Note: Amazon affiliate links in the table open in the new windows, feel free to check them for most up-to-date prices and offers.
There are many other battery models and manufacturers on the market, these are most common and most popular.
Which battery to choose? Choose according to your storage area and power/energy requirement. If you are not sure about it, but have room to accommodate 31 size battery and money to pay it, go for Exide XMC-31 or VMAXTANKS SLR125. Also, these batteries are quite heavy and when positioned low in the small boat, they lead to much improved stability. Position them high and it is accident waiting to happen.
Although classified as 'Deep Cycle', Exide XMC-31 (improved Exide MC-31), with MCA rating of 1000 Amps is nice 'Dual Purpose' marine battery. If you don't require large starting current, then XMC-31 can be used as 'Dual Purpose'. If you do require larger starting currents AND good trolling motor battery, consider Optima D31M.
Warning - some manufacturers 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!
Marine Lead Acid Batteries - Little Bit of Theory
Marine 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)
Marine 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.
Electric Trolling Motor Batteries - Little Bit of Math
You need 12V battery with at least 200Ah capacity that is capable of providing 1000A of start current, but you are unable to find it locally?
However, you can buy good gel cell 12v/110Ah/800A batteries. Are they good enough? Well, they are good, since connecting them in parallel, you will get a battery pack that has 12V voltage, 220Ah capacity and is capable for 1600A for starting (btw, 1000A is HUGE current and 1600A is ... uffff!!! :o) - this is an example, but ...). Due to slight differences in voltages among lead-acid battery types, be sure to buy exactly the same batteries from the same manufacturer and preferably from the same batch - when starting your engine(s), you don't want one battery to provide 700A and another battery to provide 300A! Also, be sure to connected them with properly sized cables and individual high-current switches and (automatic) fuses.
Keep those batteries away from heat sources, keep them apart at least 5-10 cm (2-4 inches) so that sides can be cooled by air, too.
If possible, attach temperature sensors to both of the batteries and monitor their temperature just as if you monitor temperature of your engine (cooling water, oil etc).
You need 48V/120Ah/200A battery (voltage, capacity, starting current), but you can buy only 12V/110Ah/800A battery.
Well, sorry to say, but you need 8 such batteries. First, connect four pairs of batteries in parallel and then connect those pairs in series. This will create a battery pack that has 48V/220Ah/1600A - this can cost a lot and cost of professional services are just a fraction of such costs.
Or, create 2 groups of 4 batteries connected in series and then connect them in parallel. This second option is usually preferred one, since one can use batteries in groups of four same batteries, but they can vary from group to group (various capacities from group to group, but the same capacities in the group and the same battery types in whole battery pack).
Again, such battery pack is very strong - it can provide 4 kW for about 2 hours (batteries drained to 20%) or ~77 kW for 5-20 seconds without any cell damage!
Since we have batteries connected in series, there is a danger of cell reversal - be sure not to drain such packs bellow 20% of their capacity, just to be sure.
You have SLA 12V/110Ah/800A battery and 12V trolling motor that requires 14A at maximum power and 8A continuously. You don't plan on using it on maximum power, except for short periods (10-15s - which is negligible).
You want to know, for how long you can use this battery safely.
First, 110Ah capacity is actual capacity when battery is drained for 20h at little more than 5A; but we need 8A, so we don't have 110Ah. If you have drain tables of your battery model, be sure to check capacity of your battery when drained at 8A - in such tables one can also find for how long that battery can be discharged at 8A to 10-20-30-40 etc percentage of its capacity.
If you don't have such tables, instead 110Ah, consider that you have 100Ah battery and when discharged to 20% of its capacity, we have 80Ah at our disposal for our 8A trolling motor. 80 divided by 8 is around 10 hours of continuous trolling with fully charged battery.
Note that if you have older battery and/or older model of battery, you can't expect to have 100Ah at 8A, so instead of 10 hours of trolling, you should expect less - maybe 8 hours, but this again depends on state of your battery, temperature, peek usage etc.
This calculation provides you with information what you can expect from your battery/trolling motor combination, but for real, accurate data, you will have to test the battery with 8A load and monitor temperature and voltage of the battery.
Deep cycle trolling batteries will do their job well, if used properly. Don't misuse or abuse your battery, or they will abandon you when you need them most ... 'Murphy's Laws' at their finest!