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Tech Talk
with Mike Telleria

This month's topic: Understanding Premature Battery Death and How To Avoid It

The cost for one Lifeline 8D battery is about $650. A typical Nordhavn N55 will have eight of these in its house battery bank. A Nordhavn N68 will have 12. That makes the replacement cost for the entire house battery bank between $5,200 and $7,800 for these two boats – not exactly chump change.

Thankfully the Lifeline AGM batteries we use in our boats are incredibly resilient and can withstand a reasonable amount of less-than-ideal treatment. However, there is only so much abuse they can tolerate until they begin to lose capacity, which can mean a shortened life expectancy. While most owners can count on at least 6 to 8 years before battery replacement, there are a number of situations that can shorten this time to 3 to 4 years or less. The good news is a little awareness can help prevent premature battery death.

What is Battery Death?
Many boaters don’t consider their batteries dead until the lights don’t work and the engines won’t start. The more general rule, however, is that a battery has reached the end of its service life once it can only hold 80% or less of its rated capacity. Unfortunately the procedure for conducting a full discharge test to determine the actual capacity of a battery is rather complicated – so the more approximate indication of “my batteries don’t seem to hold a charge as long as they used to” is a good sign that it’s time to ask a professional to come aboard with a battery analyzer and give you an idea of the health of your batteries.

Also, it’s important to not be fooled by seemingly acceptable voltages here. Your voltmeter can register a solid 26 volts after a charging cycle indicating that your batteries are 100% charged. However, if your batteries are at the end of their service life and can only hold say 60% of their rated capacity (that’s only about 150 amp-hours for a 255Ah 8D battery) the 100%-charged figure now means you only have 150Ah instead of 255Ah, and you’ll quickly discover that your “fully charged” batteries don’t deliver near the amount of energy they used to. (For the sake of simplicity, it’s useful to think of amp-hours as a capacity like a gallon of fuel, which can be used up quickly or slowly depending on how large the demand is.)

What is Premature Battery Death?
This is a dodgy question. The folks at Lifeline say you should expect 6 to 8 years or more out of your batteries given routine use. The definition of “routine” is important here. Even though Nordhavn owners tend to use their boats much more than owners of other kinds of boats, Nordhavns are still used infrequently compared to a commercial boat that is being used hard day in and day out. This is why a commercial boat operator might expect only 2 or 3 years of service life out of his batteries. This is also why those Nordhavn owners who do run their boats hard for years at a time can end up replacing their batteries in less than 6 to 8 years.

So, the answer to the premature battery death question now becomes one that must be considered within reason of how the boat is being used. Heavy use with deep daily discharging and recharging cycles is going to shorten longevity for sure, while periodic use with light, infrequent discharging and recharging cycles can actually result in 10 years or more out of a set of batteries.

Still, whether your routine is likely to wear down your batteries in 3 years or stretch them out to 10 years, there are things that can be done to keep your batteries from sputtering out before their time. This is where charging and discharging practices come into play.

Going Deep
The amount you discharge your batteries can have a direct impact on battery longevity. The technical term is Depth of Discharge, which is expressed as a percentage of total battery capacity. A 12-volt 8D battery has a capacity of 255Ah – use up half of that (127.5Ah), and you’ve reached a 50% Depth of Discharge.

Lifeline estimates that an average Depth of Discharge of 50% will yield a service life of 1,000 discharge/recharge cycles, and this is the sweet spot Nordhavn electrical systems are designed for. For instance, the Nordhavn 55 has eight 12-volt 8D batteries split into four 24-volt, 255Ah pairs for a total house battery bank capacity of 1,020Ah (255Ah x 4). However, the intention is for the batteries to never be discharged more than 50%, so the effective house battery bank capacity is actually only 510Ah, which is the amount we’ve determined necessary for this size of boat between charges.

However, if an owner routinely exceeds this 50% threshold, he can expect to be pricing a new set of batteries earlier than he may like. An average depth of discharge of 70% reduces service life to about 650 cycles and an average depth of discharge of 95% reduces service life to about only 400 cycles.

The reality for most Nordhavns is a much more shallow average depth of discharge with occasional deep 50% or more discharges while at anchor or moored. Most of the time energy is being provided by an alternator (while motoring), generator or shore power – and there is often little to no demand on the batteries during these times. But there are those times when an owner wants to have a quiet night on the hook and let the batteries carry the load – and Nordhavns have plenty of reserve capacity to handle these times, as long they don’t routinely include large loads that are going to pull the batteries below a 50% depth of discharge.

Going Fast
Another discharging particular to be aware of is how rapid discharging can affect battery capacity. The faster the rate of discharge (the more amps going out), the lower the Ah capacity of a battery will be. As noted earlier, an 8D battery has a published capacity of 255Ah. What this really means, however, is that you can expect this battery to deliver a steady 12.75 amps for 20 hours before it goes flat. This is what is known as the C20 rate, which is the typical figure manufacturers use when they publish the capacity of their batteries.

In the example above, you would reach a 50% depth of discharge in 10 hours. However, the relationship between increased discharge (more amps) and capacity is not linear. Say you quadruple your load from 12.75 amps to 51 amps. You might think you could sustain this load for 2.5 hours until reaching a 50% depth of discharge, but the truth is you’ll be at 50% in barely 2 hours, which has the effect of reducing the capacity of that 255Ah battery to about 230Ah. And if you keep going higher (like by adding air conditioning and water heaters to the inverter circuits), the reduced battery capacity is even more dramatic. At 100 amps discharge the time to 50% depth of discharge is about 1 hour, reducing the capacity of that 255Ah battery further to about 200Ah.

So, large sustained loads will not only draw down your batteries more quickly, they will also work to reduce the effective capacity you have in the first place. This increases the likelihood of frequent 50% or greater depth of discharge cycles, which, again, can shorten battery longevity.

This is a minimal concern for most Nordhavns. Only those with water heaters, air conditioners and galley appliances like ovens and cooktops powered by the house batteries via inverters are likely to produce a high enough sustained demand to create a significant acceleration in the discharge rate.

Charging, Charging, Charging
As with discharging, there are a few battery charging practices that can lead to premature battery death. And, in what might seem surprising to some, most Nordhavns are subjected to one or all of these practices from time to time. However, the service history of hundreds of Nordhavns shows that the Lifeline batteries stand up very well to some occasional strain.

Most Nordhavns have two charging sources: the main engine alternator while the engine is running and the charging function of the inverter/charger while the boat is being powered from generator or shore power. Also, many owners will add a dedicated battery charger to the system – primarily to provide limited shore power while in a foreign 230VAC 50Hz port (more on this later).

At the Dock
Like most boats, it’s fair to say the majority of Nordhavns spend most of their time at the dock plugged into shore power. This is pretty ideal from the batteries’ point of view. The inverter/charger will maintain a healthy charge on the batteries while it also passes shore power through to power inverter loads (refrigerators, outlets, etc.).

As most Nordhavn owners know, the charging function of the inverter/charger uses a three-stage process to charge the batteries. In the Bulk stage a constant current (amps) is applied to the batteries until a battery voltage of 28.6 volts is reached – the batteries will be about 80% charged when this threshold is reached. This may happen quickly or slowly depending on how deeply discharged the batteries are and how much charging current is available.

After the Bulk stage the charger will go into a timed Absorption stage where it holds the voltage at 28.6 volts while allowing the current to diminish. Less current is required to maintain the voltage as the batteries approach a full charge. The absorption time is typically set at 2 hours for Nordhavns, which is done to minimize generator run time when using the generator to charge the batteries. However, owners who are comfortable with changing the settings on their inverters may want to change the Absorption time to 4 or 6 hours when leaving the boat at the dock to allow for a more complete Absorption stage, which will help ensure the batteries reach a 100% charge.

The batteries are fully charged when the charging current drops to 0.5% of the battery bank’s rated capacity (that’s about 5 amps for a typical 1,020Ah eight-battery bank on a Nordhavn N55) – a quick look at the NET BATTERY AMPS meter will tell you how many amps are flowing into or out of the battery bank (a positive reading means charging and a negative reading means discharging). A good rule of thumb to follow on most Nordhavns is your batteries are charged anytime there is a charging source online and the NET BATTERY AMPS meter is hovering at about 2-5 amps.

The final charging stage is the Float stage where the voltage is reduced and held at 26.6 volts to maintain the charge and minimize overcharging. Because batteries are so sensitive to temperature, the temperature of the batteries is continuously monitored and charge voltage is adjusted accordingly to maximize charging and prevent damage.

One point worth comment here concerns the current that is applied during the Bulk phase. For repetitive deep cycling, Lifeline recommends at least 20 amps for every 100Ah of battery capacity. For our Nordhavn 55 example with 1,020Ah, that would be a charging capacity of 204 amps, which is more than the 85- to 120-amp charging capacity found on most of our inverter/chargers (but not much of a problem for twin inverter boats with 170- to 240-amps of charging capacity). This isn’t a concern for most Nordhavns because most owners are not deep cycling their batteries that often, but owners who are might want to supplement their charging capacity with an additional battery charger for use while on shore power or generator power.

On the Genny
There are differences between charging the batteries using the generator and using shore power. Time is the biggest. It doesn’t make economical sense to let a generator run 6 hours or longer to bring your batteries up to 100%. Most owner’s like to wake up their boats with a few hours of generator run time to make water, do laundry, cook food and charge the batteries.

The important thing to remember here is that the inverter/charger will only charge the batteries with whatever incoming power is left after satisfying the needs on the inverter distribution panel (refrigerators/freezers, power outlets, a.c. lighting, etc.). If the a.c. input limit on your inverter is 30 amps at 120VAC, and it has a maximum d.c. charging capacity of 85 amps at 24VDC, then you want to make sure there’s enough incoming a.c. to produce a full 85 amps. Some simple math helps: 85 amps x 24 volts gives you 2,040 watts (about 2kW), 2,040 watts ÷ 120 volts gives you 17 amps. So, you’ll need at least 17 of the available incoming 30 amps at 120VAC to power the charger at full capacity, which leaves you 13 amps of pass-through a.c. to power loads on the inverter panel. Anything more than 13 amps on the inverter panel will decrease the amount of d.c. current available for battery charging.

Still, two or three hours on the generator probably won’t be able to bring a battery bank with a 50% depth of discharge all the way to 100%. So, if you are doing a lot of deep discharge cycles you might only be charging your batteries up to 80-90% using the generator. Typically this is not an issue because most boats will be put in service and the main engine alternator will finish off the charge. And most boats also find their way to a dock at least once or twice a month where they’ll get a nice, long charge on shore power.

However, repeated deep-discharge cycles along with unchecked undercharging without a full-charge cycle every 30 days or so can erode your batteries’ life span. The main worry here is sulphation where the active material inside the batteries transforms into small crystals that grow and harden. Again, this is not a common problem on Nordhavns because frequent undercharging is rare. But, a few months on the hook with nightly deep discharges and only short genny recharges without an occasional full-charge could cause this to happen. Even so, all is not necessarily lost. Many times an equalization (or conditioning) charge can be applied in an effort to regain some of the lost capacity.

The Almighty Alternator
Assuming you run your boat as a “battery boat,” meaning you only run the generator when necessary while cruising (make water, do laundry, etc.), it’s up to the main engine alternator (or alternators – some have more than one) to supply all the 24VDC for the boat, including inverter loads, and keep the batteries charged. For most Nordhavns, the alternator(s) can provide more than enough energy to satisfy all the house loads while cruising without having to draw from the batteries too often.

However, even though you might have a big 175-amp dedicated house alternator, it’s unlikely you’ll ever see 175 amps on the meter. By design Nordhavns are intended to be operated at low rpm, fuel-saving speeds, which prevents the alternator from spinning fast enough to reach its full rating in terms of current and voltage. On the smaller boats, like the N47 and N55, this typically goes unnoticed because the alternator is still able to produce plenty of energy to handle the daily cruising loads without having to borrow from the batteries too often.

However, some of the larger boats, like the N68 and N76, push the limits of at battery boat concept and often have to borrow energy from the battery bank during peak loads while motoring, which can result in a constant cycle of shallow discharges (or even deep discharges if large loads like air conditioning and water heaters are being run off the inverters). Still, history has shown there’s no problem as long as the discharges are not too deep and there’s enough time in between for the batteries to recover (and history has also shown the opposite to be true on boats with large battery bank and multiple inverter installations where the alternators couldn’t keep up with the house demand and look after the battery bank at the same time, resulting in rapid battery decline). In fact, owners with large 24VDC and inverter panel cruising loads in the 4kW to 5kW range (not uncommon on the N68 and N76) could be better served by running a small 6kW generator instead of trying to meet the load with their alternators and batteries. This would prevent the alternators from working so hard they overheat and die an early death, and it would certainly bolster the life expectancy of your batteries.

For the many boats that do rely on the alternators for battery charging while cruising, one point often talked about is whether to use an intelligent voltage regulator on the alternator that basically mimics the three-stage battery charger. While most boating books recommend doing this, the vast majority of Nordhavns have accumulated countless nautical miles while happily charging their batteries using low-tech constant-voltage alternators without any ill effects.

The concern here is overcharging, which can certainly spell early death for an AGM battery because overcharging can cause them to gas and dry out. While this is not a major issue for standard flooded-cell lead-acid batteries where water can be replaced, a sealed AGM battery has no way to replace the water once it has gassed. The Float stage of a three-stage battery charger is designed to prevent this very thing from happening by lowering the voltage.

A constant-voltage alternator, however, does not lower the voltage once the batteries have reached their peak – seemingly opening the door to overcharging and drying out the batteries. So why don’t the batteries on Nordhavns dry out from overcharging on a regular basis? Slow operating speeds and the long wire run from the alternator in the engine room to the battery bank in the lazarette reduce the voltage seen at the batteries. Also, a slower-turning alternator tends to run a little hot, which will also cause its voltage output to decrease. The combined effect is a lower float-type voltage typically being applied to the batteries during long passages. If anything there might actually be a danger of slight undercharging here, but daily boost charges while running the generator to make water and such, and occasional longer charges at the dock help make up the difference.

One for the House
A number of Nordhavn owners have a separate house battery charger in addition to the inverter/charger. In most cases the charger is primarily intended to allow the owner to plug the boat into 230VAC 50Hz overseas power (these units will accept a wide range of voltages and frequencies), which will allow an owner to power limited loads on the 24-volt and inverter panels and charge the house batteries. However, because the Lifeline batteries can take large in-rush charging currents (as much as 5,000 amps for our 1,020Ah battery bank - way more than would be practical for us to even try to do), it also makes sense to use the battery charger to speed up charging when running the generator. Also, in addition to increasing charging capacity, cutting down on generator run time and helping to properly load a generator, a dedicated high-capacity multi-stage house battery charger can serve as a backup supply of 24VDC charging power in the event the inverter/charger malfunctions.

There is one important note to remember while using the battery charger (a.k.a. the “Poor Man’s Atlas”) to power 24VDC and inverter loads while in a 230VAC 50Hz location. For economical reasons the charger typically used is the Victron 24/100 Centaur, which is a fine unit with a three-stage charging profile and settings for AGM batteries. However, history has shown there are some limitations to using this charger as the only vessel power source for months at a time in a 230VAC 50Hz location. The combination of 24VDC loads and inverter loads as well as battery charging can prevent the charger from going into the Absorption or low-voltage Float stage. With the batteries “stuck” in the Bulk stage, they can be exposed to higher voltage and current for extended time periods, resulting in overcharging, heating and drying out. Also, these chargers sense temperature internally instead of at the batteries, which could result in poor voltage compensation if the temperature difference between the batteries and charger is high enough. To avoid problems here, every 24 hours the operator should shed loads on the 24VDC system and inverter panels to allow the house batteries to fully charge and go into Float mode, or run a generator to allow the charger and inverter/chargers to properly run the batteries through a full charging cycle. Also, continuous ventilation of the lazarette would help balance the temperature inside the space.

So Far, So Good
We’ve only had a few boats experience premature battery death where one or more of the scenarios discussed here caused the batteries to give out before their time. We’ve also seen a couple of boats that have been run hard non-stop with deep daily discharges have their batteries replaced in a few years after they’ve given their all.

To our knowledge the rest of the Nordhavns out there have enjoyed a house battery life of 6 to 8 years – or much more – thanks to the resilient construction of the Lifeline batteries and the fact that even though these batteries can take a good deal of abuse, most Nordhavn owner’s never really put too much of a hurt on them in the first place. Still, it’s worth knowing what practices can cause premature battery death and how to avoid them.

Mike Telleria is P.A.E.’s technical writer, responsible for creating the thoroughly detailed, superbly informative and highly coveted Owner’s Manual that accompanies each new boat. Mike holds a degree in marine systems engineering from the U.S. Merchant Marines Academy as well as an unlimited U.S.Coast Guard 3rd Assistant Engineer's license. Prior to joining Nordhavn, Mike served as editor of popular boating magazines such as Lakeland Boating, Go Boating, and Sea magazine.

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