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When it comes to solar, you need the types of solar batteries that can provide dependable power under any condition. The batteries should have high capacity, good discharge rates, high round-trip efficiency and can be recharged quickly.
All these traits are especially important for those who live off-grid or in recreational vehicles (RVs). The best batteries for said living conditions are deep-cycle batteries.
There are a few main types of deep cycle batteries used for solar: flooded lead-acid, sealed lead-acid, and lithium. Within these types, there are various subcategories that will be covered in this article.
We will also touch on the pros and cons of the main types as well as briefly answer a few commonly asked questions about deep cycle batteries.
What is a deep-cycle battery?
A deep cycle battery is a type of battery designed to have a high Depth of Discharge (DoD). This means it can release high amounts of stored electricity compared with its storing capacity. A deep-cycle battery is also built for fast recharge and provides a steady stream of power over a long period of time.
Examples of its range of applications include recreational vehicles (RVs), marine applications and golf carts. For RVs, the deep cycle battery is good for power-hungry appliances like RV air conditioners, RV refrigerators, induction cooktops and microwave ovens.
The deep cycle battery should not be confused with a starter battery. The latter is for releasing short bursts of power to start engines but not to sustain deep discharge and recharge.
A deep cycle battery, on the other hand, can function as a starter as well as a sustained power supply for long periods of time. That is why it is also considered a dual-purpose battery.
Can deep-cycle batteries be used as solar batteries?
Solar applications require batteries with high capacity, steady round-trip efficiency of around 90% (ratio of battery output to the energy stored), and a reasonable DoD rate.
Deep-cycle cells possess these characteristics, and they are thus used for solar purposes too. Deep-cycle solar cells are amongst the most efficient and the best batteries you can use for your solar needs.
What is the difference between a deep cycle and a regular battery?
A regular car battery is designed to provide a short, sharp burst of power to start the vehicle’s engine. However, a deep cycle battery has thicker plates and denser active material to withstand repeated charge and discharge cycles.
Types of deep cycle battery
In terms of capacity range, there are two: 6 volts and 12 volts. Of these, there are 3 basic types used for solar, which are:
1. Flooded lead-acid battery
Sometimes called lead-acid, FLA, or wet-cell, it is constructed of lead plates or grids in a container filled with a liquid electrolyte; usually, concentrated sulfuric acid.
The FLA is quite heavy, considering the ratio of battery weight to the amount of energy it provides. Consequently, the popularity of this type of battery is waning.
Being the cheapest and oldest battery technology, it has been popular among the budget-conscious and practitioners of off-grid living.
Pros and cons of flooded lead-acid batteries
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2. Valve Regulated Lead-Acid (VRLA) battery – Gel and AGM
This type of battery is the generation after flooded lead-acid batteries, created to overcome issues associated with the FLA. Instead of free-flowing liquid electrolyte, the VRLA has a thicker electrolyte. It is also sealed, thus giving its other name sealed lead-acid battery.
There are two varieties of VRLA – gel and absorbed glass matt (AGM). Gel batteries contain a gelled electrolyte. AGM deep cycle batteries use an electrolyte in a glass mat. As these are not free-flowing liquids, there is no need to add water.
Pros and cons of gel deep cycle batteries
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Pros and cons of AGM deep cycle batteries
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3. Lithium-ion battery
Sometimes called a lithium battery, it consists of a cathode and anode plates or cylinders dipped in a solution of lithium salts. It is available in many chemistries and is the safest for RV use. Although lithium batteries are generally the most expensive among the range of deep cycle batteries, they last much longer than lead-acid types.
Pros and cons of lithium deep cycle batteries
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Other categories of deep cycle batteries
1. Deep cycle marine battery
This type of battery contains lead sponge plates that are coarser and heavier than starting battery plates but not as thick as true deep cycle battery plates. It is a kind of hybrid between a starting battery and a pure deep cycle battery.
2. Group 31 deep cycle battery
This is a group of batteries which has a capacity range of 75 to 125 Amp hours. It is very common in off-grid, marine, automotive and similar applications.
3. Group 27 deep cycle battery
Also large and powerful like Group 31, it has a slightly lower Amp hour range of 66 to 110 Ah. The application is similar to Group 31.
4. Group 24 deep cycle battery
This group consists of an Amp hour range of 70 to 85 Ah. The group numbers of a deep cycle battery are ranked size numbers outlined by the Battery Council International (BCI). The numbers are assigned based on the battery’s size and description.
How to charge a deep cycle battery properly?
For a solar installation, deep cycle batteries require a battery charger of some sort such as a converter, solar inverter and solar charge controller. These must match the correct charging profile for the type and model of battery.
The voltage points must also be set to the correct voltages for each type of charging cycle. The best way to do all these is to check with the manufacturer for your specific battery.
How long does a deep cycle battery last?
It depends on a few factors but still less than factors than those of a lead-acid battery. They are:
- Proper maintenance
- Discharging and charging correctly
- correct use during below-freezing temperatures
- Temperature in the battery storage area
A more accurate way of measuring how long it lasts is by charging cycle. Lithium deep cycle batteries last between 3,000 and 5,000 charging cycles depending on the brand, model and the factors mentioned above. In terms of years, it can range from 3 to 20 years.
Flooded lead-acid batteries that are maintained correctly can last up to 6 years. The same goes for AGM batteries. Gel batteries can last a little longer, from 10 to 20 years.
Can I connect different batteries together?
No. You can only connect batteries of the same type, model, capacity, and age. The wiring used to connect them must also be of identical size and length. They should also be placed together.
You can connect the battery bank to the RV engine starter battery, but have the battery bank in a different part of the RV. You will need a battery isolator, battery combiner or charging relay so that the house bank can be charged via the alternator or the engine battery charged via solar.
Things to consider while buying a deep-cycle battery
Now that you have considered adding a deep-cycle solar cell, here are the characteristics to know before you make your first purchase:
A 12V deep-cycle battery is standard for portable applications, RVs, and boats, whereas home energy storage requires a significantly higher voltage capability of 24V or 48V. What we mean to say is that different panels require different voltage support. While larger voltage batteries require fewer charge controllers and thinner cables, it is requisite to get one that is compatible with your solar needs.
Battery capacity is referred to in amp hours. The higher the amp hours, the higher the energy it can deliver. It is another vital consideration as different use cases require and support batteries with varying amp hours. For example, having a higher-capacity cell for a portable setup would be infeasible as higher capacity means added weight and dimensions.
As for deep-cycle batteries, we denote their discharge rate in 20 hours of measurement. So, for example, a 300ah battery can discharge at a rate of 15 amps for 20 hours until it is fully discharged.
DoD, or depth-of-discharge, describes the battery capacity you can safely use without potentially harming it. As for deep-cycle batteries, their DoD varies depending on their type. For example, Flooded lead-acid cells, being the oldest, have a DoD of only 50%, and users should never discharge them beyond 80% of their capacity.
In contrast, Gel and AGM cells are 100% DoD rated, i.e., even draining them completely won’t cause long-term damage, but it is suggested to limit their usage to 80% for optimized performance.
Battery cycles refer to the charge cycles which a cell supports. Usually, a gel battery can last 1,000 cycles at half the usage (50% DoD), but only 450 cycles at 100% DoD. An AGM cell can last 500 charges at 50% usage but only 200 cycles at 100% usage.
It means that battery life has much to do with DoD, and its useful life ends when you deplete its capacity to below 80%.
Price is an important consideration for any solar component you buy, and deep-cycle batteries are no different. FLA variants can be found for around USD 100 in most markets, but they are the least efficient. In contrast, a lithium-ion (li-ion) cell can push you back by approximately USD 1,500. While FLA requires high maintenance, Li-ion ones are easier to manage. So, finding the right balance between price and care is the way to go.
Each type of deep-cycle battery has its pros and cons. Lithium deep-cycle batteries are the most suitable for solar systems in RVs since these are the most powerful, lightest, and smallest. However, their price tag can be a deterrent for those on a stricter budget.