
A word about batteries
Date: Monday, January 06, 2025. 1843Z — Sunrise: 08:37:54, Sunset: 16:22:34 |
Location: 53°18'0.3N, 6°8'18.3W — Dublin, IE |
WX: Wind 270° at 16.0 knots. Scattered clouds, 2.35°C, 987mb.. |
I am of course being sarcastic when I say “a word” about batteries, as entire books have been written on the subject, and many more could still be written. As we had some excitement with two flat batteries last year, I thought it might be a good idea to add to the extensive number of “words” written about batteries. This is the first of two parts, on the subject. I will start with an introduction to the technology, and a fairly detailed discussion of lead acid batteries. In Part Two, I will talk about Lithium Iron Phosphate batteries.
Still here? Keep reading…
First off, I will keep this exclusively about batteries in a marine environment, to try and rein in the subject matter. I will also try and steer clear of discussions of electric motors in sailboats as that is also worthy of a full blog post. There are two or three main categories of batteries in use today. The classic, and most common, is the old reliable “lead acid”. Essentially some lead plates suspended in a water solution with a hint of sulphuric acid. A battery in the true sense is actually a collection of cells. You could think of “battery” as the collective noun for cells. A lead acid cell has a nominal voltage of two volts (2V). So your average car and boat battery has six cells in series. The second category is Lithium Ion. Here the cell voltage is 3.7V, and you can combine 3 of these in series to get 11.1V, or if you dare, 4 cells to get 14.8. Your mobile phone and your laptop both use Lithium Ion cells. They’re not appropriate for boats, so I won’t discuss them too much further. It is debatable whether Lithium Iron Phosphate or LFP batteries go under the heading of “Lithium batteries” so I’m keeping them as a third category. Lithium Iron Phosphate batteries are often named for their chemistry, which is LiFePO4. LFP is easier to say, and easier to type! They have a nominal cell voltage of 3.2V.
Just to elaborate, the reason Lithium Ion batteries are not appropriate is because they don’t like seawater, and they are unstable. If they catch fire, it is extremely difficult to put out the fire. Convinced, yet? Also, they can catch fire if physically damaged, overcharged, or allowed to under-charge. Generally, your phone and laptop use something called a Battery Management System (or BMS) to make sure the Lithium Ion battery never get overcharged, or drained too much. But they’re a hazard on board a sailboat. Avoid their use and choose LFP instead.
As another segue, I want to discuss some terms here, which get interchanged incorrectly. Voltage is what a battery gives you. It represents the strength (in a way) of the battery. Current is what you take from the battery. Think of this as the load (again, this isn’t exactly accurate but useful for our purposes). The product of voltage and amperage is power. It is possible to design a very small battery which produces a high voltage, but is unable to turn a starter motor because it cannot deliver the current. By way of contrast, a stick welder uses a very high current and a very low voltage. Both of these are low power examples. A diesel generator can produce power anywhere from a couple of kilowatts up into the megawatt range.
Energy is power multiplied by time. The most common unit of energy that we are familiar with is the kilowatt-hour. In actual fact, the kWh isn’t an official unit of anything, but it is widely used. The official metric equivalent would be the Joule (one Watt-second). One kilowatt-hour then is actually 3,600,000 Joules. People regularly get confused between power and energy, and it’s understandable. Think of it this way. If you drive your car really fast, from point A to point B, you will need a lot of power. By contrast (and ignoring air resistance and friction), if you drive the same distance much slower, you would need much less power but would use the same amount of energy. You used a lot of power but only for a short time, in the first example. In the second example, you used far less power but needed to use that power for a lot longer. A lot of batteries are rated in Amp-hours. This, as you can imagine, is a form of energy specification but it is deceptive because it is independent of voltage. A 2 volt cell which has a capacity of 100 amp-hours does not store as much energy as a 12 volt battery which has a capacity of 100 amp-hours. For a car battery, multiply the amp-hours by 12 to get the watt-hours which is a more realistic energy measurement.
Lead acid batteries provide quite the kick in terms of amperage. They were designed, don’t forget, to turn the starter motor on a cold engine. As the ubiquitous motor vehicle has been with us for quite a while now, and it is far and away the #1 client for a lead acid battery, those batteries are designed with the car in mind. The inrush current of a starter motor can be up on 400 amps. That electric heater by your feet is consuming 10 amps and it’s one of the bigger current draws in the house, so that should put 400 amps into perspective. The running load of a starter motor is around 200 amps. Also bear in mind that car engines have their own electricity generating capability via the alternator. When the engine is running, the car is self-sufficient, electrically speaking. So, you end up transporting a starter motor and heavy lead acid battery in your car, purely for the two or three seconds you’re starting the engine. I’m probably labouring the point here, but a lead acid battery exists to provide a high current for a very short space of time. The fact that we use them to keep the fridge cold on a sailboat is an anomaly. Lead acid batteries from the car world are almost always 12 volt (yes, I once had an MGB which had two 6V batteries in series, but what’s your point?). Their difference is in their energy capacity and “cold cranking amps.” A quick look online shows a typical car battery with 85Ah of capacity and 800 Amps of starting current. In theory, it could deliver that 800 amps for only 6 minutes, before it was completely flat. As we’ll see, the reality is very different.
Even though watt-hours is a more official measure of energy capacity in a battery, as long as we’re talking about 12 volts, we can multiply all the numbers by 12 in our head, and just refer to amp-hours so that it is possible to compare actual batteries. Given our example starter battery has 85Amp-hours (or Ah), it would be easy to assume we could pull that energy in any way we wanted. Unfortunately, this isn’t true. When they measure amp-hours, they do it over ten hours. So they load up this example battery with an 8.5 amp load, and it can provide power for ten hours. This is known as the C10 rate. Giving us 8.5A x 10 hours, or 85 Amp-hours. What if we had an 85 amp load? In theory, it could power this for a full hour. It doesn’t, though. The efficiency falls through the floor. We might get 45 minutes of discharge, giving us an effective capacity of 63.75Ah. We might not even get that. Sometimes, capacity is specified at the C20 rate, which means you can get 4.25 amps consistently for 20 hours.
On the other end of the scale, if we only took a tenth of an amp, could we expect the battery to last 850 hours? Possibly (your mileage may vary, etc, etc). But this isn’t perfect either. Batteries have an internal leakage resistance. Which means a fully charged lead acid battery, with nothing connected to it, will lose energy. Not a lot, but not zero, either. Furthermore, lead acid batteries do not like being discharged fully. It is hard to charge the battery over 80 or 90% (this takes a long time) and discharging below 30% is bad for them. So, after all that, our energy capacity for the battery is only half of the rated 85Ah and only if we consume it at no more than 8.5 amps at a time. There are alternatives called “deep cycle” batteries which are much more agreeable to being fully discharged and recharged.
Before we talk about practical uses, it’s worth discussing how you charge a lead acid battery. Ideally, we would also charge it at the C10 rate, which would mean an 85Ah battery should be charged wit no more than 8.5 amps. We’re going to ignore that, though. Most high output alternators can put out over 100 amps of current, which (in theory) would charge the battery in less than an hour. To discuss charging, we need to consider some of the voltages, as well. This is why a digital voltmeter is essential equipment on a sailboat which has an engine, or a battery. Measuring the voltage across the terminals on battery, with no load, can tell you a lot about the state of charge of the battery and a little bit about the overall state. A fully-charged battery, with no load (in other words, the isolator switch is off), should read around 12.5 volts. There is some difference of opinion around what a dead battery should read, but it is not 0.0! Some will say that anything under 12 volts and the battery is flat. Personally, I tend to be more optimistic. If the battery is reading 10.5V though, everyone would agree it is totally discharged. How is it possible that a battery which is saying it has 10.5 volts of potential difference, does not have a charge? Surely it must have some energy if it can move the meter needle or the DMM display? What you will find is that a battery with that kind of voltage, cannot provide any kind of load whatsoever. Digital Multimeters consume milliamps or less. The battery will not turn the engine, nor drive a cabin light. By the way, never leave a battery in a fully discharged state.
On the opposite end of the scale, do not put more than 14.4 volts across the terminals of a lead acid battery. Imagine if we had a nuclear power plant, capable of insane power output. We ask it to provide 14.4 volts at any current demanded, and hook that across a flat lead acid battery. The battery would consume “infinite” current, in theory. In reality, due to internal resistances, it wouldn’t manage that. It would also get extremely hot. We would do untold damage. So it’s not enough to limit the voltage to 14.4, we also want to limit the current. If we were nice, we would limit it to 8.5 amps (for our typical example). But we could limit it to 50 amps as per the alternator. Current limiting like this involves reducing the voltage from the charger so that the demand current never exceeds our designed maximum. This is called the bulk-charge stage of battery charging. It will consume the maximum current until such point as the voltage within the battery itself starts to come up high enough that we can no longer get that maximum current into the battery without also raising the voltage above 14.4V (which of course we would never do!). So now that the voltage has reached 14.4, we will notice the current demand drop away as the battery continues to “fill up.” This is known as the absorption stage. Eventually the battery will take only a small trickle of current. If we were charging the battery using an external charger, it is now that we would need to intervene and remove the charger. If we continue to maintain 14.4V at the battery terminals, even though the battery is only taking a small current, it will boil dry. We need to switch to the third phase of charge, called the float stage. An interesting and useful aspect of lead acid batteries is if we maintain a voltage of 13.7V at the terminals, in room temperature, the battery will neither charge nor discharge. This phenomenon is used by alarm companies. They include a “Sealed Lead Acid” battery inside the alarm, and apply exactly 13.7 volts at the terminal. The battery will stay charged, and the equipment will be powered by the 13.7 volts. Should the AC mains fail, the battery will start to discharge but will continue to provide power to the alarm circuits until it goes flat or AC power is restored. A couple of caveats here; firstly, that float voltage is temperature-dependent. High-end charging systems will measure the battery temperature and adust accordingly. If your boat is in the Caribbean, expect the float charge to be less than 13.7 and in fact, you will overcharge your battery using 13.7 volts. Secondly, a floating system like this is very slow to re-charge the battery as it doesn’t have a bulk- or absorption- stage.
Using a voltmeter then, if the battery is reading exactly 13.7V then we can assume the charging system is floating the battery voltage and the battery is not being charged or discharged. It’s probably safe to assume the battery is fully charged. If the voltage is higher than 13 volts then the battery is either floating or being charged. If the voltage is 12.5, then we are running off the battery alone, and it is fully charged. Anything lower than 11 volts and we have an issue.
I mentioned that voltage gave us a slight indication of battery state. Let me elaborate. As mentioned, a flat battery can read 10.5V or even 11V and be without any charge. If we try the starter motor it might make a clicking noise, but little else. To really test the battery, we need to measure the “open circuit” voltage as well as the “under load” voltage. Garages have a battery tester which is just a defined load and a voltmeter. They measure the voltage while the battery is under significant load. These are handy, but probably not worth carrying on board. You could measure the battery voltage with everything off, and then measure it as the engine cranks. If the battery is flat, the voltage will drop down to zero (or close) as you try to turn the starter. It is also useful even if the battery is fully charged. Measure the voltage as the starter motor turns and you will have a very good indication of how much life is left in your engine battery.
Any kind of fixed load is good, but ideally it would be fairly stressful for the battery. Personally I think a 10 amp load is sufficient, and doable. If you’re in port or on the hard, it could be worth buying a battery load tester to keep ashore. A somewhat good test is to listen to how well the engine turns, when the battery is fully charged. If it springs to life, your battery sounds like it is OK. If the engine has all the enthusiasm of a teenager going to school on a Monday morning, it is quite likely that your battery is past its prime. Bear in mind though that old and cold diesel engines can sometimes add a significant load to even the best batteries. Get into the habit of listening to how “lively” the engine starts, and think about either replacing or load-testing the engine battery if that starting enthusiasm fades away.
Now that we’ve gotten the technical stuff out of the way, and at the risk of writing an entire book on the subject, let’s switch to talking about the typical boat configuration. Leaving out solar panels for now, a small 24 foot-er with an inboard engine would have a single battery with perhaps a capacity of 120Ah. You’re running the navigation lights, interior cabin lights, instruments and VHF off the same engine battery. While this isn’t ideal, it probably isn’t worth switching to a two battery solution. Think about it - you arrive in a secluded anchorage after a long day’s sailing. You turn off the instruments, turn on the interior lights and the anchor light. You wake up in the morning, turn off the anchor light, and find that the engine won’t start. This is a catch-22 because you need the engine to charge the very flat battery. In my Achilles 24, this was a regular occurrence. Luckily, it was possible to hand-crank the Yanmar 1GM10. So, after breakfast, the first job of the day was to crank and crank and crank until the single cylinder engine roared to life. At that point, the alternator was spinning and charge was being restored to the battery. On a larger boat, you’re not going to be able to hand-crank the engine in all probability. So you split the roles. You have an engine battery which is connected to only two things; the starter motor and the alternator. Ideally it is a completely closed system. At most, there might be an isolation switch, but beware of switches which connect the two batteries together. It shares a common ground with the other battery, but never run any of the instruments or lights off that battery. A second battery, ideally a deep cycle one, connects to the internal switch panel (via an isolator switch).
We can see how this approach means there is always a fully-charged battery available for starting the engine, even if we run the cabin lights and anchor light all night long. If it’s a deep cycle battery, it won’t mind too much if it is completely discharged overnight. The problem is, how do we charge the house battery? Some boats use a four way switch and a common power rail. The power rail can be connected to battery #1 (the engine battery), battery #2 (the house battery), both together, or nothing at all. Turn it to off when you’re not on board. Ultimately the weakest point here is when other people turn the battery switch to the first position (engine), or to both without realising that the domestic draw is now coming from the one place it shouldn’t be - the engine battery. A solution is to remove the four-way switch, install two isolation switches, and a voltage-activated relay. This is a device which sits between both batteries and is activated by any voltage above 13 volts. The engine battery stays isolated, but when the engine is running and the alternator is producing power, the relay automatically engages, and shares the power with the house battery. Some of these can operate in both directions, so if a solar panel is connected to the house battery, the relay will also operate so that the solar panel can provide a bit of charge to the engine battery. A slight concern here is if you’re starting the engine while this relay is engaged, it is very unlikely to be able to handle the enormous starter motor current, should that be provided by the house battery rather than the engine battery. It is entirely probable that 50% of the starting current will be pulled across that relay unless there is something to prevent it being activated while the engine is started. A separate isolation switch connecting the two batteries together can be handy in the rare or unusual case that the engine battery is flat and the house battery is not. If you’re using any technology other than lead acid (like LFP for example) as your house battery, make sure you never, ever connect the two together. Also, don’t let your LFP battery near the alternator without some form of protection. More on that in part two.
So, bringing a solar panel or two into the equation, it is best to connect these via an MPPT controller, to the house battery. The reason for this is twofold. Firstly, it is the house battery which will exhibit the most charge range, dropping from 100% to 20% for example, over the course of a day or two. The engine battery, if properly isolated, will always be at or close to full charge. Secondly, you want to use the solar power for more than just charging the batteries. Ideally your fridge would run directly off the solar power. In fact, I make a point of turning off the fridge on Nikea at around 4PM or 5PM on the basis that the solar panels aren’t producing any more power effectively, and it is cool enough that the fridge will stay cold until first thing the following morning. In an ideal world, you can calculate the amp-hours needed for the house battery by removing items such as the fridge because it will be powered by the solar panel. You can then prepare a smaller energy budget, based on expected usage for lights and instruments alone. Once you know what size battery, you can then determine how much solar power is needed to charge it over six to eight hours of daylight, with room to spare. Add in the power needed to run the fridge. Add a healthy margin to allow for cloudy days and now you know what size solar panel you need.
At this juncture, it is worth mentioning the state of the world of isolation switches. It is a good idea to connect the solar panel MPPT output directly to the house battery (with an inline fuse) rather than the other end of the isolation switch. The reason is that you want the battery to charge even when you’re not on the boat, and especially over winter. Similarly, on many boats the alternator is connected directly to the battery so that there is no risk of running the alternator without a load. In case you didn’t know, running an alternator in an open circuit configuration is fatal for the alternator. In the engine battery case, the only thing the isolation switch is doing, is isolating the starter motor. You could easily argue that the isolation switch should really disconnect absolutely everything and isolate the battery entirely, which is what it is supposed to do. I am of the view that the engine battery should be 100% isolated by the switch. I would like to think that turning the engine battery isolator to “off” meant there was zero possibility of an electrical short while I was working on the engine. That is not the case if the alternator is still “hot”. Also, it would be very unusual to start the engine without actually turning on the isolator switch as it connects to the starter motor. So personally I’m happy for the engine battery to be completely isolated when the switch is off, and I’m happy for the solar panel to have a direct connection to the house battery so it will charge even when all the isolation switches are off. I would recommend a fuse or a small on/off switch between the MPPT output and the house battery, just in case you want to completely isolate the battery. I would go further and put a small, three-way toggle switch between the MPPT output and the two batteries (assuming they are both of the same technology). That way, I can completely isolate the battery if need be, and under some circumstances, I can choose to send the solar power to the engine battery. See my blog post about flat batteries to see why that is a good idea. I would however mount that switch near the batteries and away from regular use. People can often flip a lot of switches if they think something isn’t working and you don’t want to find that both your batteries are dead at the start of the season because someone turned off the output from the MPPT controller.
FYI, people often think it is bad for the alternator to be permanently connected to the engine battery because it adds a continuous drain, but this is incorrect. The battery circuit has an independent rectifier (six diodes) which prevent any battery power from feeding back into the field coils. However, leave the ignition switch on as well, and you will find that the field coils are pulling power from the engine battery. If you do connect the alternator directly to the engine battery, make sure it is just the charge voltage from the alternator and not the ignition power.
The only thing left to mention about lead acid batteries and solar, is winter lay-up. I like the idea that the house batteries are being topped up by winter sunlight. I can also choose to run some small Arduino-like board to monitor the boat while I’m away, without worrying that it might be the cause of a dead house battery! But what of the engine battery? Wouldn’t it be nice if it got a bit of a top-up over the winter, too? It is tempting to connect both batteries together to serve this purpose (and again I repeat the warning about not doing this with dissimilar battery types), but personally I’m happy for the engine battery to be fully charged at lay-up and to leave it completely isolated until the new season. If you have access to the boat over the winter, and it’s not in some foreign marina, you can flip the MPPT output switch to engine battery every now and then, ensuring that it is getting a bit of charge.
A final note about lead acid batteries. Unlike their “sealed” cousins, they can off-gas some dangerous gases while charging. Make sure they are vented to the outside world, and strapped down so they don’t go jumping around as you launch the boat off the top of a wave.
Anyway, I think that is probably all I could possibly say about lead acid batteries on board sailboats. I will cover the revolutionary Lithium Iron Phosphate technology in part two.