Why Weight Matters for Marine Applications?
When selecting batteries for boats and marine applications, the weight of your boat is an extremely important consideration. Carrying excess battery weight in a vessel can substantially impact performance by making the boat heavier, slower, and more challenging to maneuver. For smaller boats like inflatable boats, kayaks, canoes, and other lightweight watercraft, heavy batteries can make it tremendously difficult to achieve proper balance and agilely navigate the boat. That’s why choosing lightweight, compact marine batteries for boating is absolutely essential for many boaters.
Specifically, heavier batteries raise the boat’s center of gravity, which can make it far more prone to tipping over. This makes the boat more unstable and less seaworthy. Extra battery weight may also require placing rear ballast to counterbalance the heavy battery bank, adding even more weight to the vessel. Too much weight diminishes the boat’s power-to-weight ratio, meaning the engine must work harder to achieve the same speed. This results in substantially increased fuel consumption, shorter range per tank, and slower planning speeds to reach peak velocity. Any reduction in battery weight translates directly into improved speed, handling, fuel efficiency, and safety.
For smaller recreational boats and watercraft under 20 feet long, shaving just 10-20 pounds of battery weight can have a noticeable impact on responsiveness and agility. Larger vessels have more capacity to absorb battery weight, but even an extra few hundred pounds of batteries will incrementally reduce performance. When batteries start tipping the scales over 200 pounds or more, the speed and handling penalties become significant. Boat weight directly influences how easily a boat gets “on plane” and how efficiently it cruises at its rated horsepower. So keeping batteries as light as possible pays dividends across all types of marine craft.
What is the Lightest Marine Battery Option?
Without question, the lightest marine batteries available on the market today are lithium-ion batteries. Lithium marine batteries typically weigh a staggering 60-70% less than comparable deep-cycle lead acid batteries of an equivalent capacity and energy storage ability. For example, a 100Ah lithium marine battery may weigh around just 20-30 lbs, compared to a lead acid battery (learn: What is the Battery Acid PH Level?) of similar capacity that weighs from 60-100 lbs. Some especially lightweight lithium batteries may even dip below 20 lbs for a 100Ah capacity.
Here is a comparison table showing the vast weight differences when looking at batteries of comparable amp-hour ratings and overall energy storage capacity:
|Battery Type||Typical 100Ah Battery Weight|
|Lithium Ion||20 – 35 lbs|
|AGM Lead Acid||60 – 80 lbs|
|Gel Lead Acid||60 – 80 lbs|
|Wet Cell Lead Acid||80 – 100 lbs|
As the table highlights, lithium batteries offer monumental weight savings ranging from 50-80% less than traditional lead acid batteries. This makes them the undisputed champion when it comes to the lightest marine battery option by a wide margin over any form of lead acid battery. Additionally, lithium batteries have a higher energy density, making them more efficient and longer-lasting than lead acid batteries.
What is the Typical Weight Range of Marine Batteries?
Marine batteries can vary tremendously in weight, ranging from featherlight options under 20 pounds for the most compact lithium batteries all the way up to backbreaking beasts over 120+ pounds for extra large capacity and heavy-duty lead acid batteries. Here is an overview of the typical weight range across the most common marine battery types:
- Lithium-ion: 16 – 32 lbs
- AGM Lead Acid: 30 – 60 lbs
- Gel Lead Acid: 35 – 75 lbs
- Lead-acid (wet cell): 50 – 120+ lbs
So lithium batteries, specifically lithium iron phosphate, are the clear lightweight winners, when it comes to choosing a type of battery for marine use. AGM and gel lead acid options occupy the middle ground, and traditional flooded wet cell lead-acid batteries are generally the heaviest marine battery option by a wide margin.
While the numbers above capture the general weight ranges, it is important to note that battery weights can vary quite a bit within each category based on the capacity, group size, terminal configuration, and robustness of the particular model. But the takeaway is that lithium batteries are far and away the lightest, followed by AGM and gel lead acid, with wet cell lead acid being heavyweight.
What are the Weight Differences Between Battery Types?
There are several key reasons for the considerable weight discrepancies between the major marine battery technologies. Here we will talk about the weights of 4 types of batteries:
Lead-Acid Battery Weights
Flooded lead-acid batteries are the heavyweight champions because the dense lead plates and liquid sulfuric acid electrolytes make these batteries extremely heavy for their size. A standard Group 27 deep cycle marine lead acid battery can weigh from 60-90 lbs or even more. The thick lead plates account for the lion’s share of that substantial heft. In fact, lead weighs approximately 0.41 pounds per cubic inch, making it one of the densest materials used in battery construction. The lead must also be formed into very thick and sturdy plates to withstand vibration, shocks, and pounding in marine environments. The number of charge cycles that a flooded lead-acid battery can handle is limited, and it is important to maintain and monitor the battery to ensure that it lasts as long as possible.
While lead has excellent electrochemical properties for storing energy, it comes at the cost of significant weight. Lead acid battery makers try to optimize the design by shaving down the plate thickness without compromising performance and longevity. But there are limits to how thin the lead can be while maintaining durability and cycle life. Expect 0.5-1 lbs per amp-hour of capacity for a lead acid battery. So a 100Ah wet cell battery will typically tip the scales in the 80-100 lb range depending on construction. In summary, heavy lead and liquid acid mean serious weight, but also deliver proven reliability and low cost for marine applications.
AGM Battery Weights
AGM batteries replace the liquid electrolyte used in flooded lead acid batteries with a fiberglass mat that absorbs and immobilizes the acid. By suspending the electrolyte into a semi-solid form, AGM batteries eliminate some of the weight associated with a liquid solution while still utilizing traditional lead plates to store energy. However, the fiberglass matting does add a small amount of extra weight compared to a pure flooded cell with just free-flowing electrolyte.
In an AGM battery, the lead plates themselves can be slightly thinner and lighter since the fiberglass mat provides some additional structural support. But the mat material offsets some of those savings. Overall an AGM battery will be approximately 10-30% lighter than an equivalent flooded cell lead acid battery. For a 100 Ah capacity AGM, expect a weight typically in the 60-80 lb range, shaving off roughly 15-20 lbs compared to a flooded battery. While not as astoundingly light as lithium-ion, the AGM design does noticeably trim some heft off versus wet cell lead acid models, making it a maintenance-free option.
Gel Battery Weights
Gel batteries use silica and other additives to turn the liquid electrolyte into a stabilized gel, which effectively eliminates the free-flowing liquid and thus shaves off a small amount of weight compared to wet cell batteries. However, the gel agents and other chemicals add back some mass. A typical 100Ah gel battery weighs around 60-80 lbs, putting it in the same ballpark as an AGM battery.
The gel does allow slightly thinner lead plates to be used since the gel holds its shape and provides support. By transforming the electrolyte from liquid to stabilized gel, overall weight drops by around 5-10% compared to an equivalent wet cell lead acid battery. While a low-end gel may hit 60 lbs, most are in that 65-80 lb range for 100Ah capacity. Better than a wet cell but not game changing. The gel improvement may give back 5-15 lbs, but does not dramatically alter the reality that you need strong back to move most lead acid batteries!
Lithium-Ion Battery Weights
Lithium-ion batteries are the undisputed featherweight champions because they use ultra-lightweight lithium cathode materials rather than heavy lead plates. This chemistry allows lithium batteries to weigh less than half of comparable lead acid batteries with an equivalent energy storage capacity.
A 100Ah lithium marine battery may weigh as little as 25-35 lbs, while a similar capacity lead acid battery is typically 80-100 lbs. This massive 50-75% weight reduction is possible because lithium-ion batteries do not use lead plates. Instead, they use highly energetic lithium metal oxide cathode materials layered with a liquid electrolyte and graphite anode.
Lithium weighs only 0.53 ounces per cubic inch, versus over 0.4 pounds per cubic inch for lead. By avoiding the use of heavy lead and acid, lithium marine batteries eliminate the bulk of weight associated with traditional batteries. The lightweight lithium cells also enable simplified and lighter supporting structures or casings since the batteries are not as heavy overall. The remarkable weight savings unlock performance benefits and new applications that are simply not possible with lead acid batteries.
What is the Weight of Different Battery Group Sizes?
For lead acid batteries, the physical group size and terminal configuration have a direct impact on total weight, with larger batteries that can output more power being considerably heavier. Some common group sizes for marine batteries are:
- Typical capacity range: 70-100Ah
- Weight range: 55 – 85 lbs
- Typical capacity range: 90-125Ah
- Weight range: 60 – 90 lbs
- Typical capacity range: 105-150Ah
- Weight range: 80 – 120+ lbs
So a Group 31 battery will generally be the heaviest, followed by Group 27, with Group 24 being the lightest of the three. The larger physical size required for increased power and capacity necessitates more lead material and electrolyte. This translates directly into a climbing scale weight as the group size gets larger.
Lithium batteries again break the mold, with weight staying relatively constant for a given capacity, rather than ballooning with group size. But for lead acid batteries, sticking to the smallest group size that meets your power needs is a good way to avoid unnecessary extra weight.
What are the Weight Differences Between Starting & Deep Cycle Batteries?
Deep cycle lead acid batteries designed for continuous discharge applications like trolling and accessories tend to be substantially heavier than starting batteries used primarily for engine ignition. The lead plates in deep cycle batteries need to be considerably thicker and more robust to withstand very frequent and deep discharge cycles without damage.
Starting batteries have thinner lead plates tuned to deliver strong bursts of power for ignition, but their plates cannot survive repeated deep draining like a true deep cycle battery. As a result of the thicker and more durable plates, deep cycle batteries weigh significantly more, often upwards of 50% greater than starting batteries with a comparable capacity rating.
For example, a 100Ah marine starting battery may weigh around 50 lbs while a 100Ah deep cycle battery can hit 80-100 lbs, despite having the same energy capacity on paper. The considerable extra weight of a deep cycle battery is necessary to achieve enhanced longevity and resilience for repeated heavy discharges under the taxing demands of trolling motors and electronics. So if speed and lightness are a priority, choose the lightest starting battery that can meet your power needs, and avoid beefy deep cycle models meant for cycling applications.
Does a Fully Charged Battery Weigh More?
When a lead acid battery is fully charged, it will weigh just a tiny bit more than when it is fully discharged. We’re talking about a few pounds difference at most between a depleted and freshly charged battery. This negligible weight fluctuation exists because when a lead acid battery is charged up, the lead plates chemically combine with sulfuric acid to form lead sulfate. Lead sulfate has a slightly higher density than pure lead, so fully charged plates are marginally heavier.
However, the total weight variation is small in the context of a 60+ pound battery. The fixed weight of the outer casing and inner supporting structure account for the vast majority of mass, with the variable lead material making up just a fraction. So the charge state has a minor influence on overall weight. For most practical purposes, the rated weight of a given lead acid battery model will stay constant regardless of the charging state. It is not worth splitting hairs over a few pounds either way. Evaluate battery weight based on the specs from a fully charged state for the most accurate idea.
Lithium batteries exhibit even less weight fluctuation from charging as lithium ions are extremely light. Choose lithium batteries based on the published specs without regard to charge level.
How Does Battery Weight Affect Marine Performance?
Heavier marine batteries can substantially slow down a boat, diminish handling and agility, and force an engine to work harder, especially on smaller class recreational watercraft. The excessive battery weight raises the vessel’s center of gravity, which can make it far more prone to tipping over or capsizing in rough seas or hard turns. Large banks of heavy batteries may necessitate placing rear ballast to counterbalance the weight, adding even more load to the boat.
All that excess weight also means the engine must labor intensely to reach planning speeds, cruising velocity, and peak speed, dragging the heavy boat along. This forces increased fuel consumption, raises the engine temperature, and reduces the overall range per tank of gas on board. Any reduction in total battery weight results in measurably improved speed, acceleration, handling, and safer operation across almost all types of marine craft.
Specifically, properly sized lithium batteries can optimize electrical power while slicing hundreds of pounds from the total load compared to lead acid batteries. With a lighter battery bank, the boat will plane faster, be vastly more agile, have substantially extended range, and be far easier to balance for safe handling. There are exponential performance and safety dividends by leaning out battery weight while still meeting your electrical demands.
Is There a Large Weight Difference Between Wet-cell and Dry-cell Marine Batteries?
Absolutely, there is a very significant weight difference between traditional flooded wet-cell lead acid batteries versus more modern dry-cell lead acid designs like AGM and gel batteries. Flooded wet cell batteries use a liquid sulfuric acid electrolyte solution, so they require much more robust external containment and internal structural elements to safely house the fluid. This adds considerable weight.
In contrast, AGM and gel batteries utilize a treated electrolyte medium that immobilizes the acid. By converting the battery to a “dry” internal design, AGM and gel batteries eliminate much of the heavy external housing and thick internal components needed in a wet cell. Compared to an equivalent wet cell, an AGM or gel battery will be 15-30% lighter.
For example, a 100Ah wet flooded cell lead acid battery will likely weigh around 80-100 lbs. An AGM or gel deep cycle battery (learn: What is A Deep Cycle Battery [Full Guide]) of the same capacity would hit 60-80 lbs, for a 20-30 lb weight decrease by ditching the liquid electrolyte. While still heavy machines, dry cell AGM and gel are decisively lighter than traditional flooded lead acid batteries.
How Does Marine Battery Weight Compare to Car or RV Battery Weight?
When looking at batteries of the same type, capacity rating, and construction, a marine battery will virtually always be moderately heavier than an equivalent car starting or RV house battery. Starting batteries for cars emphasize burst cranking power for ignition over longevity or resilience. Therefore they can get away with thin but fragile lead plates to save weight at the expense of cycle life.
RV house batteries see intermittent use and are not subjected to constant jarring and vibration like marine batteries. As such, RV batteries are designed more for lightness and space efficiency rather than ruggedness.
High-end marine batteries meant for deep cycling applications incorporate thick, fortified lead plates to withstand the demands of constant pounding in boat environments. This super sturdy construction adds weight compared to an automotive or RV battery, even if they have the same capacity on paper.
On average, expect a given marine battery to be 5-20% heavier than its car or RV counterpart, assuming the same battery type and rating (Related Article: Marine Battery vs Car Battery: A Comprehensive Comparison Guide). Lithium batteries are again the exception, with very similar weights for marine, car, or RV service given equivalent watt-hour capacity.
Here is a comparison table showing estimated weights for 100Ah batteries by application:
|Wet Cell Lead Acid||80 lbs||75 lbs||95 lbs|
|AGM Lead Acid||55 lbs||50 lbs||65 lbs|
|Gel Lead Acid||60 lbs||55 lbs||70 lbs|
|Lithium-ion||28 lbs||26 lbs||30 lbs|
So while marine lead acid batteries tend to be a bit beefier and heavier, a 100Ah lithium battery will be featherlight regardless of deployment.
How to Calculate Total Battery Weight for Your Vessel?
To properly estimate the total battery weight for a marine vessel:
- Make a list of all batteries required, along with their rated capacity. For example:
- 1 x 100Ah deep cycle trolling motor battery (learn: Best Trolling Motor Battery [Top 10 Picks in 2023])
- 1 x 75Ah cranking battery
- 2 x 50Ah accessory batteries
- Look up the individual weight of each specific battery model. For example:
- 100Ah deep cycle: 95 lbs
- 75Ah cranking: 45 lbs
- 50Ah accessory: 35 lbs each
- Multiply the number of batteries times the weight of each model.
- 1 x 95 lbs = 95 lbs (deep cycle)
- 1 x 45 lbs = 45 lbs (cranking)
- 2 x 35 lbs = 70 lbs (accessory)
- Add up the weight of all batteries:
- Deep cycle: 95 lbs
- Cranking: 45 lbs
- Accessory: 70 lbs
- Total: 95 + 45 + 70 = 210 lbs
This provides a realistic estimate of the total battery weight your boat will be carrying, so you can evaluate the impact on performance and handling. Repeating for alternative battery types or capacities lets you compare weights.
How to Choose Between Weight, Performance & Cost?
When choosing marine batteries, boaters must balance tradeoffs between battery weight, performance, and upfront cost. Lithium batteries deliver stellar performance and featherlight weight, but at a steep price. Basic flooded lead acid batteries are generally the most affordable and time-tested, but weigh a ton.
Finding the optimal marine battery means identifying the sweet spot with the best blend of reasonable cost, low weight, and sufficient performance for your intended application. Prioritize your needs:
- If budget is paramount, standard lead acid offers proven reliability at a low cost, despite the bulk.
- If performance matters most, lithium batteries are unrivaled, justifying their premium price for many buyers.
- If reducing weight is critical, lithium or AGM (learn: AGM vs. Lithium: 6 Differences to Consider) lead acid provides the best compromise of moderate cost and lightness.
There is no universally “best” marine battery. Choosing the ideal power solution for your boat comes down to judiciously balancing weight, performance, and budget based on your needs.
Marine battery weight varies greatly, from under 20 pounds for lithium to over 100 pounds for large lead acid batteries. While heavy, lead acid remains popular for its low cost and durability. AGM and gel technologies trim lead acid weight by 10-20%. But lithium batteries are in a league of their own, weighing 60-70% less than lead acid. Reduced mass improves speed, handling, and fuel efficiency, so go as light as your budget allows. Use battery weight specifications and group sizes to calculate the total load. And consider performance and cost tradeoffs to select an optimal marine battery setup for your vessel.