Above the Waterline

Marine Outfitter

805 455 8444

Optimizing batteries and charging systems.

You find yourself at a quiet, secluded anchorage.   Dolphins play in the waves, a seal surfaces right beside you.   Your beautiful woman takes a dip in the turquoise waters off the swimstep.   You are where you want to be.  Peace and quiet envelopes you. 

But you know you have to disrupt all of this because your batteries are calling you.   If you don't attend to them soon, your ice will melt, you won't have lights or a stereo or any of the other creature comforts you have brought with you.  Well you can't have it all - such is the dilemma of a sailboat on anchorage without adequate solar power.   

It is time to start the engine to charge the batteries.   So you can have it all tonight, the next two, three or four hours you will be spending in the company of your diesel engine's sound and pollution.   

What we are talking about here is a duty cycle.   How long does it take to charge your batteries and how long will they run for before needing recharging?   How can we build a system that charges quicker and runs longer?

Energy is created, stored and then used on a sailboat.   The duty cycle is improved by generating more, storing more efficiently and using less.   

The first improvement I suggest to my customers is to upgrade to AGM batteries.   Liquid lead acid batteries are inefficient and dangerous on a boat.

AGM batteries charge 2 to 3 times faster than flooded lead acid batteries.   They don't require any maintenance, are sealed, don't spill, last longer, and don't explode.  Their terminals don't corrode.   They can be mounted on their sides, have low self discharge rates, and charge more efficiently.   It takes less energy to keep them floating once fully charged.   

I always recommend a larger battery bank.   If you double the size of your battery bank you get more than twice the storage capacity because batteries get more efficient the less they are used. Not to mention your batteries will last more than twice as long at anchorage.  Bigger battery banks have a longer life span because they don’t have to work as hard.   AGM batteries can last more than 10 years if properly cared for.   

Discharging a battery below 50% decreases the lifespan of a battery.  The first night at anchorage you will get to use 50% of the batteries capacity, but each subsequent cycle will only yield 35% of the batteries energy.   This is because the last 15%, when the batteries are in the absorption part of the charge cycle, takes forever and is impractical while on anchorage. 

I recommend putting a Blue Sea Systems digital voltmeter with alarm on each battery bank on a boat.   These voltmeters have alarms for both low and high voltage conditions.   I set the low alarm at 12 volts and the high alarm at 15 volts.   12 volts is a little less than 50% (12.2 volts) but usually there is a load on the battery when the alarm goes off.    AGM batteries will not be harmed by a short duration charge up to 15 volts.  This will eliminate nuisance alarms caused by temperature compensation but still catch a high voltage condition prior to ruining the batteries.

There is a caveat to upgrading to AGM batteries.   The charging system has to be compatible.   Although AGM batteries are quite resilient, they will last longer if the correct charge voltages are observed.   Most alternators will work with AGM batteries but you should check them out first by putting a voltmeter on the batteries.  AGM batteries charge best at 14.3 volts and float best at 13.2 volts.  I recommend Balmar regulators that have Bulk, Absorption and Float Charging and a program specifically designed for AGM batteries. 

If you have a small alternator with a giant battery bank you can end up destroying your alternator because of the high acceptance rate of the AGM battery.  What happens is the alternator works at full capacity too long and overheats.    In situations like this I derate the alternator to 90 % to avoid the alternator from running at full capacity for extended periods of time.

Multiple charging sources can be used on the same battery bank effectively if and only if they have the same charging algorithms.   If one charging source is at 14.5 volts and the other is at 14.2 volts then the higher voltage charging source will become the dominant charging source.   So if you are going to put two alternators on the same battery bank, then make sure they have the same regulator with the same program and they will be compatible.

On sailboats equipped with a genset - the genset alternator should be set up with the same charging algorithm as the inverter charger and then both will be charging at maximum output.   If you want to significantly reduce charging time while on a genset then you can use multiple battery chargers from the same manufacturer.   If you already have an inverter charger you can add an additional battery charger.   Use the same brand because they use the same charging values.   When the batteries are deeply discharged they will work together.   As the batteries come up to absorption the chargers will drop off individually at the end, leaving the most dominant charger.

Most sailboats have more than one battery bank.   It is desirable to charge all battery banks together and discharge them separately.  Avoid battery isolators -  the half a volt drop on the diodes causes the charging system to become sluggish.  Instead set the system up with a battery combiner.  Connect the alternator directly to the house bank and echo charge to the start bank.    I recommend Blue Sea Systems 7610 or 7620 combiners.   Avoid the battery combiners with coil economizers -  they are very noisy and can cause other electronics to stop working.  

Battery chargers measure the internal resistance of the battery to determine how much charge to put into the battery.  If there is a loose connection on the battery or anywhere between the charger and the battery this will affect the charging algorithm.   Use tinned battery post terminals.  Lead battery terminals loosen up over time.    Replace the bolt that squeezes the terminal with a stainless bolt and attach the cable end to that bolt and nut.   Don't use the stud for a heavy cable because it will not give you a firm connection.   Always use nuts and lockwashers.   Never use nylock nuts on a battery terminal or any electrical terminal.   The nylon will catch fire if high amperage causes extreme heat at the terminal.    Never stack multiple cables onto a battery.   If you have more than two connections on a battery post add a power post to a bulk head and take your connections off the power post.   Never put a small connector on the same stud as a large connector.   All the current will end up passing through the small connector creating resistance and heat.    Dissimilar metals from multiple connections on a battery terminal creates corrosion.   Optimally you have only one and a maximum of two large battery cables attached to each battery.

Your charging system is only as good as its' weakest link so look over every connection.   Apply a thin coat of vaseline to your connections after they have been fully installed.  I avoid corrosion sprays because they are dielectric and they creep into the connection causing additional resistance.   Anywhere you have resistance to electrical flow you create heat and enough heat creates fire.   Fifty two percent of all fires on boats are electrical in nature so beware and keep you electrical system in good condition, inspect it regularly, and correct any problems immediately.

Batteries should be securely fastened down to the boat.   A boat should be able to survive a full knock down and roll over.   Most batteries come loose from their mountings in a full rollover.   Imagine having a 168 pound 8 D battery coming loose from its mounting and falling across the cabin in a rough sea.   Imagine that battery being a liquid lead acid battery and spilling its acid all over your belongings.  If a boat gets knocked down, this is the last thing you want to think about.   Consider mounting your batteries as if you were mounting them to a ceiling rather than a floor.    A boat without electricity is dead in the water so pay the strictest attention to how the batteries are fastened down.   If the batteries move around it affects the sailing characteristics of the boat.  If they are firmly attached to the boat they act as ballast.   Move the batteries to the lowest  and most central part of the boat so the ballast is the most effective.   Check the waterline when you first place the batteries in the location.   Use the batteries to balance the boat on its waterline.   Make sure any tanks that are off center to be half full when checking the waterline for a new battery placement.

The optimum battery and charging system can recharge itself in one hour because AGM batteries can accept a charge of up to 40%  of their amp hour rating and you only use 35% of their amp hours on a regular basis.    Size your battery bank so you can last at least 24 hours without recharging and optimally 48 hours or more.    Size your alternator to be up to 40 % of the size of the battery bank for example 40 amps for every 100 amps hours of battery capacity.    

A typical 30 footer might have 2 4D batteries in the house bank yielding 400 amp hours.   35 % is useable - 140 amp hours so you can use an average of 5.8 amps for any 24 hour period.   Your alternator would be up to 40% or 160 amps derated 10 % to 144 amps.   

You, your boat neighbours at anchorage, and mother nature will all be happier if you optimize your charging system.     Once you have your batteries and alternator optimized consider installing a solar system.  That is if you want more time with your beautiful woman and the torquoise waters she is swimming in.    

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Email James at       or telephone 805 455 8444

Above the Waterline, Ltd.,    6 Harbor Way # 226, Santa Barbara, CA, 93109, USA