If you want to know little bit more about starting, dual-purpose and deep cycle batteries, continue to read:
Deep Cycle Marine and Automotive Batteries
Deep cycle batteries can be divided on lead acid (deep discharge) 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 (voltage chargers!) and IMHO, they are safer and more robust.
Non-lead acid batteries, especially lithium batteries are very expensive, can have huge capacity (Wh/kg), 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 (so-called 'drop-in replacements'), 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 5-6x more energy per kg of battery, when compared with lead acid batteries.
If you need battery for use on kayak or any similar small boat and you have issues with weight, consider 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.
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 chargers optimized for AGM lead acid batteries and they even forbid charging them with chargers designed for flooded cells lead acid batteries - what ever you do, be sure to read the manuals! Don't lose warranty for batteries that can cost hundreds of dollars/euros because of a cheap charger!
Lead Acid Batteries - Little Bit of Theory
Marine/automotive lead acid batteries are strong, sturdy and reliable type of batteries that are mostly used in fixed installations and are used as starter batteries, trolling batteries etc.
They have the same chemistry as car batteries and they are based on lead (hence the first part of their name) and its compounds and sulfuric acid (hence the second part of the name).
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 upright position and to prevent electrolyte from leaking. These batteries must be monitored and if needed, distilled water added when electrolyte levels fall down bellow certain level. Since during operation flammable hydrogen is released, they must be kept in 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.
Absorbed Glass Mat (AGM) batteries use electrolyte that is held in the glass mats made out of woven, very thin glass fibers.
Both gel cell and AGM batteries are often made as valve-regulated lead–acid (VRLA) batteries - safety valve inside battery opens at certain pressure (usually at 2 psi - 0.14 atm) and let oxygen flow from positive plate to negative plate to recombine with hydrogen and create water - no need to add any water during their operation. Such Sealed Lead Acid (SLA) batteries are maintenance free batteries and are used more and more on fishing boats.
Lead acid batteries lose capacity rapidly if discharged using high currents, but they are pretty immune to memory effect - they can be charged regardless of their initial discharge state, since battery will not 'remember' its previous discharged level. They are very cheap, when compared with other battery types. But, they are rather heavy in terms of energy-to-weight and energy-to-volume ratio.
Typical marine deep cycle lead acid battery has 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 period of 20 hours. Actually, when giving capacity, manufacturers usually provide capacity information when battery is discharged slowly for period of 20 hours, but they also provide tables about battery characteristics when 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, or as power source 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 instalations these batteries can act as voltage regulators preventing voltage surges - not recommended for their longevity, but ...) - continuous (float) charging voltages are:
- 2.23 V for gel cell batteries
- 2.25 V for AGM batteries
- 2.32 V for flooded (wet) batteries
It must be noted that these voltages are given at 20 °C (68 °F), and must be adjusted by −0.0235 V/°C for temperature changes (for 12V (6 cells) batteries). Also, 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 life of the battery.
Voltages required for daily charging, equalization charging and gassing threshold depend on temperature and vary among manufacturers, but generally they are:
- daily 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 additional weight will NOT be a problem.
These batteries are relatively cheap and those newer deep-cycle low-discharge high-power (starting) sealed batteries can last for years.
One of the best things about lead acid batteries is that they are charged using 'voltage' chargers and most of the other types of batteries are charged using 'current' chargers. Hence, 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:
- connecting batteries in parallel one creates battery pack of the same voltage but higher capacity. Due to slight difference 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 the exactly the same capacity from the same manufacturer, preferably from the same batch! And the best solution is to obtain single battery of the required capacity.
- connecting batteries in series one creates 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 danger of cell reversals. Again, try to use the same type of batteries, the same model with the exactly the same capacity from the same manufacturer, preferably from the same batch! And the best solution is to obtain single battery of the required voltage.
I know that this sounds 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 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)
Dual Purpose Marine and Automotive 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) batteries, All Terrain Vehicles (ATV) batteries, Uninterruptible Power Supply (UPS) 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 to 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, truck, RV vehicles etc., not only for powering small electronic devices, but also as main boat battery/battery pack.
If you need 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 and increase the stability of the boat.
On the other hand, if you need light and powerful battery and don't mind spending 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 very short period of time in order to be able to start an internal combustion engine.
Starting batteries feature thinner plates with larger active surface - required for large 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 dual battery system, starting battery is used only for starting the main engine and in such role good starting battery can last for years.
Marine and Automotive Lithium Batteries
Sealed lead-acid AGM, gel cell and lead-acid flooded cells are the most common marine batteries because of their safety, price and performances. But, lithium batteries are slowly entering this market too, due to their lightweight design and excellent capacity-to-weight and power-to-weight ratio.
Rechargeable lithium batteries come in several, slightly different chemistries, and some of the most important ones are:
- IMR batteries feature Lithium Manganese Oxide (LiMn2O4) chemistry. Their nominal voltage is 3.6 - 3.7 V per cell, with maximum recommended charging voltage of 4.2 V. IMR lithium batteries commonly have smaller capacity when compared with other lithium rechargeable batteries, but are capable of delivering larger currents. IMR lithium batteries are used for starting batteries.
- INR batteries feature Lithium Manganese Nickel (LiNiMnCoO2) chemistry. INR batteries are very similar to IMR batteries, in respect to voltages, currents and capacity. They are commonly used for starting batteries.
- IFR batteries feature Lithium Iron Phosphate (LiFePO4) chemistry. Their nominal voltage is 'only' 3.2 - 3.3 V per cell, with the maximum allowed charging voltage of 'only' 3.5 - 3.6 V. Lithium batteries MUST be charged with the chargers recommended by their manufacturer - IFR batteries, due to their lower voltage, can be fire hazard if charged with lithium battery chargers set to 4.2 V per cell charging!
- ICR batteries feature Lithium Cobalt Oxide (LiCoO2) chemistry. Their nominal voltage is 3.6 - 3.7 V per cell, with the maximum allowed charging voltage of 4.2 V. ICR batteries commonly have higher capacities, but maximum allowed discharging currents are often limited to just few C. Thus, ICR batteries are commonly used as deep cycle and low-current starting batteries, since 100 Ah ICR batteries can provide 300-500 Amps (depends on the battery!) for shorter period of time.
Other lithium battery chemistries are present on the market too, including hybrid technologies like Lithium Nickel Cobalt Oxide (LiNiCoO2), Lithium Nickel Cobalt Aluminum Oxide (LiNiCoAlO2), etc.
Many lithium ion batteries come with built-in electronics and are designed to be drop-in replacement for SLA batteries, requiring no changes to the boat/car/truck electrical system.
Main advantages of the lithium ion drop-in replacement batteries is their lightweight design (3-4x lighter than SLA batteries), and their higher number of charging/discharging cycles (up to 4-6x times more charging/discharging cycles than SLA batteries).
Main drawback of lithium batteries is their cost, making their initial purchase rather expensive. But, due to the number of charging/discharging cycles, their actual price during the use of the boat/vehicle is lower of the price of required SLA batteries.
Note: never, but really never charge your lithium batteries with chargers not explicitly recommended by their manufacturer.
Best Trolling Motor Batteries - Little Bit of Math
If you are looking for the best battery for trolling motor, it is important to know what kind of trolling motor you are going to use, how long you intend to use it and how large reserve you need, just in case.
Depending on their thrust, electric trolling motors feature maximum currents in the 30-50 Amps range. Electric trolling motors usually use 12 V, 24 V and 36 V electric systems:
- 12 V are used for a thrust up to a 60-65 pounds,
- 24 V are used for a thrust up to a 100 pounds,
- 36 V are used for motors that provide more than 100 pounds of thrust.
For more about electric trolling motors, please check our Electric Trolling Motors for Small Boats, Pontoons, Kayaks and Canoes article.
Best trolling motor batteries are marine deep cycle batteries, then marine dual-purpose batteries and only then starting marine/automotive batteries. But, if you can, avoid using starting batteries in deep discharge and cycle applications - they will not last long, and that can be quite costly.
One of the best and quickest ways to determine if the battery is good or not for a trolling or deep cycle applications is to check its Reserve Capacity (time in minutes that the battery can supply 25 Amps, at 80°F (~25°C) and maintain the voltage above 10.5 volts) value and how many deep discharges battery can withstand prior loosing 20% of its nominal capacity.
Or, just check its Reserve Capacity and if the battery is designed for cycling applications (it must NOT be labeled as starting battery!).
24V Deep Cycle Trolling Motor Battery
For example, one has 24 volts trolling motor with the maximum current of 50 Amps and 60 Amps circuit breaker, and needs a battery or battery pack that is independent from boat's electric system.
One also requires at least 2h at half throttle (25 Amps) with occasional full throttle (50 Amps) use when and if required, and that the battery is rugged, reliable and tough as much as possible.
First of all, one needs two 12 V batteries connected in series - when connecting the batteries, be sure to use thick cables and to use the same batteries (AGM, Gel, flooded, lithium ion etc) of the same manufacturer of the same model, preferably of the same age and batch. This way one can be pretty sure that the batteries feature more or less same performances (capacity being the most important one here).
In this case, one needs two batteries with RC value of at least 120 minutes, and with some power reserve, this means RC of at least 150 minutes, preferably 170 or more minutes.
If money is not of an issue, then IMHO the best choice is a par of Odyssey 31M-PC2150 Marine Dual Purpose Batteries - BCI Group 31, AGM SLA 12 volts battery featuring nominal capacity of 100 Ah, 205(!) minutes RC, 1370(!) MCA and 2150 Amps 5s pulse.
It can withstand 400 cycles down to 20% of its capacity and is extremely rugged and can operate even at very low temperatures. But, it comes with very nice price tag, especially if you need a pair of them :)
Slightly cheaper choice are a pair of Optima D31M BlueTop batteries, featuring 75 Ah nominal capacity and 155 minutes RC.
Optima D31M BlueTop are dual purpose AGM SLA batteries that are also very vibration resistant and can tolerate larger number of deep discharge cycles.
If the user wishes some cheaper solution, using reliable batteries that feature 'just' 1-year warranty, then the best choice is probably a pair of UPG UB121000 Deep Cycle AGM Batteries, featuring 100 Ah capacity and RC value of ~170 minutes.
A pair of these batteries often costs like a single Optima D31M and significantly less than a single Odyssey 31M-PC2150 battery.
Note: it is also possible to use a pair of lithium 'drop-in' replacement batteries - this can be very costly, but saved weight can be used for supplies and other fishing/camping/outdoors gear. One of such batteries is Battle Born 100 Ah 12 Volt LiFePO4 Deep Cycle Battery.
Similarly, if the user has 12 volts trolling motor with similar amperage requirements, everything (math) is the same, except that the user requires just one battery, not two of them.
Note: 'power reserve' or 'energy reserve' - never underestimate Mother Nature and what she can throw at you (winds, currents, rain etc.). So, having some extra power (stronger electric trolling motor) with some reserve energy (larger battery) is good to have - just in case ...
Kayak Trolling Motor Battery
Kayak trolling motors are usually small electric trolling motors with rarely more than 30 pounds of thrust. Picking a right lightweight battery that will be used as a small trolling motor battery is mostly limited by the allowed weight of such battery.
For example, if your battery must be lighter than 10-15 kg, then possible models include batteries like:
- Battle Born 100 Ah 12 Volt LiFePO4, 13.2 kg, 100 Ah, RC unknown, but probably RC>180 min,
- Mighty Max ML35-12 Battery, ~9.6 kg, 35 Ah, ? RC,
- Odyssey PC680 Battery, 7 kg, 16 Ah, 24 minutes RC,
- VMAXTANKS V30-800 Battery, 10 kg, 30 Ah, 55 min RC,
- VMAXTANKS V35-857 Battery, 11.3 kg, 35 Ah, 75 min RC.
There are many other similar batteries on the market, so choose according to your needs and personal preferences.
Note: lithium batteries are expensive initially, but since they can be recharged almost 10x more than AGM SLA batteries, in the long run, they are actually cheaper than AGM SLA batteries. And they can save You a lot of weight ...
Deep cycle trolling batteries, dual purpose batteries and starting batteries will do their job well, if used properly.
Don't misuse or abuse your batteries, or they will abandon you when you need them most ... 'Murphy's Laws' at their finest!