania subsequently commented, "Our experience with parallel batteries, even when wired correctly, is don't try ... even when they are identical type, age and capacity ...".

The biggest problem with parallel batteries is that they will tend to charge/discharge unevenly, which can lead to sulphation and stratification of the electrolyte.

If you must place batteries in parallel, you should limit this to two parallel strings. Great care needs to be taken to ensure that there are equal current routes through both strings. The positive lead should come off one string and the negative through the other. The cable size must be identical, as should the crimped terminations.

Australian Standard 4509.2 stipulates that "... the number of parallel strings be minimised ...". The standards also stipulate that each parallel string must be fused and earthed (or fused on both positive and negative if the system is floating). So to conclude, we would suggest that you try and avoid connecting batteries in parallel, and to seek the advice of an experienced and qualified solar installer in this regard.

Read more about interconnecting batteries.

Battery Failure

The capacity of a battery indicates the ability of the battery to deliver an electric current at a given rate for a specified time. If the battery appears to have lost capacity it may be because it hasn't been recharged thoroughly, or it may be sulphated or else the battery may be approaching the end of its useful life. A battery will lose active material from the plates due to frequent cycling, movement and vibration. This material will settle in the sediment chamber and thus ceases to play an active part in the battery's function.

This results in a gradual loss of capacity throughout the battery's useful life. A battery may stop functioning very suddenly if an internal short circuit is caused. Such a short circuit may be the result of the sediment in the sediment chamber coming into contact with both positive and negative plates. Otherwise it may be that the plates have come into contact with each other as a result of buckling of the plates or "treeing" between the plates.

"Treeing" is the result of a very slow recharge rate (500 hours or more) when the spongy lead deposits on the negative plates in a "tree" like formation. This may eventually bridge the gap between the positive and negative plates. Another possible cause of failure of a lead-acid battery is when the contacts between plates, straps, terminals and/or inter-cell connectors are broken.

Maintaining Electrolyte Levels in Batteries

It is important to check the electrolyte level in your batteries on a monthly basis (assuming your regulator is working correctly). The electrolyte level gradually becomes lower due to the electrolysis of the water into oxygen and hydrogen. This is a normal and necessary process in charging a lead acid battery.

The lead plates inside the battery must always be kept below the level of the electrolyte, otherwise the battery will be irreparably damaged. Many batteries have some sort of indicators for the minimum and maximum electrolyte levels. Generally this will be 1 to 2 centimetres over the plates. Do not overfill your battery or the electrolyte may splatter out when charging. Be aware that when approaching full charge, the volume of the electrolyte will expand due to little bubbles of gas (hydrogen and oxygen) moving to the surface throughout the electrolyte (some get trapped under and between the plates. Speaking of gas, this is a very explosive mixture if ignited by a spark or flame. Never smoke near a battery or use a match to help you see inside the battery! Turn on any torch some distance away from the battery vents, in case the switching action creates a spark.

You should also be aware that the water, which you add, will tend to initially float on the surface of the heavier electrolyte (sulphuric acid). This will give you a pessimistic reading on your hydrometer. It is suggested that you top up your batteries when they are being charged, as the bubbling action will help stir up and mix the electrolyte.

We strongly recommend that you only use distilled or de-mineralised water, which is sold in automotive and department stores. Do NOT use bottled drinking water, tap water, tank water or water from a creek or river or lake, or collected off a tin roof.

The electrolyte is a strong acid and it is recommended that you wear goggles while you top up the battery. If you do get acid on you, wash it off immediately with large amounts of fresh water for several minutes.

De-mineralised water is usually sold in 4-5 litre plastic containers. Pouring it into the battery from the container can be an awkward task - it is easy to get acid splashing up, or to miss the hole and wash contamination on the outside surface of the battery into the acid reservoir. This can easily contaminate your battery causing severe damage.

Battery Replacement

Extract from

If there are any signs that the battery bank is not performing well, first check whether or not the batteries are fully charged.

The battery probably needs replacement if:

  1. one cell voltage is far below the others
  2. the battery fails to charge
  3. the battery fails to hold its charge or voltage

An investigation by a Rainbow Power Company Battery Technician can determine whether the batteries need replacing. The battery age and usage pattern may give some clue as to the outcome of such an investigation.

Battery Enclosures or Boxes

For reasons of safety, Australian Standards require that batteries be enclosed in a ventilated, lockable box which is also vermin proof.

This is to allow explosive gases to escape, to restrict access to authorised persons, and to keep rodents etc. from getting in and eating the cable insulation or similar. Contact our office for full details.

Buy Battery Enclosures

We sell pre-fabricated battery boxes for our most common battery banks. These boxes are 'flat pack' DIY kits that include pre-cut, pre-drilled components and all necessary hardware. We also build custom designed battery boxes for none standard solar battery banks. Contact us for more information.

Battery Lifespan

Factors which can effect the life of a battery include:

  • the type of battery - eg is it a true deep cycle type for use on solar applications;
  • the quality of the battery, partially expressed as expected cycle life versus "Depth of Discharge" (DoD). See diagram #8.
  • the number and depth of discharges; eg 10 -15% DoD on a good day is good;
  • the length of time spent in discharged states;
  • quality of water added to it- eg use only mineralised or distilled water;
  • ambient temperature; high temperature shortens life span;
  • regulator (charge controller) quality or suitability; eg monthly equalise function to minimise stratification.