Battery choice and maintenance

Quality battery boxes with easy access for maintenance and safe storage

In this article I will touch on battery choice and some of the basics to maintaining them.

There are basically two tasks for boat batteries (your DC system) -starting your engines and running your electrical components such as lighting, refrigeration, bilge pumps etc. Most boats would ideally have two separate battery banks for these two different tasks. Your engine starting battery bank must deliver high bursts of power to start your engine, while your ‘house’ battery bank is required to provide a smaller amount of power, delivered over a longer period between charging.

Battery types and applications

As mentioned, batteries on your boat handle two very different tasks, starting an engine and running your ‘house’ needs.

Starting batteries crank your boat’s engine. They are measured in CCA’s or Cold Cranking Amps and are recharged quickly by your engine’s alternator. They are constructed with alternating layers of negative and positive plates with insulation between them. Starting batteries have thinner and more numerous plates, providing extra surface area to generate high amp bursts of current. The two drawbacks of this construction are that the plates are relatively fragile in high-impact environments, and that starting batteries do not tolerate deep discharges, which reduce their operating lifespan.

Deep cycle batteries for your boat’s ‘house’ needs deliver a lower amount of amps but can maintain this for a sustained period of time. Compared to starting batteries, deep cycle batteries recover fully after being heavily discharged over longer periods because their design features thicker plates with a high content of antimony. Overnight, their use might deplete 50% (maximum 50% for flooded and 40% for AGM) of the battery capacity, depending on the house loads of the boat. When the batteries are recharged, energy is re-deposited into the bank, and the process, or cycle, starts over again. As a general rule, deep cycle batteries should be sized to store three to four times the expected amount of energy to be used between recharge cycles. However, it should be noted that while you are out at sea, you will not recover your batteries to 100%. This can take a long time on a low current for standard technology batteries (Flooded, AGM & Gel). Somewhere between 75 and 80% is not unreasonable for a quick top up. This means the actual capacity that is usable day after day while at sea is a lot less than the number on the specifications sticker!

There are dual purpose batteries on the market and some of the boats we look after have these installed, but generally these are more suited to smaller boats where you might have limited storage or weight restrictions for two different battery banks.

Battery Types

Marine batteries are available in four chemical types: flooded, gel, AGM (Absorbed Glass Mat) and lithium phosphate. Which type you choose is based on your needs (deep cycle vs. starting), the capacity and lifespan you are looking for and of course your budget.

Flooded batteries
are the most widely used due to their price point. Flooded or lead –acid batteries require maintenance — regular inspection and topping up with distilled water. They handle overcharging better than gel and AGM batteries, because of their hydrogen venting and because they are not sealed like the other types. They self-discharge at a higher rate of 6 to 7 % per month. Wet cells must be installed in an upright position and don’t tolerate high amounts of vibration. Their initial cost is lower than similarly sized AGM or gel batteries, and significantly lower than the new type of lithium batteries. Properly charged and maintained, wet cell deep-cycle batteries are a good choice when being mindful of budget. We provide our customers with battery inspections and top ups as part of our routine maintenance programs however we can install watering systems for customers who like to handle this task.

Gel Batteries
Sealed, valve-regulated (SVR) gelled-electrolyte batteries offer advantages over regular flooded batteries. They self-discharge at only 3 % per month, handle the highest number of lifetime charging cycles, and are maintenance free, spill proof and leak proof.  A pressure release valve keeps their internal pressure at a slightly positive level, but they can release excess pressure if needed. The SVR design nearly eliminates gassing, so they are safer to install around people and sensitive electronics (but gel and AGM batteries still need to be vented). Gel batteries, because they’re sealed, are manufactured to very high quality standards. They need carefully regulated smart charging to prevent damage.

AGM Batteries
Sealed, valve-regulated AGM (Absorbed Glass Mat) batteries feature fine, highly porous microfibre glass separators compressed tightly between the battery’s positive and negative plates, which are saturated with just enough acid electrolyte to activate the battery. During charging, precision pressure valves allow oxygen produced on the positive plate to migrate to the negative plate and recombine with the hydrogen, producing water. In addition to providing equal saturation across the entire surface of the battery’s positive and negative plates, the fibres in the dense glass mats embed themselves into the plates’ surface like reinforcing rods in concrete, providing more plate support and better shock and vibration protection than in conventional batteries.

High-density AGM batteries have lower internal resistance, allowing greater starting power and charge acceptance, up to 45 % of the battery’s total capacity, and quicker
recharging than other types of deep cycle batteries and they only discharge a low 3 %. However we have experienced some issues with AGM batteries. They can fail if regularly pulled down below 70% capacity, if their float voltage is not correct, and they say to equalise charge at 15.6v every 12 months. Of course, there is no warranty if these conditions aren’t met.

Lithium Phosphate batteries -One of the emerging “super battery” technologies, lithium batteries have a high energy density and are excellent for deep cycle applications. Compared to flooded batteries, lithium batteries handle very large amounts of current, so they can be recharged faster than any other type. They are 100% efficient so all the power put back into the battery can be reused. The only real downside to these batteries is the high cost.

Battery charging and monitoring

I occasionally get calls from customers to investigate why their batteries appear to have died after only one or two years of service. In most cases it’s either because they have the incorrect sized battery for the task and they are running them down too flat, or they haven’t been charging them properly or maintaining the fluid levels. With the right sized batteries properly charged and maintained, four to five years is the expectant life in most cases.

When you walk around the marina, most boats are using shore supply. In my opinion this is a must, as having a good quality battery charger is crucial for healthy long lasting batteries. There are a wide range of chargers on the market and certainly the technological advances in recent years have seen ‘smart’ chargers become the standard. Some of the better models have remote monitor screens that can be mounted on the dash for easy viewing. Being able to see at a glance the state of your batteries and if your charger is in ‘bulk’ (charging) mode or ‘float’ (maintenance) mode is very helpful. For example, if you haven’t been onboard for a week or two and the monitor is still in ‘bulk’ mode then this indicates an issue.

Also, you wouldn’t go to sea without knowing how much fuel you have in your tanks, so likewise with your battery banks. Why risk losing lights, refrigeration and possibly starting. The battery meter is a simple device that connects to a shunt (brass bar) in line with your house battery cable and lets you see volts, amps, amp/hours and percentage at a glance. This will enable you to stop the batteries from being over discharged and damaged, saving you money in the long run.

Loss of shore supply alarms

Loss of shore supply is something that needs to be checked on a regular basis. Every now and then a plug might accidently be bumped and pulled from its socket, or the breaker switch might trip during a storm, or something going wrong on-board can also cause it to trip. For whatever reason, if it isn’t checked regularly you may end up running your batteries too flat and destroying them. Also, many owners who use their boats regularly are tending to keep refrigeration running with basic provisions. If the refrigeration is direct to the 240v source then you simply end up with a stinking fridge. However, many fridges run direct from the batteries or through an inverter if they are 240V. In this situation, not only do you get the stinking fridge, but you also run the risk of destroying your house battery bank by running them flat.

This situation can easily be avoided by installing a simple and relatively inexpensive alarm system. The unit we have installed on a few customers’ boats simply plugs into a 240v outlet and if power fails for whatever reason, it sends an SMS alert to your mobile phone. They also have the ability to send a picture when they detect movement. These units in most cases cost less than a single battery with the only downside being the minimal monthly mobile plan for the SIM card – there are minimal cost plans these days.

Battery tips for best performance

  • When choosing new batteries, we always recommend
    customers meet our qualified electrician onboard to inspect and load test your
    batteries, discuss the way you use your boat to help gauge battery size and then
    professionally install them. Understanding the best position for batteries, resistance through cables being too long is all crucial.
  • Stay with one battery chemistry, (flooded, gel or AGM) each battery type requires specific charging voltages. Mixing battery types can result in under or over-charging. This may mean replacing all batteries onboard at the same time.
  • Never mix old batteries with new ones in the same bank. While it seems like this would increase your overall capacity, old batteries tend to pull down the new ones to their deteriorated level.
  • Regulate charge voltages based on battery temperature and acceptance (manually or with sensing) to maximize battery life and reduce charge time. Ensure that your charging system is capable of delivering sufficient amps to charge battery banks efficiently.
  • Keep batteries clean, cool and dry.
  • Always have new batteries marked with date of installation for ease of identifying age
  • Check terminal connectors regularly to avoid loss of conductivity.
  • Add distilled water to flooded lead acid batteries when needed. How often depends on many variables but certainly after installing a new bank I’d be checking weekly until you work out how often they need topping up. For some boats it might be monthly or others 6 monthly. Keep them charged. Leaving them in a discharged state for any length of time will damage them and lower their capacity.
  • Clean corrosion off terminals and keep protected with dedicated anti-corrosion lubricant.
  • Don’t let your shore supply cable hang in the water – this can cause the circuit breaker to trip.
  • Shore supply cable wound up tightly can cause heat, and in some cases a magnet to form causing resistance and greater amperage which may trip the circuit breaker. If your cable is long then figure 8 the cable, or big loose coils, or better still have the electrician make you one to the correct length to fit your berth, and then have, a second longer cable carried onboard for when you are away. Don’t use a 30m cable, when 10m will work.
  • Always use a quality heavy duty shore supply cable. Buy the best you can and not those cheap ones designed for home use.
  • If our customers are planning an extended cruise then we always have their batteries load tested. Having them fail sitting in a coral lagoon somewhere can be a serious inconvenience to say the least, and sourcing new batteries in a small port and installation can take days out of your cruise.

Scoll down for more photos and click on image to enlarge.

An example of an overcharged battery

An example of good battery chargers - important for the ongoing maintenance of your battery banks

An example of a remote sensor that can SMS your mobile as soon as 240V shore supply is lost. They can also detect movement and send pictures.













There are fluid top up systems on the market

Monitors mounted on your dash are a great idea.

Having a quality shore supply lead suitable for your boats needs is crucial

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