Deep cycle battery for a solar system: types, size, cost, maintenance

The deep cycle battery might look like an automotive battery, however, the principles behind them are quite different. Automotive battery or a starting battery serves to give out a short high-current burst to power the engine. Deep-cycle battery is made to maintain a low flow of energy over a long period of time. They can look almost identical so you have to make sure that you are buying the right one.

The distinctive feature of a deep cycle battery is that it's designed to be deeply discharged and then charged again. The percentage of charge from the battery that was used is called Depth of discharge (DOD). Some models can be discharged by 80%, however, it's not healthy for the battery. Manufacturers recommend going down no less than to 45% of charge — it simply prolongs the lifespan of a battery. For example, if a deep cycle battery that is being discharged by 80% everytime can survive up to 250 cycles, then the number of cycles can go up to 750, if it's discharged by 50% instead. The number of cycles doesn't mean the occurrences when it goes to its minimum charge and returns to 100% — it's more like the number of times a battery can undergo the process of discharging and recharging.

As of now, deep cycle batteries have a wide range of use: traffic lights, electric vehicles, computers that require UPS (uninterrupted power supply), boats of different kinds and, most importantly, solar installations, generally of a smaller, residential scale. The prices for deep-cycle batteries depend on size, voltage and type, and therefore can lie in the range from 50$ to 1000$.

So how does a deep cycle battery work? In the past almost all deep-cycle batteries used in PV systems were lead-acid batteries — and this type still offers a good cost-efficiency ratio. The principle of their work is based on chemical reactions between lead and sulfuric acid. A battery consists of cells with positive lead dioxide (PbO2) plate and negative lead (Pb) plate with an insulator between them. These plates are submerged in water (H2O) and Sulfuric acid (SO4) — electrolyte which conducts electricity. When battery discharges, lead reacts with acid and it produces lead sulfate (PbSO4), some water and around 2V of electricity. With 6 cells in a battery you get 12 V. The reaction reverts when you charge the battery, but with too much lead sulfate it just can't go on. This is why you can't charge and discharge your battery forever and why manufacturers do not recommend to discharge batteries below a certain degree.

Deep cycle batteries have thicker plates, thicker separators and higher-density active paste materials than ordinary lead acid batteries. Generally, alloys that are used for deep-cycle batteries contain more antimony — it improves the properties of lead and tin. Plates are placed further apart and some free space added underneath them so that when lead sulphate starts to form, it falls down and lets the reaction go on for longer.

Batteries are not 100% efficient and some amount of energy is going to be lost in the process no matter what — it goes into heat, which makes batteries warm when being charged. The efficiency of lead-acid batteries varies around 80%-90%, which is fair considering how old the technology behind them is. A fresh battery at first isn't going to perform as good as expected (about 5%-10% lower): it takes 10-30 reuses for it to reach its full capacity.

When we are talking about rechargeable lead-acid batteries, there are three prominent types that are being used nowadays. These are flooded (or wet cell) batteries, gel batteries and AGM (Absorbed Glass Matt) batteries. First type is a classic: it is very close to the first battery ever invented. Though it has its disadvantages, it still is a perfectly fine working battery. Gel battery takes the concept of wet cell battery and upgrades it. AGM batteries are the most modern development and their main drawback right now is the cost.

Wet cell batteries consist of cells filled with electrolyte: water and sulfuric acid. Since there is liquid in its free form, the owner must check its level from time to time and maybe add some distilled water. Obviously, you have to be careful when working with such batteries, because sulfuric acid, especially in such density, can be very dangerous for your skin and clothes. Such wet cell batteries must be handled carefully to avoid any spilling or internal breakage. There are maintenance free wet cell batteries but they don't last that long and their capacity (DoD) is lower. In addition, all wet cell batteries are afraid of extreme climates: water can either evaporate in hot areas or freeze when it's cold.

Gel batteries are a transitional point between wet cell batteries and AGM batteries. Technically, it's a wet cell battery, but a silica additive makes the electrolyte stiff, so it doesn't have a risk of spilling. They are also much better at withstanding extreme temperatures. The problem with gel batteries is their voltage sensitivity: they really don't like being overcharged and it causes almost certain malfunction.

AGM batteries have several names: they are also known as "dry cell", "non-spillable" and "valve-regulated". The principle behind them is that there is a special fiberglass mat between two plates that absorbs the electrolyte. As a result, you don't have liquid water in a cell, which makes an AGM battery very safe. Moreover, it isn't as affected by extreme temperatures as a flooded battery. These batteries lose very little of their energy while being stored somewhere in the garage. Their disadvantage is again their vulnerability to overcharging and cost: AGM batteries tend to cost twice as much as a good wet cell battery. They are the most prominent choice when it comes to solar panel systems.

While lead-acid batteries are still a common choice, in the last few years lithium-ion batteries slowly became a new standard for solar systems. Lithium-ion batteries count as deep-cycle by default — all of them are meant to be discharged and then charged back again. Their popularity is not a surprise: they are better than traditional lead-acid batteries in almost every way.

• The depth of discharge reaches 90-95%, though on a daily basis it's better not to use more than 70-80% of their load.
  
  
• They last longer — the warranties are up to 10 years.
  
  
• They are more efficient than lead-acid batteries and lose only around 5% of energy during their work.
  
  
• They are smaller and lighter — in fact, a lithium-ion battery on average occupies 75% less space and weighs 75% less than a comparable lead-acid battery.
  
  
• They are charged faster: it usually takes about 1-2 hours to fully charge a lithium-ion battery compared to 2-4 hours for a lead-acid battery.
  
  
• Partial state of charge isn't an issue.
  

The only problem with lithium-ion batteries is their high cost, which can range from 7,000$ to 30,000$, while prices for a lead-acid battery vary around 5,000$-15,000$.

When buying a battery for PV modules you first need to make sure it fits your solar system and meets the needs of your house. The voltage of the battery better be equal or lower than the voltage of your panels. In any case, you should always use a solar charge controller to keep your battery safe. It lowers the voltage of panels down to the battery level, prevents the overcharge and disconnects the battery when discharged below a certain point. At night batteries without a regulator tend to give some of the accumulated energy back to panels — the charge controller doesn't let that happen.

To size a battery for your home, at first you would need to calculate your daily energy consumption. Lead-acid batteries are measured in Amp-hour. Lithium-acid batteries capacity is measured either in Amp-hour or in kWh (kilowatt-hours). While you're connected to the grid, your energy consumption can be calculated using energy bills for the last year. Or you can take the current draw (measured in Amps) of appliances, multiply by the amount of hours of runtime and you get Amp-hours. There are also special devices that can tell you the amount of energy you're spending everyday: plug-in power meters, submeters, special inverters. Kilowatt-hours can be measured through Amp-hours and vice-versa: Watt-hour = Amp-hour x Voltage of the battery.

To find out the right battery capacity for your home, you would also have to take into calculation the allowable depth of discharge of a battery. Let's say, your daily energy consumption is 50 A/h. The depth of discharge for a deep cycle lead-acid battery can go to 80%, however it is safer to keep it below 50%. Therefore, a 100 A/h battery would probably suit your daily needs. Would your house need a day or two of energy autonomy in case, for example, of bad weather? Then the size of your battery should go up to 200-300 A/h.

The shelf life of a battery heavily depends on the way it is treated: batteries should not be overcharged or left in a hot place for an extremely long time. Moreover, battery properties depend on the type and model you get. However, deep cycle batteries tend to last longer than starting batteries. The AGM battery is estimated to last from 4 to 10 years. Gel batteries live from 2 to 7 years. Wet cell deep cycle batteries can serve for up to 6 years with proper care. Lithium-ion batteries have the longest lifespan that can surpass 10 years. However, it's very easy to kill a battery. Here's a few tips for battery maintenance.

• Batteries should not be exposed to high heat. In fact, their lifespan shortens dramatically if they are exposed to temperatures higher than 86 F° for a prolonged period of time.
  
  
• Batteries don't like not being used, so it's a bad idea to buy a battery and save it in the garage for a rainy day. Especially they don't like being stored empty, so even if you don't plan to use a battery for some time, charge it fully before putting it away.
  
  
• Wet cell batteries have to be checked every once in a while. If the level of electrolyte in a battery dropped, it should be refilled with distilled water.
  
  
• If it's possible, it's better to charge your battery slower, because faster charging means more heat for a battery.
  
  
• Batteries need to be washed with water and baking soda solution — just don't use any cleaning products, as it may provoke an unexpected reaction.
  
  
• Often the problems with batteries are caused by cable connections — they need to be cleaned and tightened regularly.
  
  
• Once again — don't overcharge your batteries. Solar charge controller should help with that and its display (if it has one) shows you the charge and current state of the battery.
  

Lead-acid batteries are meant to be almost fully recycled — the only thing that is good for nothing after the death of a battery is an insulator between plates. Make sure to turn in your used batteries for reprocessing.

This concludes our deep cycle battery guide. If you need a more thorough guide on batteries in general, check out our new article: Solar batteries for your solar system: basics to make the right choice