Marine lead acid batteries are still going strong as main boat battery, for trolling and similar, but in many circumstances they are replaced by other types of batteries.
Batteries are used for various purposes, but generally, batteries can be divided in two groups:
- small primary or secondary (rechargeable) batteries that are used to power various small devices like lights (hand lights, boat lights etc), radios, sonars, GPS devices, cell phones etc.
- large, fixed (rechargeable) batteries that are used as starter batteries for main engine(s), for navigational and other lights, for powering trolling motors, electric winches, electric pumps etc.
To use batteries safely and to avoid undesired situations, it is good to know some information about batteries, that we often taken for granted.
Primary cells are not used often - when they are discharged, they must be replaced with new one. But, when not in use, they can last for years even decades due to their low self-discharge rate - great for stand-by devices and devices with low current consumption, like LED lights and similar.
Most secondary (rechargeable) marine batteries have following type of chemistry: Lead Acid batteries, Nickel Cadmium batteries, Nickel Metal Hydride batteries, Lithium batteries etc.
Nickel Cadmium Batteries (NiCd)
Due to cadmium present, nickel cadmium (NiCd) batteries are not environment friendly type of battery (nor are lead acid batteries, but lead batteries are easily and commonly recycled). Their relative capacity (power-to-weight, power-to-volume, capacity-to-weight and capacity-to-volume) is better than in lead acid batteries, especially when using high discharge currents (more than 15C currents).
In fact, NiCd batteries are one of the best batteries for insanely high discharge currents - up to 30-50C, although these currents are rarely (if at all) used on boats, except for starting main engines, but even that lasts not more than 3-5 seconds and lead acid batteries can handle such load with ease.
They are more expensive than lead acid batteries and prone to memory effect. Also, their nominal cell voltage is 1.2V, so for 12v battery, 10 cells are needed. They are not good for connecting in parallel, since they require current chargers (not voltage chargers) - when charging NiCd, NiMH and Lithium batteries, be sure to use proper charger, otherwise bloating and explosion of battery can occur - commonly! Also, they have rather high self-discharge rate and if left for few weeks, they will have to be charged again before usage.
NiCd batteries were often used on small portable devices, underwater scooters and power tools, but these days they are phased out by NiMH and lithium batteries.
Nickel Metal Hydride Batteries (NiMH)
Nickel metal hydride batteries are often found in portable stand-alone devices and in cordless appliances in general. Most modern NiMH batteries are self-discharge NiMH batteries that can stay several months charged and lose only few percent of their charge. Since that they don't have cadmium nor lead, they are environment friendly. However, be sure to recycle them.
They have higher capacity than lead acid and NiCd batteries and almost no memory effect. When discharged at low currents, there are NiMH batteries with up to 100-120 Wh/kg capacity. They tend to lose capacity when discharged at higher currents, but even then one can expect up to 80 Wh/kg.
Newer NiMH batteries can be drained even at 30C and that makes them comparable with NiCd and lead acid batteries in terms of surge currents.
Their cell voltage is also around 1.2V and for 12V battery one needs 10 cells.
They also require 'current' chargers and they are not good for connecting in parallel.
Low self-discharge, low price, moderately high currents and other characteristics has made NiMH preferred choice over NiCd batteries in most appliances.
Lithium batteries come in various types: lithium ion, lithium ion polymer, lithium sulfur, lithium phosphate, thin film battery etc batteries are 'high-end' batteries.
They are superior to other battery types in terms of capacity-to-mass, capacity-to-volume, power-to-mass, power-to-volume ratios, they (almost) have no memory effect, can be discharge with relatively high currents (model dependent, but certain models tolerate 50C currents), they last long and they are one of the most expensive types of batteries used today.
Often, one can see letters IMR, IFR and ICR as part of labels on 18650 and similar lithium batteries. These letters describe battery chemistry:
- IMR batteries are based on LiMn204 (Lithium Manganese Oxide) chemistry, their nominal voltage is 3.6 - 3.7 V, maximum recommended charging voltage is 4.2 V. IMR batteries commonly has smaller capacity, but are capable of delivering stronger currents.
- IFR batteries are based on LiFePO4 (Lithium Iron Phosphate) chemistry, their nominal voltage is 3.2 - 3.3 V per cell, maximum recommended charging voltage is 3.5 - 3.6 V.
- ICR batteries are based on LiCoO2 (Lithium Cobalt Oxide) chemistry, their nominal voltage is 3.6 - 3.7 V, with maximum recommended charging voltage 4.2 V. They commonly have higher capacities, but maximum allowed currents are limited to just few C, if even that.
- INR batteries are based on LiNiMnCoO2 (Lithium Manganese Nickel) chemistry. Very similar to IMR batteries, they can provide plenty of currents, with slightly lower capacity.
Note: unprotected ICR batteries vent out easily, but other chemistry batteries must be treated with respect, too! Also, there are other technologies, including hybrid technologies like: Lithium Nickel Cobalt Oxide (LiNiCoO2), Lithium Nickel Cobalt Aluminum Oxide (LiNiCoAlO2), etc.
Lithium batteries are very sensitive to charging conditions - when charged properly, some models can be charged fully in under 15 minutes. But, when charged wrong way, they are accident waiting to happen - explosions and fires can occur easily. Connecting them in parallel? Forget, unless special circuitry is used (charging individual cells connected in series separately and discharging them all together).
Larger lithium batteries often come with built-in electronics - smart batteries. Such electronics protect battery from overcharging, from too high discharge currents, they monitor temperature and similar. Such batteries are often made as one-to-one replacement of SLA batteries and they provide more power and higher capacities, at higher price.
They are fantastic batteries for high-end stand-alone devices like laptops, sonars, GPS units, radios etc, but for use as main boat battery, I am not so sure.
Lead Acid vs. Lithium Battery - Battery Comparison
Lithium batteries for RC models commonly achieve 150 Wh/kg with models achieving 200 (or even more) Wh/kg being more and more common and when used properly and charged with proper chargers, they can last for more than 1000 (thousand) cycles. Marine lithium batteries are designed to be robust and support high discharge currents, with good deep cycle characteristics. Such batteries achieve relative capacities of 45 to 65 Wh/kg, but cost less than RC lithium batteries.
If you like trolling, but you are limited with mass of carried equipment, go for good (although expensive) lithium battery.
For example, you have 12V trolling motor that requires 10A for continuous operation (120W) and you have 15kg limit for your battery. Lead acid battery that has 30 Wh/kg limit, can power such trolling engine around 3 hours, 90 Wh/kg NiMh can power such trolling engine for 9 hours and 180 Wh/kg lithium battery can power such motor for 18 hours!
Great, isn't it?! Well, wait till you see the price of such batteries and controllers :)
Marine lithium batteries with capacities of 60-65 Wh/kg cost more than SLA batteries, but far less than lithium batteries for RC models.