Deep Cycle Battery, Everything You Need to Know
Before we dive into what is a deep cycle battery, it is first important to understand a few battery basics of what a cycle is, what depth of discharge means, and what it really means to deep discharge a battery.
One full cycle is considered a full discharge and recharge of a battery. What is meant by a full discharge? Discharge is measured by the capacity removed from the battery – the depth of the discharge (DoD) is used to indicate how much of the battery capacity has been used during a single discharge. A full discharge is 100% DoD.
DoD is inversely related to state of charge (SoC), which is how much charge remains in the battery. 100% DoD = 0% SoC.
With this understanding of DoD and a battery’s cycle, you may be left wondering what truly classifies as a deep discharge. A deep discharge is classified as 80% to 100% of capacity discharged from the battery.
What is a Deep Cycle Battery?
This answer may seem obvious, but a deep cycle battery is a battery that has been designed to continuously handle deep discharges of 80-100% DoD. Does this mean high-rate batteries can’t be used for a deep discharge? Yes and no. One or two deep cycles will not hurt the battery (if maximum discharge and charge rates are strictly adhered to), but this is where the design of the battery matters because a high-rate battery would break down severely over time if it were continuously cycled in a deep manner.
In SLA (sealed lead acid) batteries, the electricity is generated in the plates. In high-rate batteries, there are many thin plates to allow for more surface area for quick generation of energy. In deep cycle batteries, the plates are thicker than those inside a high-rate battery because the energy-inducing chemical reaction goes into the plate and therefore needs to be thick to be able to handle the reaction. If the plates were thin, like they are in high-rate and starter batteries, the lead plates would degrade very quickly and would not hold up to multiple deep discharges over time.
Two subcategories of deep cycle SLA batteries would be deep cycle gel and AGM (absorbent glass mat) deep cycle. Gel batteries use a substance that turns the electrolyte acid inside the battery into a thick gel, therefore making it non-spillable. The gel is better at heat distribution and off-gassing than the AGM, and therefore has better performance throughout its life. In AGM batteries, a glass mat is placed between the negative and positive plates to keep the electrolyte in place. The mat is what makes the battery non-spillable and optimizes the surface area of the plates for better electrolyte distribution over conventional batteries.
In a lithium deep cycle battery, energy cells are used. Energy cells are designed to deliver sustained current over a long period of time, making them ideal for use in cyclic and deep cycle applications. In addition to the cell makeup inside a lithium battery, the circuitry also matters as the protections may set limits to how much current can be drawn from the battery. Different lithium batteries will have different levels of protection, so make sure to check the technical documentation and specifications for further information.
As well as different types of chemistries, deep cycle batteries can come in variety of voltages and capacities, most popular voltages being 12V deep cycle batteries and 24V deep cycle batteries.
Deep Cycle Battery Applications
Before selecting a battery, you will need to have a clear idea of what needs to be powered, for how long, how often, and how much power it needs. In general, deep cycle batteries are cyclic applications where the user tells the battery when it needs to be used.
For example, a medical cart used in a hospital will be away from an electrical outlet and need battery power for the entire time it is unplugged. In this instance, the nurse or doctor tells the battery when it will be used by turning on the medical cart. It will be used every single day for many hours each day. The battery needs to provide consistent power for a long amount of time, meaning this is a deep discharge use and therefore needs a deep cycle battery.
Another example of a deep cycle battery is the battery inside your cell phone. This battery, depending on age, is designed to last all day on a single charge and to be used every day. You tell the battery when it is time to be used by turning on the phone or unplugging it from the charger. If you’re like most users, you wait to plug your phone back in until it is completely dead – the DOD is 100%/SOC is 0%. You expect this battery to provide consistent power all day long, so a deep cycle battery must be used.
There are many other applications that utilize deep cycle battery technology and these include marine, leisure (including RV batteries), mobility scooters, all types of electric vehicles and solar applications.
To differentiate, high-rate batteries are typically used in back-up or emergency-use type applications. This would be an application where a battery is sitting in an elevator waiting to provide back-up power in the event of a power failure. This battery will need to deliver a large amount of energy very quickly in emergency situations. These batteries may even need to be replaced before they are ever used. Think of a high-rate battery as your power’s insurance policy, delivering a lot of power all at once in emergency situations, whereas a deep cycle battery is going to be your work horse that delivers consistent power very frequently – as often as a full discharge cycle daily.
Types of Batteries
Batteries are divided in two ways, by application (what they are used for) and construction (how they are built). The major applications are automotive, marine, and deep-cycle. Deep-cycle includes solar electric (PV), backup power, traction, and RV and boat house batteries. The major construction types are flooded (wet), gelled, and sealed AGM (Absorbed Glass Mat). AGM batteries are also sometimes called starved electrolyte or drybecause the fiberglass mat is only 95% saturated with Sulfuric acid and there is no excess liquid.
Flooded may be standard, with removable caps, or the so-called maintenance free (that means they are designed to die one week after the warranty runs out). All AGM gelled are sealed and are valve regulated, which means that a tiny valve keeps a slight positive pressure. Nearly all sealed batteries are valve regulated (commonly referred to as VRLA. Valve Regulated Lead-Acid). Most valve regulated are under some pressure. 1 to 4 psi at sea level.
Battery Lifespan
The lifespan of a deep cycle battery will vary considerably with how it is used, how it is maintained and charged, temperature, and other factors. It can vary to extremes. we have seen L-16’s killed in less than a year by severe overcharging and water loss, and we have a large set of surplus telephone batteries that see only occasional (10-15 times per year) heavy service that was just replaced after 35 years. We have seen gelled cells destroyed in one day when overcharged with a large automotive charger. We have seen golf cart batteries destroyed without ever being used in less than a year because they were left sitting in a hot garage or warehouse without being charged. Even the so-called dry charged (where you add acid when you need them) have a shelf life of 18 months at most. (They are not totally dry. they are actually filled with acid, the plates formed and charged, then the acid is dumped out).
These are some typical (minimum-maximum) expectations for batteries if used in deep cycle service. There are so many variables, such as depth of discharge, maintenance, temperature, how often and how deep cycled, etc. that it is almost impossible to give a fixed number.
- Starting: 3-12 months
- Marine: 1-6 years
- Golf cart: 2-7 years
- AGM deep cycle: 4-8 years
- Gelled deep cycle: 2-5 years
- Deep cycle (L-16 type etc): 4-8 years
- Rolls-Surrette premium deep cycle: 7-15 years
- Industrial deep cycle (Crown and Rolls 4KS series): 10-20 years.
- Telephone (float): 2-20 years. These are usually special purpose float service, but often appear on the surplus market as deep cycle. They can vary considerably, depending on age, usage, care, and type.
- NiFe (alkaline): 5-35 years
- NiCad: 1-20 years
Starting, Marine, or Deep-Cycle Batteries
Starting (sometimes called SLI, for starting, lighting, ignition) batteries are commonly used to start and run engines. Engine starters need a very large starting current for a very short time. Starting batteries have a large number of thin plates for maximum surface area. The plates are composed of a Lead sponge, similar in appearance to a very fine foam sponge. This gives a very large surface area, but if deep cycled, this sponge will quickly be consumed and fall to the bottom of the cells. Automotive batteries will generally fail after 30-150 deep cycles if deep cycled, while they may last for thousands of cycles in normal starting use (2-5% discharge).
Deep cycle batteries are designed to be discharged down as much as 80% time after time and have much thicker plates. The major difference between a true deep cycle battery and others is that the plates are SOLID Lead plates. not sponge. This gives less surface area, thus less instant power like starting batteries need. Although these can be cycled down to 20% charge, the best lifespan vs cost method is to keep the average cycle at about 50% discharge. Unfortunately, it is often impossible to tell what you are really buying in some of the discount stores or places that specialize in automotive batteries. The golf cart battery is quite popular for small systems and RV’s. The problem is that golf cart refers to a size of battery case (commonly called GC-2, or T-105), not the type of construction. so the quality and construction of a golf cart battery can vary considerably. ranging from the cheap off brand with thin plates up to true deep cycle brands, such as Crown, Deka, Trojan, etc. In general, you get what you pay for.
Marine batteriess are usually a hybrid, and fall between the starting and deep-cycle batteries, though a few (Rolls-Surrette and Concorde, for example) are true deep cycle. In the hybrid, the plates may be composed of Lead sponge, but it is coarser and heavier than that used in starting batteries. It is often hard to tell what you are getting in a marine battery, but most are a hybrid. Starting batteries are usually rated at CCA, or cold cranking amps, or MCA, Marine cranking amps. the same as CA. Any battery with the capacity shown in CA or MCA may or may not be a true deep-cycle battery. It is sometimes hard to tell, as the term deep cycle is often overused. we have even seen the term deep cycle used in automotive starting battery advertising. CA and MCA ratings are at 32 degrees F, while CCA is at zero degrees F. Unfortunately, the only positive way to tell with some batteries is to buy one and cut it open. not much of an option.
Deep Cycle Battery as a Starting Battery
There is generally no problem with this, providing that allowance is made for the lower cranking amps compared to a similar size starting battery. As a general rule, if you are going to use a true deep cycle battery (such as the Concorde SunXtender) also as a starting battery, it should be oversized about 20% compared to the existing or recommended starting battery group size to get the same cranking amps. That is about the same as replacing a group 24 with a group 31. With modern engines with fuel injection and electronic ignition, it generally takes much less battery power to crank and start them, so raw cranking amps is less important than it used to be. On the other hand, many cars, boats, and RV’s are more heavily loaded with power sucking appliances, such as megawatt stereo systems etc. that are more suited for deep cycle batteries. We have used the Concorde SunXtender AGM batteries in some of our vehicles with no problems.
It will not hurt a deep cycle battery to be used as a starting battery, but for the same size battery they cannot supply as much cranking amps as a regular starting battery and is usually much more expensive.
What is the range of an e-bike battery?
The range of a battery pack depends on the amount of energy packed inside of it and is measured in Watt-Hours (Wh). Watt?
Watt-hours are calculated by multiplying the battery capacity, in Amp-hours, by the battery Voltage, in Volts.
Let’s assume that, on average, 1-mile requires about 25Wh of energy. So a 14Ah, 36V battery should get you about 25-miles per charge.
Keep in mind that the weight of the rider, outside temperature conditions, and the amount of pedaling will make a significant difference in range.
A word of caution: the range that e-bike manufacturers provide should be taken with a grain of salt. That number is generated from tests that are run in perfectly tailored lab conditions. Do you charge any of your electronics in an incubation chamber set at 28° C with a lab-grade charger that applies the perfect current while charging? Yeah, I don’t either. And so, We should assume that the manufacture-specified range is delivered only if the battery is charged and discharged under ideal conditions i.e. not real world conditions.
For a more realistic estimate, shave off 15% of the manufacturer specified range and assume this padded number to be your real range.
If you’re looking for a longer range, choose a battery that has higher capacity (Ah). If you’re looking for more power, choose a battery that has higher voltage (V). Learn more why voltage and capacity matter.
What is the lifespan of an e-bike battery?
There are several factors that affect the lifetime of a battery such as:
- environmental conditions: temperature during charging discharging
- charging rate: how fast or slow your battery is charged
- charging voltage: what voltage the battery is charged to
- depth of discharge (DoD): what voltage the battery is discharged to
The list above isn’t exhaustive but, in general, batteries decay as a function of time in the charged state. Period.
Day 1: You get your new e-bike and charge it up to 100% and go on a bike ride. When you come home, you charge the bike back up to 100% and you’re excited to ride it again soon.
Day 2. 364: Life get’s in the way and you still haven’t been out on your bike since that first ride.
Day 365: One year later, it’s the perfect day for a bike ride and you finally have some time on your hands. You head to your basement, unlock your bike, and excitedly turn it on. 80% charge. What? You clearly remember charging your bike to 100% last year before moving it to the basement!
The truth is, we can’t beat thermodynamics. I’ll say it again: batteries decay as a function of time in the charged state.
Now, because you left your battery at 100% for a whole year in a basement with no temperature control, you inadvertently caused your battery to lose a certain amount of irreversible capacity. Your range will be ~20% lower and you’ll likely have to replace your battery sooner than you expected. The table below shows you how much recoverable capacity exists in a battery after storing it at different temperatures and different charge states for 1-year.
This is why a lot of electronics come with batteries that are only partially charged. to help slow down this decay. That being said, it’s hard to track how long e-bikes and their batteries have been sitting in warehouses before being delivered to your door so you could get a battery that has been decaying for a year or two.
Manufacturers also tend to overrate their batteries and will make claims about certain batteries having a lifetime of at least 1,000 cycles. Show.me.the.data.
The lifetime of a lithium-ion battery is described as the number of cycles until the capacity (Ah) drops below 80% of it’s initial capacity. In general, this is roughly 250-400 cycles (depending on battery chemistry and other factors) which amounts to roughly 1.5 to 2 years if you charge discharge daily and care for your battery properly.
How to charge your e-bike battery to make it last longer
- The thing that will kill your battery faster than anything else is leaving it charged at elevated temperatures. If it’s 80 degrees outside and you have your e-bike fully charged, move it indoors where it’s cooler and try to drain the battery as soon as possible.
- Charge your battery at room temperature as often as possible.
- When sourcing an e-bike battery charger, the slower the charge rate the better. For example, if you have a 2-Amp charger, and your battery is a 14 Ah battery pack, you are charging at 14 Ah / 2-Amps = 7-hours. This is a nice, slow charge which will certainly improve the longevity of your battery pack. Avoid charging at rates that are faster than 2-hours for a full charge.
There’s a lot that goes into choosing the best battery for you e-bike, and there certainly isn’t a one-size-fits-all approach. But if I were buying an e-bike battery today, here’s what I’d do: LFP or NMC, slow charge, avoid storing or charging in hotter temperatures, and leave the battery at around 30% charge if you don’t plan on using it for a while.
Have questions? We’d love to help. You can get in touch using the contact form or find us on @somerville_ev
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How You Plan to Use Your Electric Bike
All in all, the general consensus is that charging rates are affordable for the vast majority of potential electric bike owners. However, these costs also depend on how much use you expect to get out of your bike. For example, you should know whether you can ride an electric bike on the road or through tougher terrain, as gentler riding will be more power efficient and therefore require less recharging.
Finally, there are several other factors that can affect how much it costs to charge an electric bike. Some of these factors are energy efficiency settings on the battery and charger and any discounts offered by your utility company or local government. Some states offer free charging to incentivise the use of these vehicles–be sure to research these options before purchasing an electric bike so you can maximize savings while getting the performance levels you need.
