Nowadays, More and more people like to build an off-grid solar system for their small traveling boats, a complete off-grid solar power system, generally contains solar panels, lithium ion batteries, charge controllers, solar generators, and inverters, It is impossible to imagine modern yachts without a safe and reliable power supply on board. Not only comfort depends on it – a cold beer from the on-board fridge is a fine thing – but also safety. Without power, navigation with plotter and GPS looks pretty old. You also have to do without radio or radar if the on-board battery has given up the ghost, not to mention AIS.
But although almost every boat over 5 meters in length has a battery on board, the right choice of marine battery type remains a mystery for many owners. This is not least due to many half-truths, often coupled with personal experience reports, which make the rounds in forums and on Facebook. Of course, it’s good to share experiences. But the choice of battery types on board depends on many factors, so apples are often compared with oranges.
Overview of battery types
Generally, two basic types of rechargeable batteries can be found on board: Lead-acid batteries or the more modern lithium phosphate batteries. In the case of lead-acid batteries, an additional distinction is made between closed (wet) and sealed batteries (AGM, gel and lead crystal). This naming is admittedly quite confusing. In the following we bring some light into the darkness:
Closed (“wet”) lead-acid batteries.
These are the “classic” lead-acid batteries, also called “wet cells”. The battery cells have a closure with a valve through which the oxygen and hydrogen produced during the charging process can escape. In the older design of this type of battery, distilled water must be added at certain maintenance intervals to compensate for the loss of liquid due to gassing.
In the newer, “maintenance-free” models, refilling is no longer provided for. Accordingly, the caps of the cells are no longer accessible. These maintenance-free batteries are designed to contain sufficient fluid for their intended life. This is estimated at 3-5 years for typical starter batteries.
It should be noted that sealed batteries designed as starter batteries are generally not particularly cycle resistant, as they are discharged very little during a normal starting procedure. They are therefore only recommended to a limited extent as consumer batteries(12v lithium battery or 48v) on board. However, there are also cycle-resistant sealed batteries, for example the so-called armored plate batteries.
Sealed lead acid batteries
Sealed batteries are a further development of closed batteries. They no longer contain liquid electrolyte, but the electrolyte is bound in glass mats (AGM, lead crystal) or a gel. In English, this type of battery is also called VRLA (Valve Regulated Lead Acid). These batteries should not gas during normal operation. Therefore, no “normal” valve is installed, but only a pressure relief valve for emergencies. If this becomes active, the battery is destroyed. For this reason, the maximum charging voltage should not be exceeded with sealed batteries.
AGM batteries have the electrolyte bound in fiberglass mats. They can theoretically be installed in any position (although an upright position is recommended, at least when charging) and have low self-discharge. Depending on the manufacturer, they can be left at low temperatures for several months without recharging without being damaged. AGM batteries can deliver high currents, so they are well suited for large consumers such as bow thrusters or electric (anchor) winches. They also work well as starter batteries. (However, the maximum capacity of the starter battery for the corresponding engine type should not be exceeded).
Many AGM batteries are designed for applications in emergency power systems. There, a high cycle stability is rather secondary, which is why these batteries are only conditionally recommended as consumer batteries. In case of doubt, the number of cycles should be compared with other battery types before purchase.
In gel battery, the electrolyte is bound in a gel binder, for example silicic acid, so that it acquires a gel-like consistency. This also makes gel batteries leak-proof and prevents acid stratification. Gel batteries have the advantage of a relatively high number of cycles and the self-discharge is even lower than with AGM batteries.
However, they usually have a higher internal resistance and thus cannot deliver as high currents as AGM batteries. Even though they can theoretically also be used as starter batteries, the strength of gel batteries clearly lies in their high number of cycles. That is why they are ideal as a consumer battery, where many cycles rather low currents are needed.
The relatively new (proprietary) technology of lead-crystal batteries promises to combine the advantages of AGM and gel batteries. Here, the special electrolyte is bound in fine glass structures – similar to an AGM battery. In addition to high purity of the lead plates, the manufacturer promises a cycle number similar to that of gel batteries, low self-discharge and high current-carrying capacity. In addition, the batteries are said to be insensitive to deep discharge. However, due to the relatively new technology and the still low distribution on the market, there are still few empirical values (and no independent studies), so it remains to be seen whether the technology delivers what the manufacturer promises.
This is also a very new technology, the first exhibits of which appeared on the market in Australia in 2014. In principle, these are sealed (VRLA) batteries in which the negative plates are either coated with a carbon layer or, in the case of other manufacturers, an additional plate is inserted next to the negative lead plate and coated with a carbon compound as the active mass. Another variant is that the active mass lead is embedded in a sponge-like carbon structure.
There are very different statements about the technology and the properties, depending on the manufacturer, but most of them are still unproven. Everyone agrees that the carbon embedding reduces sulfation by far and that this alone significantly improves the service life of these batteries in terms of the number of cycles (see article on the structure of lead-acid batteries). The data vary here between a double to eight times the number of cycles compared to standard AGM batteries.
In addition, the batteries are said to be able to cope with significantly higher charging currents; here, too, the specifications vary over a very wide range. The short-circuit currents are said to be several times higher than those of standard VRLAs (keyword “ultracapacitor”), especially in those designs in which an additional carbon plate is used in addition to the “normal” negative lead oxide plate, where the positive properties of VRLAs are combined with those of a super/ultracapacitor, resulting in a very high short-term current output.
So far, there are (still) several designs. As already indicated, some of these wonder batteries come with coated negative plates, others received additional negative carbon plates, some work with glass fiber mats as electrolyte carriers, others with liquid, highly diluted sulfuric acid – all this indicates that there is still a wide development potential here. Although some manufacturers are already offering “carbon lead technology” (Exide, BatterX, Winner and others), information on this in German is still very rare – in the English-speaking world, you can find much more under the term “carbon lead acid batteries”.
Lithium batteries have fundamentally different properties than lead batteries. At first glance, lithium batteries(such as 12 volt lithium battery) are the ideal on-board power storage: significantly lower weight, very high number of cycles, performance decreases less at low states of charge than with lead-acid batteries.
Due to the different charging characteristics, it is important that the charging technology is designed for the characteristics of this relatively new type of battery. As a rule, these batteries are only sold with a suitable battery management system that permanently monitors the battery and disconnects it from the on-board system in the event of irregularities.
However, this is also one of the system’s weak points: lead-acid batteries rarely give up the ghost spontaneously (except in cases of gross abuse), but lose their power over time. Thus, even a “broken” battery can still supply a few ampere-hours in an emergency. Lithium batteries such as lifepo4 battery, on the other hand, are switched off by the battery management system as soon as the system detects a fault. The skipper has no influence on this and must expect a total loss of power supply if the worst comes to the worst.
The advantages are also offset by the significantly higher price, which is usually more than ten times that of a good lead-acid battery. In addition, lithium batteries sometimes suffer significant capacity losses at temperatures around freezing point and below. Another disadvantage is the lack of long-term experience in on-board use: while lead-acid batteries can remain in use for 12 years or more if well cared for, it is not clear whether lithium batteries can achieve a similar service life. Those traveling in remote areas should also consider reverting to established lead-acid batteries, for which replacements can be found even on the remotest South Sea atoll.
Evaluate on-board batteries
So how can I judge whether a battery is worth the money? The prices for 12v or 48v battery are officially of the same quality can vary greatly. But cheaper is not necessarily better, especially when it comes to the reliability of the electrical system on board.
There are big differences in the manufacturing quality of lead-acid batteries. These ensure that a poor battery gives up the ghost after 2-3 years despite good treatment with a cell short circuit, while a good battery can faithfully perform its service even after ten years and more.
But there are differences here, too. It is worth comparing the number of cycles specified by the manufacturers at the same temperature, depth of discharge (DoD) and discharge time (Cxx). Furthermore, as a rule of thumb, the more a battery weighs, the longer it will last. A heavier battery contains more lead, so has thicker plates, which has a direct impact on cycle life.
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