Marine Electronics – Batteries

Dear Batteries,

I am sorry. Can you ever forgive me? I have neglected you all these years, I can’t believe you haven’t given up on me. I jump aboard every week and simply expect you to do your job, getting very little in any love or attention back from me, and yet you continue to provide power to start the motor, shine lights when it’s dark, charge our phones, chill our beers and allow our stereo to produce sweet sounds while we cruise around.

And yet I ignore you. I feel so guilty.

“What kind of batteries does your boat have?” enquired Chris from Tweeds Marine to the attentive class eager to learn all about marine electronics.

“Ummmm…” I replied blankly, feeling a sudden stab of guilt. What kind of caring diligent boat owner doesn’t know what kind of batteries they have FFS…?! Everyone else in the class appeared to have a far more intimate relationship with their boat batteries than I did.

It turns out that there are a number of different kinds of batteries, and I really SHOULD be paying them a bit more attention than I currently am, as if you take care of them then they will give you years of service and save yourself lots of hassle and money in the long run.

To be fair, my batteries aren’t exactly located in a convenient location to go checking them out. Being heavy, they are both positioned centrally and low in the boat, which is the optimum position for boat stability. I have also got them strapped down and in a box so that if we had the misfortune of rolling over they wouldn’t go flying around all over the place. In fact Yachting NZ regulations stipulate that they should also be installed in acid proof boxes and not near any fuel tanks, filters or lines. They should be positioned above the level of the cabin sole, because of the possibility of conductive salt water covering them if they were in the bilge, and being a little bit higher allows a bilge pump to run as long as possible, also, if salt water should ever get inside an battery that has refilling vents, it can explode with a cloud of toxic chlorine gas.

Furthermore YNZ regulations require that all electrical work onboard complies with the current version of standards. You need to have a separate battery with the primary purpose of starting the engine (unless you have some other means of starting the engine). The Standards specify that the battery box should be ventilated and in a place where the manufacturers temperature limits will not be exceeded, fuses should be placed within 200mm of the terminals, and no more than four wires are to be connected to any battery terminal.

Batteries produce explosive Hydrogen Gas when charging, so the containment area needs to be vented to allow the gas to escape. Hydrogen goes up – not down like LPG.

Most yachts have at least two batteries. One is the Cranking Battery which is able to deliver a massive load for a short time – to turn the motor over. They have lots of thin metal plates inside them and are rated by CCA (Cold Cranking Amps) – the current a fully charge can deliver for 30 seconds before the voltage drops to 7.2v at -18ºC. (or 0ºF)

Then you have a House Battery whose main purpose is to provide all the fun things like lights, stereo’s, fridges and important things like GPS, auto helm, VHF and that kind of thing. If you have got lots of systems on your boat then you might need more than one house battery, more on that later! House batteries are classed as Deep Cycle. They have thick metal plates inside and are much heavier than a starter battery. They are rated by Ah (Amp Hours) – the number of amps that can be withdrawn over a set time – either 10 or 20 hours,

Traction Batteries are not true deep cycles. They are designed to do both cranking and deep cycle, but are not 100% purpose specific for deep cycle. They were originally designed for large earth moving equipment that needed huge starting power and on going electrical needs.


Well I know for sure that I have got two batteries, and a switch to change from Battery #1 or #2 or both, but I don’t know which is #1 and which is #2 and which is the house and which is the cranking, or whether they are both the same. I do vaguely remember buying them, must have been quite a few years ago now.

So it turns out that batteries are made up of a complex chemical reaction going on between two metals of different affinities which are exposed to acid inside the heavy plastic box. You can learn lots more about this process and much more on a cool website called Battery University.

Some other fun battery facts:

  • A battery doesn’t actually store energy – but it converts chemical energy to electrical energy by a electrochemical reaction.
  • You’ll shorten the life of your battery if you over or under charge them.
  • The voltage difference between a fully charged and fully discharged battery is approximately 1.0 cold (12.6v to 11.6v)
  • Battery capacity needs to be at least three times your usage between charges.

Lead Acid Batteries

Lead Acid batteries are cheap with a low cost per watt-hour, and easy to manufacture, they recharge well and have a low self-discharge rate. However they are slow to charge, must be kept fully charged to prolong their life-span and their life-span is limited. They aren’t great for the environment either (lead is toxic). However most of them are recycled.

There are a few different kinds of Lead Acid Batteries and they are as follows:

Wet Cells – these have filler caps on the top and need to be checked regularly and filled up with distilled water (because plain normal water may have minerals in it that can affect the chemistry inside the battery) They are good value and if cared for then they will last for a long time. If they aren’t used they can lose around 7% of their power per month. They need to be equalised every now and then (recharged with a charger at a high voltage to control sulphation). They take a long time to fully charge. They are the cheapest option for lead acid batteries.

These batteries need to be cycled. You get a problem called surface charge if they are left in float for long periods. This is where the voltage tests good, but it drops away fast when placed under load. So they needs to be slightly discharged and then charged fully again once per month. Also, the acid being heavier that water, settles to the bottom and the equalisation charge stirs that up to maintain a proper mix of electrolyte. Also something that needs doing once per month – you need a charger (and a power source – like in a marina) to do this.

On a recent sailing forum people had these things to say about wet cells:

  • “they take abuse and are cheaper, but need more care”
  • I would avoid lead acid batteries because of all the maintenance headaches. They gas-off when charging, They can spill out through the cap vents and need periodic refilling.”
  • “Flooded, then if you cook one, you are out $90 and not $900”
  • Flooded, maintenance is 20 minutes a month
  • “Flooded, easier to replace around the world. more forgiving charging”
  • “I have 8 x 6v “wet cell” batteries on our trawler. Check them religiously 1x/month. I just picked the first of each month which makes it easier to do. About every other month I need to add just a bit of distilled water.”
  • I would go with the much cheaper flooded batteries because they are much cheaper.

Sealed Lead Acid – as above but without the filler caps. They do have a valve on the top which allows for gas to be discharged. (Chris reckons that this is probably what I have got).

  • “Sealed lead acid technology that isn’t finicky about charging, doesn’t mind being left partially charged, can regain 100+% of capacity if abused/left partially charged often. Downside is only available in group 31 and is not inexpensive. But amp hour for amp hour, the lowest cost over the life of the battery. The saving in stress over proper charging – priceless!”
  • “Most of my cruising friends get 3-4 years out of flooded batteries. Haven’t come across anyone claiming 8-10 years, like you get. And I REALLY appreciate locating the batteries so I never need access to them, because they require NO maintenance. That alone is worth the cost. No distilled water, no acid, no checking, no nothing.”
  • “Sealed lead acid batteries lasted significantly longer for us than AGMs. And we religiously charged the batteries correctly.

Gel-Cells – these were popular in the 1980’s and 90’s and were designed for remote power back up applications, where they would remain fully charged for long periods and then if the power drops, the backup wold come on till the power is restored. They don’t like a lot of discharge cycles, but they are happy to remain in float charge for long periods. They are susceptible to damage if not charged properly – so they need a charging regulator. They are a sealed unit, so they don’t give off much gas and can even be submerged. They self discharge around 3% per month and have the highest number of charge/discharge cycles for lead acid batteries. The performance of these batteries drops suddenly at the end of their lifespan. Most expensive Lead Acid kind of battery.

  • A gel battery can still gas off, especially when it fails. I know because it recently happened. I visited my boat one evening and I smelled rotten eggs coming from my boat. 2 of my 4 gel cells had failed and had filled my son’s cabin with gas vapors. It if had happened at night while he was sleeping on the boat who knows what would have happened. It’s my understanding that an AGM cannot gas off even under failure. A gel cell and AGM are about the same money, so I’ll take AGM over gel batteries. If we go with flooded we are going to relocate them to under the settee in the salon so no one is sleeping I’m an enclosed cabin with them, also for ease of maintenance.”

AGM (Absorbed Glass Mat) – these batteries have a fibreglass mesh inside them. They have the quickest recharge ability, are long life and have just a 3% self discharge rate. They can be used for house and starter batteries, are spill proof and maintenance free. Their performance has a gradual decline. Middle of the price range – more expensive than wet-cell but cheaper than gel.

Here is what the forum people have to say about AGM batteries:

  • I switched to all AGMs. I don’t worry about maintenance and I can leave them in place during the Michigan winters.”
  • I went with AGMs. Before I did I managed to kill a couple of sets of wet cells from MY neglect. The AGMs fit better and I do not need to worry about sea water releasing poison gas if I get flooded to any extent. With AGM, if one goes bad I can drop in a wet cell (any place in the world). Put first set in around 1998?? lasted till around 2010.
  • Go with the AGMs – it’s an investment, but you will be happy with them
  • “Just make sure your charger is compatible with AGM and can be changed to Lead acid Batteries which are much easier to get abroad.
  • Unless you spend a lot of time at the dock fully charging AGM’s they will not tolerate the deeper discharge and less than 100% charging that happens when cruising…My lead acid lasted almost 7 years, top of the line AGM’s less than 2 years.”

You should never let lead acid batteries get below 50% of their capacity before you charge it up again as this can cause permanent damage and limit the lifespan of your battery. As they take a long time to charge up, especially the last 20% or so. Because of this you might find that you are only able to use 30% of the battery’s potential i.e. when it is between 50-80% charged.

As soon as you let a battery, especially an FLA (Flooded Lead Acid) drop below 12.6V, it starts to deteriorate. If you keep a battery fully charged and never cycle it, it will last for years. If you fully discharge a battery, to nothing left in it, it will likely never fully restore itself again. doing that about 3 times in a row kills a battery. There is no point in killing the battery within just 3 uses. The 50% discharge rule of thumb is just that. A rule of thumb. Because it is considered the point in which you get the best number of cycles over cost. However, not all batteries are created equal and this is all where it gets really difficult to understand. The facts lie in the guarantee. A real McCoy deep cycle will offer a warranty of X many years if depth of discharge is within 50%. But some offer warranties of X number of yrs only to a depth of 20% and one I’ve heard of, only if you can provide a paper printout of the history of it’s life. So care needs to be taken with what that fine print says.

It is important to not over-charge sealed lead acid batteries either. If they are over-charged then this can cause gassing and water depletion. The metal plates inside can dry out and crumble. If you have a wet-cell battery, then you can simply top up the water level. But with sealed units you can’t. So for this reason Gel and AGM cannot be charged up to their full potential.

Screen Shot 2017-05-27 at 2.22.51 PM

Lithium Ion Batteries

Lithium Ion batteries have some advantages over lead acid. They are much lighter and smaller than lead acid, they charge faster and last longer than lead acid. They have a higher usable capacity as you can discharge them to 0% without damaging them – meaning you can go longer between re-charging. They are better for the environment than lead acid, but only a small part of them can be recycled.

On the other hand however, they cost a lot more than lead acid and they also require an expensive battery charging monitoring system, as they can be very dangerous and explode if they are over charged or allowed to go below a certain safe voltage threshold.

Getting a Lithium battery bank setup with a monitoring system can cost around NZD$15,000!

Screen Shot 2017-05-27 at 4.04.50 PM

Here is what the forum people say about Lithium:

  • If money is no object go with the lithium batteries
  • “A Li battery bank for the average sailboat would run you about 10x that of any other kind. Their charge control technology and safety has improved quite a bit over the past couple years and prices are coming down, but still waaaay to expensive for most cruising budgets.”
  • “lithium, 2000 cycles to 80% and half the amp hours required. Can’t over or under charge though, if you do they are permanently damaged. Not a recommendation but I believe they will be the best option soon.”
  • “I would love to switch to lithium.

Check out this great post by SV Evenstar on the reasons why they are changing to Lithium batteries.

So which one do you choose? Chris made some good points: If you are a coastal cruiser and you are motoring on and off a mooring every time you go out, then you are probably going to be using your motor regularly anyway – which of course charges up your batteries, so lead acid would be perfectly fine in this situation.

If you are an ocean sailor then you might not want to be turning your motor on every day to charge your batteries, and using up precious fuel on a long journey. If you are planning on having your boat for a long time, or if you are a racer and weight is a consideration, then lithium batteries might be a good investment to make.

Battery Charge

As a rule of thumb every 0.1V = 10% of the battery capacity.

  • 100%  = 12.8v
  • 90%    = 12.7v
  • 80%    = 12.6v
  • 70%    = 12.5v
  • 60%    = 12.4v
  • 50%    = 12.3v
  • 40%    = 12.2v
  • 30%    = 12.1v
  • 20%    = 12.0v
  • 10%    = 11.9v

This is good to know, especially if you have a gauge that tells you what your battery levels are. I have got a couple of dials on my switch board, but am feeling even guiltier to admit that I never look at them… 😦

IMG_1915Seth doing some soldering.

Looking after your batteries

Perhaps a good start might be actually knowing what kind of batteries you have got. (I promise I am going to look when I go out sailing tomorrow…)

  • Keep them clean
  • Get to know them – how should they be charged and how do you tell when they are full?
  • Don’t let them get too hot or too cold. Keep them cool but not frozen
  • Store them fully charged – get them back to at least 80% of capacity after every use, and back up to 100% as often as you can.
  • Don’t over charge them
  • Don’t over discharge them – lead acid batteries shouldn’t get below 50% charge.
  • Make sure you top up flooded (wet) cell batteries with distilled water
  • If you over-discharge a battery recharge it rapidly as soon as possible
  • When using the windlass or inverter or other power hungry appliance always start up the motor
  • If you are plugged in to shore power, don’t leave the charger on for a long time.
  • If you have flooded lead acid, equalise them every month or so.
  • Get a measurement system – know how many volts are at the battery, how many amps are going in and out (get a ‘shunt’)

So, confession time – do you know what batteries are on your boat? Which kind do you prefer and why? How do you charge them? Any other tips or tricks for me to add to my post?

Thanks Alan Wheeler blog reader for his extra tips 🙂

Click here to read about Marine Electronics Terminology

Click here to read about wire sizes, fuses and how to design an electrical plan of your boat. 


24 thoughts on “Marine Electronics – Batteries

  1. I just replaced 2 of my 3 batteries. All are sealed wet cell, deep discharge purchased from a national auto parts retailer. Based on my sailing, age and ‘pedigree’ of boat, it didn’t make sense to spend $200 each to buy anything more exotic. My boat is docked 5 days a week normally, and we never do passage-making. We’ve never even spent a night out on the bay. I believe we have the right setup for our needs.

    Liked by 1 person

    • My current set up sounds the same as yours except I’ve just got two batteries and that is the kind of sailing we usually do except when we go away for summer holidays. Has worked really well for the last 13 years or so 🙂


  2. I’m really curious about your “voltage vs % of charge” chart. We installed two new lead-acid deep cycle batteries (our #2, or house bank) back in January. The were fully charged, but I charged them again to be sure, and their resting voltage was 12.3v. According to your chart, that would be 50% discharged. I wonder why?

    Liked by 1 person

      • I checked my gauge at the weekend and it only shows both batteries as being at 12v… yikes! I am hoping my gauge is wrong. Well the motor started so that was the main thing!

        Liked by 1 person

      • So I’m going to go out on a limb here. I think (and your battery guru can tell me I’m full of fluff if he/she needs to!) that your chart is a “maintenance charging voltage”, rather then a “resting voltage” chart.
        So in my world, a fully charged lead acid battery that’s still being lightly charged would be at about 12.8 volts. A fully charged battery that’s just sitting idle, neither charging nor providing power – would be at about 12.3 or 12.4 volts for 100% capacity, and at 12.0 volts would be down to about 50%.
        At least that’s the numbers I’ve observed from our brand new house battery bank. And at 12.0, the engine turns over just as fast as it does at 12.3.

        Liked by 1 person

      • OK here goes. Two points about that voltage v state of charge chart. a) it is generic and you should always refer to the one that came with your batteries as it will differ slightly. b) The voltage is measured with the battery at rest; this means that there must be absolutely NO load on the battery and it must have been sitting for some time. Ideally the resting voltage should be measured after the battery has been resting for a couple of hours or so.

        The tiniest load will drop the voltage a bit. If you are measuring in that state then are you sure that you have ‘fully’ charged the batteries? What type of batteries do you have; flooded, sealed, AGM, gel, etc.? They are all lead acid but with important differences.

        Liked by 3 people

  3. Hi Vicky. If you’re live-aboard sailors and use a fridge, you should consider a high capacity charging unit. We had a small one when we bought our boat, left France with a great house battery park but a small charging unit – previous skipper sailed often but never more than a week without putting the boat batteries on the harbor socket, so his batteries got full charge while he was “tranquille à la maison” – should take care about the “floating period” of your charging unit, when, your batteries being at 80% charge, your charger will send a quite small quantity of amperes into them. As a result, with us living on the boat, the fridge (huge consumer) and all the other stuff, lights, laptops, whatever, the charger had periods when there was less amperes coming in than those going out. Finally I bought a pack of truck batteries in Morocco, knowing that in maxi two years I had to change them again… the old ones were dead, and I had to move the boat. As you perfectly explained at the beginning of the article, it all depends on what you do with your boat – racing, weekender, holiday cruising or live-aboard circumnavigator. There’s a balance to do between what you need and what you can produce (but isn’t it this way that life goes ?)
    Thanks for all, it’s great reading you, especially when being stuck here at work to get some money for sailing (later…).

    Liked by 1 person

    • Thanks Roz, Is the high capacity charging unit for when you are in a marina and can plug in to mains power? My boat is on a mooring but we will be hauling out shortly so I will make sure I give the batteries a good charge up then, but that does sound like a good option for when we move in to the marina (hopefully next year when they build it!)


  4. We replaced our AGM batteries a couple of years ago because we increased the load (freezer and laptops) and therefore needed to increase capacity – all with full time cruising in mind. We could have increased the old AGM bank but bit the bullet and bought the lithium-ion. We haven’t looked back! Our advice, particularly for long term cruising, is to get the biggest battery bank and charging sources (solar, wind, water, alternator) you can. You can’t produce too much power!

    Liked by 1 person

      • On the other hand, you can buy a TON of lead-acid batteries for the cost of one Lithium. That’s not an environmentalists responsible view of things, but… I’ve run the numbers over and over, and on a pure cost-per-amphour basis, I just can’t justify the Lithium batteries. Am I missing something? Maybe our style of cruising and our fairly low power requirements are the key.

        Liked by 1 person

      • Yes absolutely I totally agree, however I think if you also look at not just the purchase price but also the cost of charging up those batteries – say if you were living aboard and had to run your motor for a couple of hours a day to keep them charged up – as opposed to being able to let the Lithium ones run right down before charging them up again, then perhaps there is added cost in regards to fuel, engine wear, that kind of thing perhaps?
        I seem to do ok with just motoring on and off the mooring at the moment – no solar or wind generators on my boat. But then we aren’t living aboard at the moment.

        Liked by 1 person

  5. I always thought that we would go for AGM batteries, because of the price of lithium – but once I started looking closer into it I realized that we can get 100 Ah for about 1000 euro (We’re based in Denmark/Germany). AGMs would be about 600 euro. And since lithiums can be drawn down completely, they almost provide double the Ah. So suddenly the price isn’t so bad. Also, our boat is really small, so the option of extra power that doesn’t add weight or take space really matters to us. Good luck choosing!

    Liked by 1 person

  6. Good to see Seth soldering, on our trip from Auckland we had the engine on two two days solid, we found there was no alternator regulator on the batteries, it cooked them both and filled the cabin with hydrocloric acid fumes, we had no sight of land anywhere, we evacuated to the desk (5 of us), shut down everything and waited….. they were very hot and swollen, lucky the fumes did not ignite, a thermal working on batteries would be a good safety feature…

    Liked by 1 person

  7. Pingback: Marine Electronics – Charging your Batteries | Astrolabe Sailing

  8. Pingback: RYA Offshore Yachtmaster | Astrolabe Sailing

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