Deep Cycle Battery

We use batteries on a daily basis. So, what do we mean when we speak about deep cycle battery? (also sometimes called deep charge battery) . In what are they different from other batteries? In particular, it is very important to use car batteries for electricity storage. Deep cycle batteries as often used in off grid power systems can be discharged and recharged many time, not like car batteries.

What is a battery?

A battery in a nut shell is a device that stores electrical energy. It can generate electrical current by utilizing a chemical reaction. The battery can generate so much current for so much time (measured in Ah – Amperes times Hours) until the chemical reaction can’t be sustained any more. Generating electricity by chemical generation by Alesandro Volta preceded the modern generators that convert mechanical energy to electricity by nearly 100 years

Re-Chargeable batteries

There are “one time batteries”, usually for small appliances that you use and throw away (to a battery disposal location). There are batteries you can charge and discharge many times. The rechargeable batteries mostly used are inside your mobile phone (a small light weight battery) and the one in your car (a bulky heavy weight one)

Do not use car batteries for off-grid solar applications

The one in your car is a lead-acid battery (lead plates dipped in diluted sulfuric acid). Amazingly, after 100 years, lead-acid is still very popular, of course there was a tremendous evolutionary progress to improve and diversify the technology. Lead-Acid batteries diversified according to the different usage areas. One point to stress you can not use the car batteries for your offgrid solar system. The way a car battery is constructed, it is supposed to stay almost fully charged and provide short bursts of high current for ignition and a low current for lighting. If the car battery (also known as starting battery) will go through deep charge/discharge cycles, they will fall apart after 30-60 cycles. They can however start the car engine thousands of cycles.

Deep Cycle Batteries

For your off-grid solar system, you need a deep cycle battery that can be fully recharged during day time by the energy of the solar panels and deeply discharge after sunset(even after a deep discharge don't let the battery to discharge to less than 20% of its full capacity) . To get any descent life span, considering there are 365 days in given year, you need the deep cycle battery to go through thousands of charge/discharge cycles. Deep cycle batteries can do just that, charge/discharge thousands cycles. These batteries are lead-acid, the lead plates are solid and thick.

There is no need to go any deeper to the battery structure aspects (after all you want to use batteries, not to build batteries). I want however to highlight the important functional characteristics for the deep cycle battery (what is called by engineers the “specifications”).

Deep Charge Batteries Specifications

Here is a list of the important specifications
  • Voltage rating. The voltage rating must fit the battery charger and the offgrid inverter rating.
  • Capacity in Ah (Amper hours)
  • Efficiency. You want the highest possible, any energy that is converted to heat inside the battery is a waste and you need larger solar panels to compensate for battery’s inefficiency. Its better to pay a bit more for good batteries rather than paying much more for larger size solar panels
  • Life span. 4-8 years is typical, premium grade will double this to 8-15 years, compare prices and calculate the annual cost. (take into account the logistics involved in replacing a farm of batteries)
  • Maintenance free, in other word sealed batteries (you don’t want to add distilled water to so many batteries)

Practical Advices

Some practical advices:

  • Keep the batteries in a shaded relatively cool place (hot temperatures are not good for batteries health and capacity drops in extreme hot and cold temperatures)
  • Keep the batteries in a naturally ventilated area or use ventilators
  • Don’t keep batteries where people live
  • Use a good charge controller from a manufacturer with a good reputation. The charge controller is the batteries insurance policy, preventing overcharge to guarantee long life expectancy
  • Try to buy the entire off-grid solar system from a one stop shop store
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