Marine Lead Acid Batteries
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)
For more on marine batteries, their types, their use etc. check these guides:
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