True deep-cycle batteries should be used in all solar systems. These are made for slow, steady, and deep discharge, making them ideal for powering domestic and workshop loads including lights, tools, and appliances. Car batteries and other SLI (starting, lighting, ignition) batteries, on the other hand, produce a massive burst of power to start the engine and are then instantly recharged by the alternator of the running motor. Discharging a beginning battery to 50% or even 80% of its capacity, as you would with a deep-cycle battery, would soon destroy it.
Types of Battery
Lead-acid, nickel-iron, and lithium-ion are the three primary types of deep-cycle batteries used in solar systems.
Due to their mix of performance, relatively long life, and inexpensive cost, lead-acid batteries are by far the most often used variety. The cheapest batteries are flooded lead-acid (FLA), but they must be filled up with distilled water on a regular basis, making them a maintenance concern. If turned over, they will also leak, posing a maintenance and safety concern. Sealed lead-acid batteries are almost maintenance-free and will not leak when tipped. Absorbent glass mat (AGM) and gel are the two types available. Gel batteries are often less robust and can be permanently destroyed if not charged correctly. As a result, AGM batteries are the most popular in the solar industry, despite the fact that they are nearly twice as expensive as flooded batteries.
Nickel-iron (NiFe) batteries, sometimes known as Edison batteries, use a potassium electrolyte, which is less toxic than electrolytes used in other types of batteries. Edison batteries are strong and long-lasting, lasting several times longer than lead acid batteries. However, they have a small storage capacity, a high rate of self-discharge, and a poor rate of efficiency. In addition, they are more costly and less readily accessible than conventional batteries. If nothing else, Edison batteries are valued for their long life and relative resistance to overcharging and overdischarging.
NOTE: FLA and NiFe batteries, for example, require ventilation to release explosive hydrogen gas created by the electrolysis of water inside the battery.
These batteries' enclosures must have ventilation, and big banks may require an explosion-proof exhaust fan to properly ventilate the gas produced.
Lithium-ion batteries for solar systems are similar to those used in cell phones, laptop computers, and electric cars. They have a lot of advantages over lead-acid batteries, but they're a lot more expensive. The question is whether the benefits are worth it for your application. Lithium batteries are roughly a third of the size and weight of lead-acid batteries with equal storage capacity, making them particularly appealing for RVs and other portable applications where space and weight are critical. Lithium batteries last nearly twice as long as lead-acid batteries, and they may be depleted up to 80% without losing performance. This means you'll have to buy fewer batteries and replace them less frequently.
A complex battery management system (BMS) is required for lithium storage, which adds to the overall cost. Lithium storage may be supplied as part of a built system for big systems that connects batteries with the appropriate charging mechanism for simplicity of installation and to improve safety by ensuring that only certified persons operate with these highly technical components.
When purchasing batteries, keep in mind that a battery's amp-hour rating, or storage capacity, is determined by the rate of discharge. When depleted over a 20-hour period, a battery rated at 100 Ah may only supply 80 Ah when exhausted in a 10-hour period. When depleted over 100 hours, the same battery may supply 130 Ah. Manufacturers supply specification sheets that list the predicted capacity at a certain rate, known as the C Rate, and frequently contain a dozen or more rates.
Cycle life, or the number of cycles a battery can produce, is another important rating element. One cycle is when a battery is fully discharged (usually no more than 50% with lead-acid batteries) and then fully recharged. The depth of discharge affects this rating. A battery may be rated for 1,500 cycles at 10% discharge every cycle, but only 1,000 cycles at 50% depth of drain, and even less at 80% depth of depletion. A year is divided into 365 cycles by one cycle every day. Under normal conditions, a 1,500-cycle battery should last roughly four years when used on a regular basis.
The stated Ah amount for deep-cycle batteries is usually based on their 20-hour rate, but some batteries are sold using their 100-hour rate, which can make them appear 30% bigger when comparing only the labels, therefore look at the C Rate parameters. A daily cycle rate is roughly equivalent to a 20-hour rate.
Tips for a Long Life
Given the high expense of high-quality deep-cycle batteries, it pays to treat them well. A five-year battery might survive just one or two years due to poor operation and maintenance practices. Here are some pointers to help you get the most out of your lead-acid batteries:
• For maximum longevity, discharge batteries no more than 50%; discharge them no more than 80%, which drastically reduces their life.
• Keep them cool: temperatures above 77 degrees Fahrenheit drastically reduce battery life.
• Check electrolyte levels in flooded lead-acid batteries once a month at the very least. Overcharging/discharging batteries rapidly depletes electrolyte levels, necessitating more frequent replenishing.
• Inspect battery cable connections for looseness and corrosion on a regular basis; both cause inefficiency and incomplete charge.