How to assess and choose the right house batteries for your boat (published November 2013)
We were reaching south halfway between Annapolis and Norfolk when the chartplotter faded and blipped off. I immediately knew the problem, low house batteries, but was puzzled because they were inspected and topped up before departure just hours earlier. Plus, we hadn’t been using the autopilot, all the lights were off and virtually nothing else had been running.
So on went the diesel and for the next few hours I watched as the batteries charged. Then, when the engine was off, they drained rapidly and died. That was it. They were toast.
I’ve come to expect my fair share of issues while captaining deliveries, but house batteries that absolutely wouldn’t hold a charge was a new one. Fortunately, a delightful cab driver and a helpful staff at the West Marine in Norfolk got me setup and I was able to quickly change out the old house batteries for new without creating too much lag time in my delivery schedule.
But what to make of these old batteries; how were they so far gone? As often happens, the owner had consistently drained the batteries below 10 volts, recharged, and then drained again. Once this cycle started it was hard to break and the batteries began to suffer. Their overall lifespan decreased exponentially as they were drained until they were basically rendered useless.
Treat your batteries this way and you are begging to be the battery customer of the day at your local chandlery; like me.
A cruising sailboat’s bank of house batteries is the heart and lifeblood of the boat’s electrical system. Whether you are merely powering lights and a small amount of electronics; or pumps, a watermaker, an inverter and refrigerator, paying close attention to your energy consumption is a critical part of cruising. But choosing the right batteries for your boat and staying on top of your energy supply isn’t as simple as it seems. Here are some helpful hints and tips to think about when buying and caring for a new set of house batteries.
SIZING YOUR BANK
Before deciding on what type of batteries to buy it is best to size up your boat’s energy consumption and ability to replenish what you have lost. If you already operate on a given battery bank for your boat and it works well for you, then you are probably set to choose batteries. But if you need to figure out how much power your boat requires, it is good to start with a simple chart on how much energy you plan to use (see energy consumption chart on page 39).
The one major problem with energy consumption charts, though, is to accurately figure out how many amp-hours you are actually expending in a day. This varies, of course, on how you use your boat. During an offshore passage you might run a full compliment of electronics, including an electronic autopilot, for 24 hours multiple days in a row, and will keep running lights on all night. When coastal cruising and hopping from one anchorage to another, you might run your electronics for part of the day and will run your interior and anchor lights at night. The best thing to do is to try and figure out the maximum amount of amp-hours you envision yourself using and go with that number. A good rule to abide by is to install an energy bank that is three to four times your actual consumption.
Deciding on batteries that are right for your boat can seem to be a somewhat intimidating process, but in reality, there are only a few types of batteries to choose from. True to form with many things in the maritime world though, it is important not to cheap out when buying new house batteries. Also, it is ill advised to try and replace one battery of a multi-battery system, as the older batteries will drag down the performance of the new one.
When determining what type of batteries to buy, you will generally be looking at three different types of deep-cycle marine batteries: Flooded, Gel and AGM (Lithium-ion batteries are also available, but due to the newness of the technology they are still quite expensive). All of these will have their capacities listed in Amp-hours (Ah), Reserve Minutes and Cranking Amps. They will also have a “group” number listed on them, which is purely describing the physical size of the battery.
Amp-hours is the most relevant number to look at when deciding on house batteries for a cruising sailboat. Amp-hours measures how much total energy the battery will deliver over a 20-hour period at a constant discharge rate before dropping below 10.5 volts. As an example, if you have two 100 Ah batteries, they can run a 10-amp load for 20 hours before dipping below 10.5 volts.
The three types of batteries vary in their design and ability so it is important to know what you are looking at before making the big purchase.
Flooded batteries are just that, they are “Flooded” with a reservoir of liquid sulfuric acid. When the battery charges it produces hydrogen and oxygen that vents out. Being that hydrogen is explosive, if you choose to go with Flooded batteries it is imperative to have a properly vented battery compartment. Also, it is important to remember that Flooded batteries require maintenance, which means they need to be topped up with distilled water every so often.
The major advantage of Flooded batteries over Gel and AGM is that they can handle overcharging better because they have the ability to vent. The problem with this though, is that the batteries have to be stored upright and they typically do not handle vibration well. They also vent more than any other battery, typically at a rate of six to seven percent per month, which means you’ll want to leave them on a charger when not using them for long periods of time. On the plus side, Flooded batteries are cheap, dependable and can handle several hundred to a thousand cycles before needing to be replaced.
Gel batteries are designed and manufactured to be maintenance free, spill proof, submersible and leak-proof. They vent at a monthly rate of three percent and can handle a high number of cycles in their lifetime. By design, they release very little gas, which makes them safer to install and have aboard. The one major drawback to Gel batteries is that they can be irreparably damaged if overcharged, so having a quality smart charger is crucial. Cost wise, Gels fall in the medium range making them a good buy based on the amount of cycles you’ll get.
Absorbed Glass Mat (AGM) batteries have become the go-to battery for the majority of live aboard, ocean-going sailors. They are maintenance free, discharge at about three percent per month, recharge exceptionally fast, can be installed in nearly any position and are resistant to damage caused by vibration. AGMs are built with porous microfiber glass separators between the negative and positive plates that are saturated with acid electrolyte to activate the battery. This basically means the batteries have less internal resistance, which allows them to charge faster and gives them a rigidity that helps with shock and vibration protection. AGM batteries are the most expensive of the three, but many sailors justify the cost based on their many positive attributes.
Now that you have purchased and installed your lovely new batteries it is absolutely imperative to properly take care of them to ensure you will get to see their full potential. Testing suggests that no matter what type of battery you choose to go with, they will all weaken and die rapidly if you don’t treat them well.
Battery care starts with maintenance. Even though batteries say they are maintenance free, it is important to keep them clean, cool and dry. Inspect the terminals regularly to avoid a loss of conductivity and remove corrosion when you first notice it.
One of the major killers of batteries is leaving them discharged for too long a period. Never discharge your batteries past 80 percent and try to recharge them after each extended period of use. For prolonged battery life, this is where having a quality smart-charge regulator comes in.
A smart-charge regulator controls the output of your engine’s alternator in three distinct stages: bulk, accept and float. The bulk charging phase is going to bring your batteries up to that first 75 percent; from there, the acceptance phase is going to take over and slowly charge the last 25 percent; and then the float phase, which is actually a maintenance phase not a charging phase, is going to keep your batteries from losing electrolytes from their cells. At the float phase AGM batteries should hover at 13.6 volts, Gel batteries at 13.8 volts and Flooded batteries at 13.4 volts.
Lastly, if you have technical questions regarding your boat’s specific electrical needs it is a good idea to consult a professional marine electrician before jumping into a project that is above your level of expertise or comfort.
The batteries I was dealing with on my delivery had been deeply discharged time and time again without proper charging in between. When we installed the new batteries in Norfolk I hooked the boat up to shore power for a final charge to make sure they were at their peak. Next, I unplugged from shore power and ran the engine to see how the alternator and smart-charger charged the new bank. Happily, there were no battery issues for the remaining 1,300 miles of the delivery.