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Solar 48v Battery Information

When used in a properly conditioned environment, batteries can store large amounts of energy that can be used when needed and last 6-8 years or longer. One application that that is rapidly growing is the demand for 48 volt DC air conditioners known as solar or DC mini split systems. When connected to a 48 volt battery bank, and a properly sized photovoltaic solar system, these solar mini-splits can run all day and/or all night without using any energy from the grid. These solar AC systems are designed for locations where utility power is unavailable or unreliable, of where people have decided to go off-grid for ecological or other reasons.
Solar Battery Wiring Diagrams

Deep Cycle Only

Deep cycle batteries can be charged and discharged repeatedly without damage to the battery when the manufacturer’s guidelines are followed. For this reason, only deep cycle batteries are recommended for solar and solar air conditioning applications. Note, marine batteries are often not “true” deep cycle batteries, so make sure to check the specifications of any battery you are considering.

The construction of a battery storage compartment or housing is critical because of the weight of the plates and insulators used to generate the proper voltage and current. All batteries consist of individual cells that generate voltage using chemical compounds to create an electrical charge. To use this charge, a conductive path is created by the electrodes and grids that are bonded internally.   The battery cells are made up of insulators and plates that have both negative and positive polarities. The polarities are separated by an insulating material that varies by manufacturer. The electrolyte (lead acid) consists of free electrons that are liberated through a chemical reaction and are absorbed in the separator material. 

What type of deep cycle battery is best for a solar or DC air conditioner?

Lead Acid Batteries: Lead acid batteries contain electrodes and grids made from lead plates. Polarity is determined by the lead oxide material that coats the grids. These are the most common and lowest cost type and are also called flooded batteries.

Flooded Batteries: This battery is the most commonly produced and purchased type and therefore it is lowest cost solution on the market, this is what we normally recommend for solar air conditioners. Outside of solar, deep cycle flooded batteries are mainly used for forklifts, golf carts, floor buffer machines, etc. and are fine for most solar applications. They do require distilled water during quarterly maintenance periods.  

Sealed Batteries: This type of battery is sealed and does not require periodic topping off of distilled water. The manufacturer fills the battery with a sufficient amount of water and acid to last through the stated warranty.

VRLA Batteries: Valve Regulated Lead Acid batteries are sealed. This is also a sealed battery that does not need maintenance. One advantage VRLA batteries have is they vent less gas than a standard lead acid battery. They are considered AGM or GEL type batteries because of the regulating valve used to vent the hydrogen gas produced when charging the battery.

AGM Batteries:  Absorbed Glass Matt batteries are ideal when the battery storage area is poorly ventilated. Other advantages are the electrolyte is absorbed in a mat which means that what is inside cannot spill out like the acid in a flooded battery. They are vented with a pressure relief valve during charging, very similar to a VRLA battery. This type of battery is fine for off grid applications and remote telecom power plants although they are more costly than standard batteries.

GEL or Gel Cell Batteries:  Because gel cell batteries are sealed like an AGM  battery, maintenance is at a minimum. Silica and Sulfuric acid are combined to turn the substance into a jelly like form. This jelly is the electrolyte and must not be exposed to high amperage situations such as fast charging or discharging. Doing so may scar the jelly and lead to battery failure. These cells are very sensitive to over charging.

General Battery Info:
Always read the battery manufacturer's instructions concerning charging the battery. Always locate the batteries in a well ventilated and approved battery cabinet prior to connecting or charging the battery bank. Hazardous and explosive gases may be generated during the charging of the battery and can be dangerous. Please dispose of unusable batteries according to you local disposal codes.

Recommended Batteries For Solar Air Conditioning http://www.trojanbattery.com/Products/GolfCart.aspx
In particular, the models T-105 6v golf cart batteries wired in series to produce 48v are highly recommended.

Solar Battery Sizing Information:
To get the longest life from a deep cycle battery the idea is to limit the depth of the discharge. Discharging a battery all the way down will damage the battery slightly each time, lower the future performance, and shorten the life of the battery. For this reason it is recommended to oversize the battery plant such that the batteries don't discharge too deeply. For example, golf cart batteries that are discharged about 50% can last 6-8 years whereas the same battery discharged at 80% may last only 2-3 years. We suggest to never discharge a battery more than 60%. In general a 50% discharge is an ideal number, meaning that the AH of the battery system should be such that it is 200% of the expected requirement.

See 48v battery wiring diagrams

See DC Solar Air Conditioner

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