A deep cycle battery is one that can be discharged (up to 80% or more) and recharged multiple times without being damaged. This is in contrast to a maximum of 45% discharge rate recommended for other batteries. It is designed to reliably supply power over extended periods of time.
What is deep cycle battery?
Deep cycle batteries, may look a lot like car batteries to people who aren’t familiar with them, but in reality, they’re quite different. A deep cycle battery is a lead battery designed to provide sustained power over a long period and run reliably until it is 80% discharged or more, at which point it needs to be recharged. It is important to note that although deep cycle batteries can be discharged up to 80%, most manufacturers recommend not discharging below 45% to extend the life of the battery.
The level of discharge is the “deep cycle” and stands in contrast to other types of batteries that provide only short bursts of energy before they need to be recharged. To be specific, a starter battery discharges only a tiny percentage — usually 2 to 5% —each time it is use.
When to use the deep cycle battery
Deep cycle batteries provide sustained energy, making them ideal for certain applications that require more than a quick start. Some of the most common uses for deep cycle batteries include:
- Marine applications
- Recreational vehicles
- Materials handling, including forklifts
- Golf carts
- Off-grid renewable energy
For some applications, particularly marine uses, hybrid batteries are another solution. A hybrid marine battery can offer both a starter burst and sustained power for marine applications, but tends to have a shorter lifespan than a battery with a dedicated role.
Type of deep cycle batteries
The deep cycle function (ability to discharge and recharge) is not limited to lead acid batteries. The function is also used with Lithium technology. However, lead acid technology is still more widely used.
For starters, deep cycle lead acid batteries can either be flooded or sealed (gel cell or AGM). Each of these battery types differs in functionality. In order to choose the best for your functions, you need to understand their features.
Lithium-Ion Deep Cycle Batteries
The Lithium option is relatively new when it comes to deep cycle batteries. They are 30% lighter than flooded batteries. Additionally, they have 80 to 100% usable capacity. They come with a fast recharge rate and can last up to 5000 charge cycles.
Deep cycle Lithium batteries supply a constant voltage as it discharges. This means that if you are using it to light up a place, the lights stay bright even as the battery discharges and will just go off once the power is out. They will not slowly dim as the charge goes down.
Lithium iron phosphate batteries frequently employ the deep cycle function. They are commonly used in power backup systems or to store solar energy.
While Lithium batteries are maintenance-free, they need a battery maintenance system.
Flooded Lead Acid Batteries
These are the oldest serving batteries. They are made by submerging lead plates in an electrolyte (a mixture of water and sulfuric acid). The resulting chemical reaction reduces the electrolyte level, which needs to be topped up periodically.
Flooded lead acid batteries’ usable capacity is between 30 and 50%. The usable capacity is the amount of charge you can use before the battery needs to be recharged to full capacity to avoid damage.
Flooded batteries require the most maintenance efforts compared to other deep cycle options. You will need to keep adding distilled water to ensure that the electrolyte level is optimal. Additionally, you’ll need to clean acid residue from terminals, covers, and surroundings.
Always keep the battery upright to avoid leakages.
AGM (Absorbent Glass Mat) Deep Cycle Batteries
This is one of the VRLA battery types. The AGM refers to the use of thin fiberglass mats. These mats are placed between the lead plates so as to absorb the electrolyte. This prevents leakage and unnecessary electrolyte movement.
The glass mats act as a damper between the battery’s lead plates. This damper action makes the battery resistant to vibrations and shock. It also gives the battery the ability to withstand low temperatures.
AGM batteries tend to be more expensive than their flooded counterparts. They are generally maintenance-free and are not sensitive to positioning. They are fast-charging, but are sensitive to overcharging, and therefore need a regulated charger.
Gel Cell Batteries
This is another type of Valve Regulated Lead Acid battery. It is made with gelled electrolyte —the typical water and sulfuric acid mix which is suspended in silica gel.
Deep cycle gel cell batteries are costly compared to flooded lead acid batteries. Additionally, they require a special regulator and charger.
Deep cycle battery charging requirements
The charging parameters vary depending on the type of battery.
Flooded acid batteries require 10% of the Ah rating. This means that if the Ah rating is 100 Ah, 10 amp charging current is needed.
Gel and AGM batteries require 20% of the Ah rating.
| ITEM | Gel Cell Battery | AGM Battery | Flooded Battery |
|---|---|---|---|
| Absorption charging | 14.2 to 14.4 V | 14.6 to 14.8 V | 14.4 to 14.8 V |
| Float charging | 13.6 to 13.8 V | 13.6 to 13.8 V | 13.2 to 13.5 V |
The battery voltage settings are at 25 degrees. The charging voltages greatly affect battery longevity and performance.
If the charging voltage is higher than what is recommended for that battery type, there is a risk of overcharging. If it is too low, there is a risk of undercharging.
Battery charging time is dependent on the state of charge. For starters, it takes 60% of the indicated charging time to recharge 80% of the battery, and 40% of the time to recharge the last 20%.
Recharging the last 20% of the battery is crucial. It is the stage where sulphation buildup is removed and the battery’s performance is enhanced.
The DC-DC charging settings should always match those of the battery. Recharging a deep cycle battery with incorrect settings can cause leakage due to excessive use of water. It can also cause undercharge, which will result in battery failure.
How long will a deep cycle battery last?
One of the main differences between a lead acid and a lithium deep cycle battery is the cyclic performance. With a lithium deep cycle battery the capacity is independent of the discharge rate, this means in deep cycling applications where the discharge rate is often greater than 0.1C, a lower capacity rated lithium battery will often out perform the equivalent lead acid battery.
When it comes to measuring how long a deep cycle battery will last the correct way is in cycles rather than time. A lead acid battery can give 200 cycles (based on 100% DOD, to 80% capacity) whereas a deep cycle lithium battery can achieve over 10 times the amount at 2000 + cycles.
Which deep cycle batteries you will choose?
The following table shows a simple comparison of four of the major types of deep cycle batteries and their features.
| ITEM | Flooded Lead Acid Battery | AGM Battery | Gel Cell Battery | Lithium Ion Battery |
|---|---|---|---|---|
| Depth of discharge | 30–50% | 80% | 75% | 80–100% |
| Charge efficiency | 70–85% | 95% | 85–90% | 85% |
| Sensitive to overcharge | Tolerant | Yes | Highly | Yes |
| Position sensitive | Yes | No | No | No |
| Lifespan | Up to 6 years | 6 years | 10 years | Up to 5000 charge cycles, 3–10 years |
| Charging time | 8 to 16 hours | 5 hours | 5 to 10 hours | 2 to 3 hou |
While each battery type has its own maintenance requirements, there are some general rules when it comes to taking care of a deep cycle battery.
- Allow the battery to cool down after a round of prolonged use so as to avoid grid corrosion
- While some batteries have up to 100% DoD, it is advisable to be cautious so that the charge doesn’t drop below 20%
- Keep the battery terminals and covers clean, dust-free, and corrosion-free
- Be sure to charge the battery even when it is not in use so as to maintain battery life and prevent sulfation
What battery has more capacity?
Capacity differs greatly between lead-acid and lithium batteries due to their inherently different chemistry. The major difference is their capacity in relation to their rate of discharge. Lithium batteries can provide up to 100% of their rated capacity regardless of their rate of discharge. Lead-acid batteries provide significantly less usable energy with higher rates of discharge. Although both battery types may lose capacity in colder weather the changes in performance are far less drastic with lithium. At 0°C, for example, a lead-acid battery’s capacity is reduced by up to 50%, while a lithium iron phosphate battery suffers only a 10% loss at the same temperature.
