Wet/Flooded, AGM and Gel-Cell Deep Cycle, Dual Purpose and Starting Lead-Acid Batteries - Little Bit of Theory
Lead-acid batteries are still the preferred choice for many marine, automotive and industrial applications, despite the presence of lightweight and powerful lithium-ion batteries on the market for quite some time.
As such, lead-acid batteries come in many sizes, forms, electrolyte types, plate alloys, intended uses etc. that sometimes finding the right battery for certain application may be a daunting task. But it shouldn't be...
Updated: November 22, 2022.
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Wet/Flooded, AGM and Gel-Cell Lead Acid Batteries
Marine/automotive lead acid batteries are a strong, sturdy, and reliable type of batteries that are mostly used in fixed installations and/or are used as starter batteries, dual-purpose automotive/industry batteries, electric trolling motor batteries, off-the-grid batteries, etc.
They have the same chemistry as "common" car batteries, which are based on lead (hence the first part of their name) and its alloys and sulfuric acid (hence the second part of the name).
The main difference between these batteries is their electrolyte - basically, there are flooded (wet) lead acid batteries, gel cell batteries, and Absorbed Glass Mat (AGM) batteries.
Flooded (wet) lead acid batteries use liquid electrolyte - it is important to keep these batteries in an upright position and to prevent electrolyte from leaking.
These batteries must be monitored, and if needed, distilled water must be added when electrolyte levels fall down below a certain level. Since during operation, flammable hydrogen is released, they must be kept in a well-ventilated area.
Gel Cell batteries use electrolyte in the form of gel - diluted sulfuric acid is mixed with fumed silica to create gel which is placed between battery plates.
Such electrolyte allows the battery to operate in (almost) any position, to be very resistant to vibrations and mechanical impacts, to feature an increased number of charging/discharging cycles, to be (almost) spill-proof, etc.
Also, gel-cell lead-acid batteries tolerate lower charging and discharging currents than wet and AGM batteries and are most often used as deep-cycle solar/wind batteries.
However, some brands offer starting/cranking gel-cell lead-acid batteries for use in motorcycles, ATVs, boats, and similar - such batteries feature excellent resistance to vibrations and mechanical impacts and are spill-proof and maintenance-free.
Absorbed Glass Mat (AGM) batteries use electrolyte that is held in the glass mats made out of woven, very thin glass fibers.
AGM batteries may be found as starting/cranking, dual purpose, and deep cycle batteries, they are virtually maintenance-free batteries, very resistant to vibrations and mechanical impacts, can operate in (almost) any position, etc.
Both gel-cell and AGM batteries are often made as valve-regulated lead–acid (VRLA) batteries - safety valve inside the battery opens at a certain pressure (usually at 2 psi - 0.14 atm) and let oxygen flow from the positive plate to the negative plate to recombine with hydrogen and create water - no need to add any water during their operation.
Also, if the pressure inside the battery rises to a critical level, safety valves release hydrogen and oxygen out of the battery, preventing the battery from swelling. This makes AGM and gel-cell batteries lose their water, reducing their operating lifetime, but at least, it prevents the battery from bursting.
Such Sealed Lead Acid (SLA) batteries are maintenance-free batteries and are used more and more on fishing and leisure boats and yachts, cars, and other vehicles, as industry batteries, etc.
Lead acid batteries lose capacity rapidly if discharged using high currents, but they are immune to the memory effect - they can be charged regardless of their initial discharge state since the battery will not "remember" its previous discharged level. Actually, it is highly recommended to fully recharge lead-acid batteries after use, regardless of their Depth of Discharge percentage (DoD%).
Also, lead-acid batteries are relatively cheap, at least initially, when compared with other battery types, especially lithium-ion batteries. But, they are rather heavy in terms of energy-to-weight ratio.
Typical marine deep cycle lead acid battery has a capacity of around 20-30 Wh/kg when discharged in less than one hour (even less when discharged to 20% of capacity in less than 10 minutes) or 30-40 Wh/kg when discharged slowly, for example in a period of 20 hours.
When labeling their capacity, manufacturers provide capacity information when the battery is discharged slowly for a period of 20 hours, but they usually provide comparison charts about battery characteristics when the battery is discharged at different currents, voltages, and temperatures.
Since lead batteries easily tolerate high surge currents, more and more deep cycle batteries are used as starter batteries, too (for smaller gas and diesel engines) or as power sources for fixed or portable power centers.
Due to their weight, these batteries are often positioned low in the boat to lower the center of gravity and increase the stability of the boat.
General voltage ranges per cell are:
- at full charge: 2.10 V
- at full discharge: 1.95 V
- loaded at full discharge: 1.75 V
One of the advantages of lead acid batteries is that they can stay connected to chargers all the time (again: no memory effect, and in simple installations, these batteries can act as voltage regulators preventing voltage surges - not recommended for their longevity, but ...).
Float charging voltages are:
- 2.23 V for gel cell batteries
- 2.25 V for AGM batteries
- 2.32 V for flooded (wet) batteries
Note: These are typical values, and they may differ not only from manufacturer to manufacturer but also among the different batteries of the same manufacturer. Intelligent battery chargers have the option to allow the user to select the actual battery type, and after charging the battery, they automatically enter the maintenance charging mode.
It must be noted that these voltages are usually given at 20 °C (68 °F) or 25°C (77°F), and must be adjusted by approximately −0.0235 V/°C for temperature changes (for 12V (6 cells) batteries).
Again, float voltage recommendations vary slightly among manufacturers - this float voltage is critical for longevity since too low or too high float voltage can significantly shorten the life of the battery.
Voltages required for daily charging, equalization charging, and gassing threshold depend on temperature and, again, vary among manufacturers, but generally, they are:
- daily/cyclic charging voltage: 2.37–2.4 V
- equalization charging voltage: 2.5 V for no more than 60-120 minutes (wet cells - be sure to open the cells, monitor level of electrolyte, add distilled water if needed, monitor cells' temperature!)
- gassing threshold voltage: 2.4 V
Even so, lead acid batteries are IMHO the best choice for boats where the additional weight will NOT be a problem.
These batteries are relatively cheap, and newer deep-cycle, low-discharge, high-power (starting) sealed batteries can last for years.
Lead acid batteries can be connected in parallel and series as one needs in order to achieve higher capacities and/or voltages. However, great care and planning must be taken when doing something like that:
- by connecting batteries in parallel, one creates a battery pack of the same voltage but higher capacity. Due to slight differences in voltages between flooded (wet) cells, gel cells, and AGM cells, it is important NOT to mix those types of batteries. In fact, it is recommended to use THE SAME type of batteries, the same model with exactly the same capacity from the same manufacturer, preferably from the same batch! And the best solution is, if possible, to obtain a single battery of the required capacity.
- by connecting batteries in series, one creates a battery pack of the same capacity but higher voltage. Batteries of the same capacity must be used, and even then, batteries should NOT be discharged less than 20% of their capacity due to the danger of cell reversals. Again, try to use the same type of batteries, the same model with exactly the same capacity from the same manufacturer, preferably from the same batch!
These warnings may sound a little bit paranoid, but these things are not toys, they can cause fires, damage, and even harm people.
If you are not sure what to do, it is better to find professionals - just explain to them what you want or need, and they will either find a solution or find someone who can find you a solution.
Such services do cost money, but in the long run, they are worth it - or you would like to jump off the fishing boat due to fire, 20 miles off the shore, while your chum and baits are in the water and sharks might be around...? :o)
Deep Cycle Marine, Automotive, and Industrial Batteries
Deep cycle batteries can be divided into deep discharge lead acid marine/automotive batteries and non-lead acid marine/automotive batteries.
Of course, both of them have their strengths and weaknesses, but generally, lead acid batteries are heavier, cheaper, can be connected easily in parallel and series (just be sure to use the same batteries and to connect them properly), and IMHO, they are safer and more robust.
Non-lead acid batteries, especially lithium batteries, are very expensive and can have huge relative capacity (usually given in Wh/kg), and when discharged with higher currents, their loss of capacity is lesser when compared with lead acid batteries.
Unfortunately, they require special lithium battery chargers that constantly monitor battery cells in order to prevent overcharging and other similar problems.
Fortunately, most modern lithium batteries have internal sensors that prevent overcharging, overheating, deep discharging, and similar conditions - sometimes, they have internal converters and can be charged with ordinary lead acid chargers, but be sure to read the manuals of such batteries.
If not sure, ask for professional help - such batteries are potential bombs! But, when used properly, they provided 2-5x more energy per kg of battery when compared with lead-acid batteries.
If you need a battery for use on a kayak or any similar small boat and you have issues with weight, consider a good lithium battery for your trolling motor. But, if there are no problems with heavier batteries (that can be used as ballast to increase the stability of your boat), then go for cheaper lead-acid batteries.
The electric motor can't tell the difference between those battery types, and it doesn't care :)
Note: some manufacturers of AGM prefer their batteries to be charged with deep cycle battery chargers optimized for AGM lead acid batteries, and they even forbid charging them with chargers designed for flooded cells lead acid batteries - whatever you do, be sure to read the manuals! Don't lose a warranty for batteries that can cost hundreds of dollars/euros because of a cheap charger!
Deep cycle batteries feature relatively thick plates and are designed for deep charging/discharging cycles. Their ability to provide large currents is somewhat limited, and generally, they should not be used as starting/cranking batteries.
On average, deep cycle batteries may endure 250-500 cycles down to 80% DoD, much more if they are not discharged below 50% DoD.
Note that discharging current is also important, and the actual number of cycles depends on other things as well, like battery temperature, quality of the charger, current spikes, etc.
Smaller lead-acid batteries are often made as general purpose batteries, being optimized for cyclic operations, but are also able to provide decent short burst currents, often given as "Maximum 5 seconds current."
Large batteries, even when designed to be true deep cycle batteries due to their size, are often able to provide hundreds of CCA Amps.
Dual Purpose Marine, Automotive, and Industrial Batteries
Dual-purpose marine batteries are 'strong enough to provide more than enough current to start internal combustion engines and are designed to tolerate frequent deep discharges without negative influence on their capacity or time of operation.
These batteries combine the best of two opposite worlds - cranking batteries and deep-cycle batteries.
Dual-purpose batteries are often used not only as main boat/car/truck batteries but also as batteries for golf carts, Recreational Vehicles (RV), All Terrain Vehicles (ATV), Uninterruptible Power Supply (UPS), as wheelchair batteries, off-the-grid batteries, sump pump backup batteries, etc.
Dual-purpose batteries are usually Valve Regulated Lead Acid (VRLA) Sealed Lead Acid (SLA) batteries - either gel-cell batteries or Absorbed Glass Mat (AGM) batteries.
They don't require to be positioned vertically like flooded lead acid batteries, they are almost maintenance-free and very tolerant to vibrations and shocks - they will tolerate electric and physical abuse, just don't push them too hard (don't hit them with the hammer, anchor, chain, rock, etc.).
Dual-purpose marine batteries don't have to be lead-acid batteries. With the improvement of technology, lithium batteries are present more and more on boats, yachts, ships, cars, trucks, RV vehicles, etc., not only for powering small electronic devices but also as the main boat battery/battery pack.
Personally, if you need a new dual-purpose battery for your boat, go for tested and reliable sealed lead acid batteries. Such batteries are heavier but are cheaper, and weight difference means little on the boat. In fact, if such batteries are positioned low in the boat, they may even increase the stability of the boat.
On the other hand, if you need a light and powerful battery and don't mind spending a few extra dollars/euros, go for good lithium iron phosphate (LiFePO4) or similar lithium marine/automotive battery - such batteries can be used on small boats, kayaks, golf carts, RV and ATV and similar vehicles, where extra few pounds/kilograms means a lot.
Starting/Cranking Marine and Automotive Batteries
Starting/cranking batteries must be able to provide large currents for a very short period of time in order to be able to start an internal combustion engine (gas or diesel).
Starting batteries feature thinner plates with larger active surfaces, which are required for providing strong currents.
However, such plates are sensitive to deep discharge conditions since they can be easily deformed (among other things).
If your vehicle or vessel features one battery electrical system, but you don't often use the battery for lights, radio, stereo, and other electronics while the main engine(s) is turned off, starting battery is your best choice.
If you regularly discharge your battery down to 70% (or even more) of the full charge, consider getting dual purpose battery.
If your vehicle or vessel features a dual battery system, starting battery is used only for starting the main engine, and in such a role good starting battery can last for years.
Long Story Short: With the broad range of batteries on the market, getting the right one should be easy - just be sure to check your old battery and write down its dimensions, BCI Group, battery type, voltage, capacity, CCA, MCA, and other specifications.
At first, really good batteries may look expensive, especially the large and powerful dual-purpose or deep cycle AGM SLA batteries, but never forget that Your safety depends on the quality of these units, so don't go cheap with batteries - better safe than sorry.