When it comes to solar, you need 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 re-charged 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 which 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?

Source: CSBattery Energy Co.,Limited

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 sustain power supply for long periods of time. That is why it is also considered a dual purpose battery.

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

Pros Cons
  • Cheapest type of deep cycle battery
  • Reliable
  • Low internal impedance
  • Can deliver very high currents
  • Tolerant of abuse and overcharging
  • Indefinite shelf life if stored without electrolyte
  • Can be left on trickle or float charge for prolonged periods
  •  Available via many suppliers worldwide
  • The world’s most recycled product
  • Very bulky
  • Very heavy
  • Typical usable capacity of 30 to 50%
  •  Charge efficiency of 70 to 85%
  • High self-discharge, 5% per month
  • Can fast charge
  • Can overheat during charging
  • Generate poisonous gases when charging
  • Require regular maintenance to top-up electrolyte
  • Low cycle life – about 300 to 500 cycles
  • Must be stored fully charged to avoid damage
  • Must be kept upright to prevent electrolyte spillage
  • Sensitive to freezing (case bursts, electrolyte spills)

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

Pros Cons
  • No maintenance
  • Low self-discharge (1 to 3% per month)
  • Charge efficiency of 85 to 90%
  • More tolerant of high heat than AGM
  • Gelled electrolyte (silica added) does not spill as easily as flooded lead-acid
  • Gel cells are sealed, so there is no risk of gas leakage or electrolyte spillage with typical use and charging
  • More expensive than flooded lead-acid and some AGMs
  • Cannot tolerate fast charging
  • Can be damaged by overcharging
  • Require own specific charging profile – different from AGM, lead-acid or lithium

Pros and cons of AGM deep cycle batteries

Pros Cons
  • Usable capacity of 60 to 80%
  • Low self-discharge (1 to 3% per month)
  • Charge efficiency of 95%
  • Can charge at much higher rates than lead-acid or gel batteries
  • No maintenance
  • Electrolyte is only in saturated glass mats, so will not spill
  • AGM cells are sealed, so do not off-gas with typical use and charging
  • Not affected by freezing temperatures
  • More expensive than flooded lead-acid batteries
  • Can be damaged by overcharging

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

Pros Cons
  • Very light and compact (half the weight of lead-acid batteries)
  • Usable capacity of 80% to 100%
  • Very low self-discharge (1 to 2% per month; lowest if stored partially-charged)
  • Charge efficiency of 99%
  • Rapid charge rate – up to 5 times faster than AGM batteries
  • Can last 2000 to 5000 cycles
  • High energy density
  • Extremely high possible charge rates
  • No battery memory
  • No maintenance
  • Can be stored at low state-of-charge with no damage
  • Very little power loss at low temperatures
  • Most expensive among all the deep cycle batteries
  • Some lithium-ion chemistries are thermally unstable, causing overheat and risk of fire or explosions
  •  Cannot be charged at low temperatures, near freezing or below freezing temperatures
  • Require battery maintenance system for protection, most lithium batteries include an internal system

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 an 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.

Conclusion

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, the lightest and smallest. However, their price tag can be a deterrent for those on a stricter budget.

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