Buying a new battery for your trolling motor can be confusing affair. Not only are there various makes and models on the market, you actually have an assortment of battery types too.
That’s why we have created this ultimate trolling motor battery buyer’s guide. Not only have we gone in depth on the options available to you, we have also reviewed 5 top rated batteries to help you with your search.
For our findings at a glance, just head to the table below. For our ultimate buyer’s guide, plus comprehensive reviews, keep on reading.
Best Trolling Motor Battery Top 5
12 Volt 35AH
Marine Deep Cycle
18-35lb Minn Kota,
Cobra, Sevylor & More
(12V 35AH, GROUP U1)
Deep Cycle Marine
BEST BUDGET BUY:
70% longer cycle
lifeup to 400 cycles
recharge in 4-6hrs
12 Volt 35 AH
(12Volt; 35 AH)
What to consider when buying a trolling motor battery
When selecting the right troll motor battery, one needs to consider the weight, thrust, and size of the boat, alongside the fishing condition and weather.
The weight and thrust of the battery are related, as a heavier boat requires more thrust, and because the trolling unit is sometimes used to aid precision maneuvering of the vessel, it is necessary for one to acquire a large battery with a high discharge capacity.
Likewise, boat size determines the thrust needed, and thus large boats need large, high capacity boats, while small boats can do well with compact batteries that can fit in their battery compartments.
When boat thrust is considered in isolation, the battery must be able to power the trolling motor unit so that it can accelerate the boat forward.
The motor power needed is usually measured as pounds of thrust, with the current recommendation being that the motor unit should provide 5 pounds-of-thrust for each 200pounds of boat weight.
The battery must be able to provide the motor unit with the required electrical power for it sufficiently accomplish this.
The weather condition also affects the operation of the trolling motor unit, with windy weather causing the unit to draw more power from the battery.
On the other hand, fishing during calm weather helps save battery power.
With regards to fishing conditions, more battery power is needed for trolling in swampy or marshy water as compared to trolling in calm lakes.
Likewise, deep water fishing requires more trolling power as compared to low water fishing. Therefore, one can align his/her fishing needs and fishing time with the approximate battery capacity available.
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Trolling Motor Battery Reviews
In the modern market, there are different brands, models, types, and sizes of trolling motor batteries.
For one to find the best trolling motor battery for his/her boat, one needs to consider the above-mentioned factors, along with the battery cost, construction quality, and design quality.
High-quality batteries are durable and long life-span.
Using the aforementioned criteria, the different battery models in the market can be rated and graded.
This allows for the identification of the best trolling motor batteries. The following review focuses on the top 5 battery models.
1. TOP PICK: VMAX857 Hi-Performance AGM Trolling Motor Battery Review
The VMAX857 is a compact, high performance 12-volt deep cycle SLA AGM marine battery with a recommended maximum depth of discharge of 60%.
Its maximum discharge capacity is 35Ah. It can also be purchased as a full kit, which means that the battery comes alongside its battery box, trickle charger, 60amperes fuse, and critical wiring.
Likewise, it is built to be stable, reliable, and durable; and its superior build quality makes it resistant to vibrations and shocks.
According to BCI battery standards, it is grouped as a U1 battery. Its lifespan ranges from 5-8years if one uses it constantly, and greater if used sparingly and stored well when not in use.
Moreover, because it uses AGM to immobilize its electrolyte and reduce the rate of electrolyte evaporation though water electrolysis, this model is considered as maintenance-free as one does not need to top-up the electrolyte level.
Additionally, because its electrolyte is non-spillable, this product is rated as non-hazardous.
This compact, medium-weight sealed AGM battery weighs about 24pounds, and is 7.7inches long, 5inches wide, and 6.1inches tall.
Its beautifully-designed battery casing features a hard/tough seal that makes it not only spill-proof, but also reliable and resistant to extreme weather condition, in addition to being heat-resistant. This case is made from high-quality ABS plastic.
The recommended charger to be used with this model is the 12-volt, 3.3-amp VMAX BC1204 charger. The recommended charging current and voltage are 3A-15A and 14.4-14.9V (volts) respectively.
The average recharge duration is 75minutes. Its terminal posts are 6mm in diameter, which allows one to easily fit in battery terminal or crocodile clips.
Once the battery is fully charged, it needs to be maintained by a floating voltage of 13.5V-13.8V so as to compensate for its low-grade self-discharge.
Construction-wise, this battery features heavy-duty electrode grids that are made from lead-tin alloy, and this not only improves its discharge performance, but it also increases its service life whether it is used in cyclic or float applications that require constant recharge.
These military-grade alloyed plates also support the electrolyte suspension system that gives this battery is versatility.
This model is design to work best with trolling motor units that can produce 18-40 pounds-of-thrust which is enough for accelerating loaded boats weighing between 360pounds and 800pounds.
When powering an 18pounds -of-thrust motor, it can run for about 9hours, and this reduces to 4hours when it powers a 40 pounds-of-thrust troll motor.
It has an MCA of 390 and a CCA of 300. Its operating temperature ranges are -4 to 140°Fahrenheit[F] during discharge as well as during storage, and 14-140°F during recharge.
This model is supported by a limited 12-month warranty for factory defects.
Even so, with a maximum discharge capacity of 35Ah, this model is unsuitable for users who aspire to enjoy long trolling trips deep into the lake or sea.
The VMAX857 V35-857 high performance AGM troll motor battery is our top pick choice for good reason.
The 12-volt, slow discharging deep-cycle SLA battery is a versatile beast, designed to run some of the best trolling motors on the market.
Whether you’re using a MinnKota, Cobra, and Sevylor units you will achieve superior performance using the VMAX.
And let’s not forget the build quality too. Its heavy-duty grids are made of military grade lead-tin alloy, and this gives it an average lifespan of 6years of continual use.
The battery is also resistant to vibrations, extreme weather, heat and shocks making it an ideal choice for out on the water.
With a relatively cheap price, long life span and high-quality design, in our opinion the VMAX857 is the best trolling motor battery on the market today.
2. Optima OPT8016-103 Trolling Motor Battery Review
This offering from Optima is a heavy-weight and high-power density 12-volt DC dual-purpose model that can operate as either a starter battery or a deep-cycle battery.
Its blue top identifies as a starter battery, as this color code is used to denote a starter lead-acid battery.
The spiral cell design gives it greater discharge efficiency, which also improves its shelf-life. It also endows it with a high energy reserve capacity that makes it ideal as a starter SLA that can provide large surge currents, that can even start the fuel-powered outboard engines of boats.
In fact, its reserve capacity is 120minutes of continuous use. As a high-quality model, it is built to be stable, reliable, and durable; and its superior build quality makes it resistant to vibrations, extreme weather, shocks and heat.
It is designed, manufactured, and warranted by the U.S-based company, Optima Batteries, Inc. Its model number is OPT8016-103, however the sub-model number is D34M which denotes its blue top.
This model is a high performance, deep cycle SLA AGM marine battery with a discharge capacity of 55Ah and a high DoD. According to BCI battery standards, it is grouped as a Group 34/78 battery, which means that its primary utility is as a starter battery.
Its lifespan is 6-7 years if used properly.
Moreover, it is described as maintenance-free as it uses high-quality AGM to immobilize its electrolyte and reduce the its rate of loss though water electrolysis.
It is also grade as non-hazardous because its electrolyte is non-spillable.
This compact, yet heavy troll motor battery weighs about 43.5pounds, and is 10inches long, 6.9inches wide, and 7.8inches tall.
It is well designed for aesthetics and a sturdy build. It is also spill-proof which allows one to place it in any orientation in the boat. Its color scheme is a blue top and a gray case.
It can be charged with any 12-volt, 10A DC smart charger. The recommended charging current and voltage are 2A-10A and 13.8-15V respectively.
The average recharge duration is 120minutes. Its dual SAE terminal posts are made from stainless steel, and they are 5/16inches in diameter, which allows one to easily fit battery terminals or crocodile clips on them.
The floating voltage that is needed to maintain a fully-charged battery is 13.2V-13.8V.
It has an MCA of 870 and a CCA of 750. Its operating temperature ranges are -4 to 140°F during discharge as well as during storage, and 14-140°F during recharge.
This model is supported by a limited 24-month warranty for factory defects.
The Optima OPT8016-103 is a compact, heavy-weight and high-power density 12-volt DC dual-purpose model that can operate as a starter or deep-cycle battery.
Its cylindrical cells house spiral electrode plates that give it a high energy reserve capacity.
Also, its high-quality build makes it resistant to vibrations, extreme weather, heat and shocks. It also has an average life span of 6.5years.
Highly recommended overall.
3. Odyssey PC680 Trolling Motor Battery Review
The Odyssey PC680 is a medium-weight 12-volt deep-cycle AGM troll motor battery with a ruggedized build that belongs to the Extreme Series of its PowerSports brand of SLA batteries.
As expected, it is designed for use in tough conditions, and as such it is built to withstand extreme vibrations, intense shocks, rapid heat changes, and extreme weather.
It has a maximum DoD of 80%, and its nominal capacity is 16Ah. This stable, durable, and reliable model uses pure lead in its electrodes, which allows for stable voltage release during the discharge phase.
It is designed, manufactured, and warranted by Odyssey Batteries. Its model number is PC680.
This model has a reserve capacity of 24minutes. Its lifespan ranges from 3-10years depending on how one uses it.
Also, because it immobilizes its electrolyte in the woven AGM fabrics, it has an extremely low rate of electrolyte loss hence requiring minimal maintenance in terms of electrolyte topping-up, and is thus considered as maintenance-free.
However, even though this electrolyte is non-spillable, this product is not rated as non-hazardous, and it comes with a Hazmat (hazardous material) marking.
This compact, well-designed light-weight spill-proof AGM battery weighs 15.4pounds, and is 7.15inches long, 3inches wide, and 6.65inches tall.
Its strong and well-molded battery casing is made from high-quality heat-resistant plastic. Its color scheme is black body with a red top.
As a spill-proof model, it can be oriented in one of 3 ways in the battery compartment; upright, upside down, and on the side.
It can also be charged with any 12-volt DC smart charger, and one needs to be careful to avoid overcharging it.
The recommended charging current and voltage are 2A-10A and 13.5-14.4V respectively. The average recharge duration is 6hours.
Its brass terminal posts are tin-plated so as to improve their conductance capability, as well as make them resistant to acid corrosion.
These terminals are designed for placing normal battery terminals and crocodile clips. The floating voltage that is needed to maintain a fully-charged battery is 13V-14V.
This battery has an MCA of 280 and a CCA of 170. Its operating temperature ranges are -4 to 75°Fahrenheit[F] during discharge as well as during storage, and 25-140°F during recharge.
This model is supported by a limited 24-month warranty for factory defects.
The main demerit of this battery is that it is unsuitable for users who want to have long trolling trips.
The Odyssey PC680 is a well-designed 12-volt deep-cycle AGM troll motor battery with a ruggedized build that is designed for use in tough conditions.
As one of the more robust batteries on the market it can withstand extreme vibrations, intense shocks, rapid heat changes, and extreme weather.
It has a maximum DoD of 80%, and its nominal capacity is 16Ah. This stable, durable, and reliable model uses pure lead in its electrodes.
With a life span that can reach up-to 10 years, this is actually the most cost effective battery in our top 5 due to longevity alone. Well worth checking out.
4. Mighty Max ML35-12 Trolling Motor Battery Review
The Mighty Max ML35-12 is a low budget, 12-volt DC AGM troll motor battery whose electrode grids are made of calcium-impregnated lead alloy grid.
It has a high DoD. It is specially designed to power the Minn Kota Endura C2 trolling motor unit. Also, it is built to be stable, reliable, and durable; as well as resistant to vibrations and shocks.
This model is designed, manufactured, and warranted by the China-based Company, Mighty Max.
As an AGM-SLA battery, its electrolyte is immobilized which reduces the rate of electrolyte evaporation, hence making this model a maintenance-free model.
Likewise, as the electrolyte is non-spillable, this product is rated as non-hazardous. The calcium in its electrode grids speeds up the gas recombination process so that gases produces during recharging are converted back into liquid water.
This well-designed, medium-weight sealed battery weighs about 24pounds, and is 7.7inches long, 5.2inches wide, and 7.1inches tall.
The battery casing features a hard/tough seal that makes it spill-proof and resistant to extreme weather and rapid heat changes. Its color scheme is black.
This model features integrated screw terminals which allows one to securely attach the motor leads to it. It can also be charged with any 12-volt DC smart charger, and this means that one must be careful not to overcharge it.
The recommended charging current and voltage are 3A-15A and 14.4-14.9V respectively. The fully charged battery needs to be maintained by a floating voltage of 13V-14V. Even so, its rate of self-discharge is quite slow.
This model uses heavy-duty catalyzed electrode grids which improves its discharge performance, as well as increases its service life.
It also uses an electrolyte suspension system that allows one to place this battery upside-down in the boat without any electrolyte spillage occurring.
This versatile, power efficient battery is design to work best with trolling motor units that can produce up-to 24 pounds-of-thrust which is enough for accelerating loaded boats weighing about 450 pounds.
This battery is supported by a limited 12-month warranty for factory defects.
Its key demerit relates to its energy rating, which makes it unsuitable for anglers who favor long trolling trips over short trips.
The Mighty Max ML35-12 is a low budget, 12-volt DC AGM troll motor battery whose electrode grids are made of calcium-impregnated lead alloy grid.
It is built to be stable, reliable, and durable; as well as resistant to vibrations and shocks. Moreover, its relatively cheap cost as compared to the other 4 models in this review makes it the best budget trolling motor battery in this review.
If you are on a tight budget, this is the trolling motor battery for you.
5. UPG 85980/D5722 Sealed Lead Acid Battery Review
The UPG is a compact, yet powerful 12-volt deep-cycle SLA battery that is built to provide 35Ah. It has a high DoD which allows it to run troll motors for short fishing expedition.
Regarding design quality, it is built to be stable, reliable, and durable. Likewise, it is resistant to vibrations and shocks.
This model is designed, manufactured, and warranted by Universal Power Group (UPG).
Like other high-quality SLA batteries that pack a lot of electrical energy and power in a compact size, the electrolyte in UPG D5722 is immobilized, and this serves to reduce its rate of evaporative loss, hence obviating the need for electrolyte refilling, thus making this model characteristically maintenance-free.
Moreover, the electrolyte is non-spillable, and hence this product can be regarded as non-hazardous so long as its top cover is still intact.
This black, well-designed battery weighs 22.5 pounds, and is 8.1inches long, 5.9inches wide, and 8.1inches tall, which gives it a characteristic rectangular shape.
Likewise, it is made up of individual rectangular cells. Its beautifully-designed battery casing features a hard/tough seal that makes it spill-proof and resistant to extreme weather and rapid heat changes. It has a lifespan of 5 years.
This battery has integrated screw terminals which allows one to securely attach the troll motor leads to it. However, this present challenges when one wants to use alligator clips or battery clamps.
Another merit of this SLA battery is that it can be charged with any 12-volt DC smart charger. Even so, one must be careful not to overcharge it.
The recommended charging current and voltage are 5A-10A and 14.5V-14.9V respectively. The fully charged battery needs to be maintained by a floating voltage of 13.6V-13.8V. Even so, its self-discharge rate is low compared to the electrical energy it stores.
This battery has heavy-duty electrode grids, and this enhances its discharge performance, as well as increases its service life.
Also, its electrolyte suspension system allows for its placement in any orientation in the boat without any instance of electrolyte spillage.
This battery is supported by a limited 12-month warranty for factory defects only.
Its main demerit is that it cannot be used in long trolling trips.
The UPG D5722 is a compact and powerful 12-volt deep-cycle SLA battery that is built to provide 35Ah.
It has a high DoD which allows it to run troll motors for short fishing expedition. It is also built to be stable, reliable, and durable.
Another worthwhile entry in our top 5 trolling motor battery list.
The MinnKota is compact, lightweight battery-holding case that is designed to be fitted on the transom ( in the stern of the vessel).
It is built to hold 2 types of batteries; the group 24 batteries and the type 27 DC rechargeable batteries.
It can safely store the battery away from the passenger compartment and fishing platform, and this gives one ample trolling space.
It can also be used as a battery carriage case that allows to safely transport a troll motor battery.
The most outstanding feature of this model are the built-in accessories in its cover plate, which transforms it from a battery case into a trolling motor power center.
These accessories include external battery terminals, built-in battery meter, accessory plugs, and reset circuit breakers. The orientation of these accessories is described hereafter.
The cover plate is hinged to the case body, and there is also a strap that wraps around the middle sector of the entire casing.
On either ends of the front panel of this (cover) plate are the positive and negative battery terminals that allows one to connect charger leads and trolling motor leads to the SLA battery without opening the case.
Likewise, on either side of the plate are the pair of 12-volt accessory plug.
Additionally, the most prominent accessories are located on the right top-side of the plate, with the 60amperes trolling motor circuit breaker located near the front edge, and directly behind it is the 15amperes boat accessory circuit breaker (for the accessory plugs).
Near the back edge is the LED battery meter that displays the real-time charge status of the battery contained therein.
The circuit breakers are designed to be reset manually. Relatedly, the handle of this casing is designed to support up-to 50pounds of weight.
It also comes with metal screws and 2 rubber strips which allows one to screw the strips onto a strong surface, and then feed the straps through the strips hence securing the battery casing in a stationary position.
Group 24 and Group 27 batteries describes the group number assigned to marine batteries by the battery trade association called Battery Council International (BCI).
BCI classifies marine lead-acid batteries based on their voltage rating, size, and whether they are deep-cycle or starter batteries.
Group 24 describes 12-volt, deep-cycle batteries that measure 11.13inches in length, 6.6inches in width, and 9.25inches in height.
The grouping covers AGM and Gel SLA batteries with a power-rating of 80Ah. On the other hand, Group 27 covers 12-volt, deep-cycle rechargeable batteries with a discharge capacity of 90Ah, and whose dimensions are 12inches long, 6.63inches wide, and 9.06inches tall.
Like Group 24 batteries, most Group 27 batteries are AGM and Gel SLA batteries.
This means that MinnKota battery power center is best suited for carrying and storing AGM and Gel SLA batteries whose length does not exceed 12inches, and whose width and height are less than 6.7inches and 9.1inches respectively.
This light-weight model weighs only a pound, and is 15inches long, 8.7inches wide, and 15inches tall which makes it ideal for placement in small boats. It is built from strong durable materials that are resistant to corrosion and acid degradation. Its color scheme is black.
The key demerit of this model is that comes without a manual or notice of how one is to use the rubber strips.
This holding case with a 12-month warranty.
Trolling Motor Battery Buyer’s Guide
What is the trolling motor?
Recreational fishing still remains one of the most popular outdoor activities.
To further improve the fishing experience, modern fishing boats come with a built-in trolling motor unit which not only obviates the need for paddling, but also allows one to move the boat at different speeds so as to reach places where there are many fish.
The trolling motor is usually designed as a self-contained unit with a propeller, electric motor, and associated controls; and is fitted in the bow or stern of the boat of an angler.
For this motor unit to work, it needs a source of energy, which is normally provided by a specially designed direct-current(DC) portable battery called a trolling motor battery.
Even so, in some unique boat models, the trolling unit is powered by a secondary gasoline-powered outboard.
Apart from trolling for fish, the trolling motor unit can provide auxillary power in the boat which that allows for precision maneuvering.
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The electric trolling motor
Most trolling motor models are electrically-powered, and the basic components of these electric models are the brushed DC motor, watertight motor housing, propeller fitted on the propshaft, and a tiller with a control knob in hand-controlled unit or a foot control in foot-operated units.
Modern models also come with wireless remote for speed control and serve-controlled steering. For all these components to work properly, they must be supplied by adequate electric power for the considerable time that one is fishing.
This means that the trolling motor battery must be able to discharge slowly, while at the same time provide enough power to consistently operate the trolling motor unit.
For this reason, the battery models that are used are those with a high depth of discharge(DoD). As expected, these batteries are rechargeable, and are thus classified as secondary batteries.
The main types of trolling motor battery
With regards to battery construction and operation, the lead-acid batteries are further categorized into 2 groups; the flooding deep-cycle batteries, and the maintenance-free sealed, valve-regulated batteries. These batteries are briefly described hereafter.
The lithium-ion batteries use electrodes made of lithium, lithium alloys, and/or intercalated lithium compounds.
It operates by moving lithium ions through a pressurized flammable electrolyte hence its designation as lithium-ion cells.
During recharge, the positively-charged lithium ions move from the positive electrode to the negative electrode, and during discharge, the movement of lithium ions is reversed.
The energy-dense batteries with low self-discharge and tiny memory effect that allows them to hold charge for ling periods of time are designed for high performance per unit weight, and hence allowing for small batteries to power large appliances.
However, they suffer from major disadvantage which is related to its safety profile, and this is the risk of fire if it is overcharged or if a short circuit is established between the electrodes.
Moreover, because the electrolytes are highly pressurized, this fire can result in a large explosion; and this is what happened to the lithium-ion batteries in Samsung Galaxy smartphones which forced Samsung to recall them in 2016.
By inference, if a battery fire occurs in the much larger lithium-ion battery powering a troll motor unit, then the resulting explosion can severely damage the ship, and even cause it to sink or in extreme cases, kill the occupants.
Another demerit of these batteries is that they are more expensive than their predecessors, the lead-acid batteries.
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The lead-acid battery is a rechargeable battery whose positive and negative plates are made of lead dioxide and lead respectively, with sulphuric acid serving as the electrolyte.
The charging and discharging process rely on the electrochemical reaction of dissolving or generating lead sulphate. In the fully discharged state, lead sulphate is formed on both the positive and negative plates, while the electrolyte is diluted due to the formation of water.
This is because electrons are conducted from the negative terminal, through an electrical appliance, and then back into the battery through the positive terminal. This is best represented by the following ionic reactions:
The Charging Process
During the charging process, current flows into the positive terminal and the reverse reaction occurs:
PbSO4(s) + 2e– + 2H20(l) → PbO2 (s) + H2SO4(aq) + H2(g)
This means that hydrogen gas is released on the positive terminal during charging, and for this reason, the battery must not be placed in a well ventilated room during charging. Meanwhile, on the negative terminal:
PbSO4(s) + 2H+(aq) + 2e- → Pb(s) + H2SO4(aq)
As is evident, charging restores the acid concentration in the battery, but loss of hydrogen ion in the form of hydrogen gas leaves the battery. In case of overcharging, electrolysis of water occurs and this results in the generation of hydrogen and oxygen gas:
2H+ + 2OH– → 2H2(g) + O2(g)
For this reason, lead-acid batteries need to be topped up with water as overcharging causes water loss.
The molecular weights of the above-mentioned reactants in the lead-acid battery add up-to 642.6 grams per mole, which means that an ideal 12-volt battery has a discharge capacity of 83.4 ampere-hours (AH) per kilogram weight.
However, in practical use, the discharge capacity is lower because the weight of case, non-reactive battery components, and water (in the battery) reduces the overall mass of reactants.
Compared to lithium ion batteries, lead-acid battery have a lower energy-to-volume ratio, as well as a lower energy-to-weight ratio, but they do compensate for this by having a higher capacity to supply large surge currents which ultimately gives them a higher power-to-weight ratio as compared to lithium-ion batteries.
This large power-to-weight ratio, and the relatively cheap cost of lead-acid batteries makes them more popular than lithium-ion batteries for use in troll motor units.
Even so, when operating troll motor units, surge currents – such as those provided by surge batteries (also called starter batteries) – are not needed, and for this reason, a special type of lead acid battery called a deep-cycle battery is used.
The Deep Cycle Battery Process
This battery is designed to slowly discharge up-to 75% of its discharge capacity, and this gives it a large DoD that makes it useful in running troll motor units which need to be operated for hours on end when one is fishing.
Deep-cycle batteries can be categorized into flooded types and sealed types.
Flooded batteries, also called flooded lead-acid (FLA) batteries use sheet plates as negative electrodes and rows of cylinders for as positive electrodes, and these electrodes are fully submerged into the liquid electrolyte hence the designation flooded.
There are also separators placed between adjacent positive and negative electrodes, and this not only prevents short circuits, but also streamline the shedding of active materials in the battery, hence creating a power-efficient closed circuit.
The geometry of the tubular positive electrodes in a deep-cycle battery is what gives these batteries a greater discharge capacity than the normal starter battery that uses only sheet plates.
Even so, some deep-cycle batteries also use sheet plates as positive electrodes, but these sheets are generally thicker than those found in starter batteries.
a) Flat-plated batteries
These category of deep-cycle batteries are called flat-plated batteries so as to differentiate them from the tubular-plated (also called Opzs) batteries. One major disadvantage of flooded batteries is spillage can occur when they are subjected to vibration or jolting.
Likewise, this requires one to place them upright on a flat surface. Another disadvantage is that it requires maintenance which is done by constantly topping up the electrolyte so as to ensure that its level is always above the electrodes.
This ensures that there is sufficient reservoir to compensate for water loss during charging. This also means that one needs to constantly inspect the electrolyte level.
The principal cause of failure of FLA batteries is insufficient electrolyte capacity.
b) Sealed lead acid (SLA) batteries
To address the construction and operational disadvantages of FLA batteries, a new generation of deep cycle batteries called sealed lead acid (SLA) batteries were developed.
These batteries are designed with a valve regulation as a safety mechanism, and they are therefore oft-times described as valve-regulated lead-acid (VRLA) batteries.
As its names suggests, the SLA battery comes in a sealed casing that incorporates a valve mechanism, and because of this construction design, one is not required to top up the electrolyte hence its designation as a maintenance-free battery.
Also, because electrolyte spillage is not possible in these sealed batteries, they can be placed in any orientation, including upside down or their sides in the battery compartment of the boat.
The electrodes of an SLA battery can be flat plates similar to the flat-plated FLA or they can be spiral rolls which make up individual cells.
Even so, the flats plates in SLA batteries can be oriented horizontally (in the so called pancake style) so as to improve their cycle life, unlike flat-plated FLA batteries whose plates must always be oriented vertically.
The dangers of a closed sealed battery compartment
Because the recharge process creates hydrogen gas which accumulates inside the battery, there is need for a valve to allow the gas to escape so as to prevent dangerous build-up of pressure inside a closed sealed battery compartment that can cause it to not only swell, but even explode in extreme circumstances.
This is caused where overcharging causes a rapid build-up of pressure due to rapid generation of hydrogen and oxygen gases.
This pressure-operated valve (or pressure valve) is set to automatically open when gas pressure levels reaches a certain level, usually 2 pounds-per-square-inch(psi) in rectangular cells and about 40psi in spiral cylindrical cells, hence the designation valve-regulated batteries.
The need for setting the gas blowoff pressure considerably high, rather than simply letting the gas vapors escape as they are created is to allow the low pressure hydrogen and oxygen gases in the sealed case to recombine and form water which restores the electrolyte mass in the battery.
Likewise, gas diffusers in the cell covers of SLA batteries allow for excess hydrogen gas to safely disperse.
The 3 types of SLA Battery
SLA batteries can be categorized into 3 types based on the nature of their electrolytes; the wet cell, the gel cell (also called a silicon battery) and the AGM(absorbent glass mat) cell.
The wet cell
The wet cell battery contains the electrolyte in its liquid form, and as expected, the electrolyte level is usually set to submerge the electrodes.
As expected, vibrations can cause the electrolyte to be splashed around inside the battery. Likewise, it is not advisable to place these wet SLA batteries upside-down, or on their sides.
To counter this problem, the electrolyte needs to be immobilized, and this can be done by mixing fumed silica with the sulphuric acid so as to create a gelified immobile electrolyte.
The gel cell
The SLA battery that uses this gel-like electrolyte is called a gel SLA cell. The immobility of the electrolyte not only improves its resistance to shock and vibration, but it also reduces the degree of electrolyte evaporation, besides preventing electrolyte spillage.
Moreover, gel cells can be oriented upright, upside-down, or on its side. Additionally, in gel batteries, calcium is incorporated in the lead plates where it catalyzes the gas recombination reaction.
Glass mats (AGM cells)
Another way to immobilize the electrolyte is to hold in glass mats made of finely woven glass fibers. This mat also serves to increase the contact surface area between the electrolyte and the electrodes.
Another advantage of using glass mats is that it allows for molding of the cells into flat, bent or rolled versions.
SLA batteries that use these glass mats are called AGM cells.
Despite the designation absorbent, the glass fibers cannot absorb the electrolyte, and this ensures that the entire electrolyte mass is always available for discharge.
Factors to consider when choosing a trolling motor battery
The main component that is powered in the trolling motor unit is the brushed motor, and the power ratings of this motor is used to determine the most appropriate battery discharge capacity that is needed to run the unit.
Even so, there are other factors apart from discharge capacity that one needs to consider and the main factors are discussed hereafter.
As mentioned above, the principal factor to consider is the discharge capacity, and this is usually stated in Ah (ampere-hours). To choose the right battery, one needs to check the power rating of the motor, especially its current draw.
This is because the power rating is a product of current and voltage using the following simple formula:
Electrical power (in watts) = Current (measured in amperes) X Voltage (in volts).
This also allows for calculation of the current draw using the following formula:
Current = Power/Voltage
As the battery is rated in ampere-hours, one can find out how long a battery is expected to last before it is fully discharged using the following derived formula:
Battery capacity (in Ampere.hours) = Current X Number of hours,
this allows for the following derivation;
Number of hours = Battery capacity/ current draw.
For example, for a 12-volt troll motor unit rated 40 watts, then a 100 A.h battery can power it for:
Number of hours = (Battery Capacity X Voltage) / Power rating (here, current draw is substituted by power/voltage).
Number of hours = (100Ah X 12 volts)/ 40 volts.amperes (or watts)
= 1200 / 40 hours
= 30 hours.
However, as the DoD is usually set for 75%, then it is recommended that the troll motor unit be powered for (0.75 X 30) hours or 22.5 hours before it is recharged.
A related factor to consider is the marine cranking amperage (abbreviated as MCA). The MCA rating is the maximum current draw that the battery can support for half-a-minute (30 seconds) at 0° Celsius/Centigrade (or 32°Fahrenheit[F]) for the voltage to drop to 1.2volts per cell (or 7.2volts for a 12-volt battery).
The MCA rating of troll motor batteries are usually designated is Cold Cranking Amperes (CCA), as power is delivered at the melting point of pure ice at sea level.
Therefore, a battery whose rating is 100CCA can provide 100amperes for 30 seconds at freezing temperature. This CCA rating allows for comparing and benchmarking batteries.
Another critical factor to consider is the voltage rating. As expected, a 12-volt DC battery cannot properly power a 24-volt DC troll motor unit.
Even so, because troll motor units have voltage ratings in multiples of 12, for example 12volts, 24volts and 36volts; one can link 2 or more 12-volt batteries in series so as to power units that require more than 12volts to operate.
Likewise, one can pair more than one 12-volt DC battery in parallel when powering an energy-hungry 12volt troll motor unit.
The third factor to consider is the recharge time, that is, how long does a fully discharged battery take to be fully charged.
This is usually determined by the current rating of the charger, and it is important for one to prevent overcharging as this usually reduces the lifespan of the battery.
Generally, lithium ion batteries have a much shorter recharge time as compared to lead-acid batteries. Recharge time also gives one a rough estimate of how to space their trolling trips.
One also needs to consider battery storage, as the troll motor battery needs to be stored in a safe place when not in use.
The battery also needs to be stored in a fully-charged state in a cool and dry storage unit. For FLA, one may need to top up the electrolyte level with distilled water.
One of the best storage units is the MinnKota Trolling Motor Power Center which we described above.
One should also consider the lifespan of the battery. Wet cell SLAs last for about 24months while AGM SLA batteries last for twice as long with constant usage, and the expensive lithium-ion battery has an average lifespan of 10 years.
Another factor is weight. If one needs a light-weight, energy-dense battery with a high DoD, then it is advisable that (s)he uses a lithium-ion battery.