I was invited to dinner on a doctor's boat. A clever man. The engine was running when I arrived and the wind generator was clattering around making quite a racket, disturbing until alcohol soothed the nerves and lubricated the conversation. I asked why the engine was running (after an hour or so) and was told it was to charge the batteries. Ah, so the wind generator is broken then? No, the rattling noise is just the hub in the middle, a little loose. Something he hadn't got round to repairing. He'd got used to the noise. The wind generator was beside the solar panel array. The engine ran on. He ran it he said 2-3 hours a day like that, faster than tickover. I suggested that if the batteries needed that much charging, perhaps his batteries were dud. No, he replied - as soon as the engine was running, the battery voltage went up to 14V.
So, a clever man with an array of solar panels and a windmill, in the Caribbean with all its sun and wind, needed to run his engine up to 3 hours a day. For what? His LED lights, a fridge and his iPad. The clever man knows very little about batteries and charging, and I've found this very common indeed amongst yachties. The subject is dull and technical and it isn't sailing, and very few people know what you need to know to live comfortably off-grid. If you'll tolerate a little technicality, and I'll try to be as brief as possible, I'll tell you all you need to know.
HandyBob's blog taught me a lot! Got me on the right track. I recommend you reading it sometime, but it is verbose. Here is a summary:
- Every time a battery goes through a discharge/charge cycle without being fully charged, it loses some of its storage capacity.
- You cannot tell a battery's condition by measuring the voltage.
- A lead/acid battery can only accept charge at a certain rate - a rate which diminishes as the battery becomes fuller. When the battery is nearly full, it still needs to be filled further, but you can only do this by feeding it in slowly.
- If a battery loses some of its storage capacity, this is caused by the plates becoming sulphated. The plates are designed to be porous, but if a battery isn't charged fully after each discharge, the plates become sulphated - that is, a hard less porous crust forms around the plates. You'll still measure a high voltage on the battery after charging, but you won't be able to draw current for long. The battery's capacity to hold charge is diminished by the crusted plates.
- A battery that has lost its capacity to some extent can be repaired by deliberate and careful overcharging (equalisation). Equalisation can be harmful in excess, and in vented lead/acid batteries, water is lost in the process and needs topping up afterwards. A battery that has been kept undercharged for too long is knackered.
- And, voltage drops caused by using undersized wires between the power source, controller and batteries can reduce the voltage getting to the batteries by enough to ensure that the batteries are never fully charged, and their useful life will be greatly shortened.
I have a friend who was literally born on a boat and has always lived on boats. He is often short of electricity and simply replaces his batteries every year, at great expense. What a waste!
Batteries need charging in stages: bulk, absorption, float and equalisation. The voltage supplied to the batteries by the charge source is different for each of these stages. Different brands need different voltages applied to them for the various stages. I use Trojan T-125 6v batteries, two in series to give me 12v. For whatever batteries you have, you need to know these voltages (the manufacturers usually publish the specifications of their batteries on a website) but they'll be similar to mine, so I'll use their voltage recommendations in the following description.
Bulk charging is used when the battery is depleted. The battery can accept all the charge you can throw at it, up to a maximum current capacity which is limited by the size of the battery - rule of thumb, the current should be no more than 20% of the battery's amp-hour capacity, that is 20 amps max for a 100 Ah battery.
As you charge a battery, the voltage across the terminals increases. When it reaches the absorption voltage, it needs limiting to that. You don't want the voltage getting above that or you'll damage the battery. With my battery setup, that voltage is 14.7 volts. That voltage needs maintaining till the battery is fully charged. Lots of chargers - engine alternators in particular - don't reach this voltage, and so running an engine to charge your batteries isn't good enough. As the battery fills, it takes less and less current. When the current is small enough, down to an amp or so, the batteries are full, and it is time to switch to the float voltage.
On my batteries, the float voltage is 13.2 volts. The battery won't lose charge if it is held at this voltage, and won't use water either.
Equalisation needs doing occasionally, and there is a good description of that here. No need for me to repeat what is written there.
Naturally, you want to spend as little time as possible watching voltages so you can do something more interesting, like watch paint dry. So you need a controller between your powers source (solar panels, windmill, whatever) and your batteries. A good controller will deal with feeding the batteries the right voltages for the right amount of time. A good controller will limit the current supplied to a battery using pulse width modulation (PWM). Others just dissipate the unused power as heat. PWM is what you want - you get less sulphation with PWM. What you also need is a controller that allows you to set the correct voltages for the batteries you have now or may switch to later. A controller that doesn't allow you to set these voltages (setpoints) is a waste of money.
I bought the cheapest controller I could find that used PWM and allowed me to set the voltages - a SunSaver Duo controller. I've got it set up and running fine, but it certainly isn't ideal. For one thing, you can't choose when to equalise the batteries with this controller. It is preset to equalise once a month. If I've had a run of cloudy days and used a lot of power, I want to equalise again when the sun comes out, not when the controller says a month is up since the last equalisation. And if there has been plenty of sunshine, and I haven't used much power, I don't want to equalise at all. And if I am out sailing and I have all my instruments running and the VHF and AIS and so on, I don't want the controller to start equalising, because the voltage will go up to close to 16v, which might well damage some of my electronics. When I equalise the batteries, I need to do it when it needs doing and when I don't have any sensitive equipment switched on. And besides this, the recommended equalisation charge for my Trojan batteries is 15.48v. When I set up the controller to use this voltage, I got an 'over-voltage' error. On contacting the manufacturer, I was told that although the SunSaver Duo can be set to this voltage, the protection circuit which is nominally set at 15.5 volts will cut the power somewhat sooner. So it simply can't supply the required voltage.
Setting up the correct setpoints for my batteries is possible using a meterbus from Morningstar, which is a thing that connects the controller to your laptop. Here you can set the voltages stages, and you can also see what state the batteries are in.
How I get round the equalisation problem is to connect a solar panel directly to the battery when I need to equalise it, and regulate the voltage myself by tweaking angle of the panel to the sun, or partially covering it up. It's a pain, but I don't need to do it very often, and I can usually manage to do it when I have other jobs to do around the boat.
Perhaps I should have simply bought the more expensive Morningstar Tristar 45 Handy Bob recommends, but here's another issue.
When a battery reaches the absorption stage, some of the available power is being dumped by the controller. My solar panels have a maximum output of around 12 amps at 13 or so volts. But usually, there is 4 or less amps going into a battery. Much of the available power of my panels isn't being used. So I now have two battery banks - previously I'd ignored the Sunsaver Duo's capacity to manage two battery banks. Every day I switch my battery selection switch to give each battery bank every other day off duty. This allows the controller to pour most of the available power into the battery currently in use and at the same time, top up with other battery with the small current needed to fully charge it. This is how I manage to ensure my batteries reach full capacity and I don't waste too much of the power available from my panels.
By the way, this is how I found a friend's batteries were badly sulphated. My friend has a similar boat, and like me, a couple of solar panels but with a higher power output. He told me one day he was pleased with the panels as they were currently giving him 5 amps. He asked how much mine were giving. I looked at my gauge, and it told me 12 amps. He found this difficult to believe. His batteries showed the same voltage as mine. What was going on? His batteries were sulphated, so they couldn't absorb much current. He badly needed to do an equalisation to restore his battery capacity.
A boat engine mechanic told me that he reckoned most of his jobs were the result of people running their engines to charge their batteries. He reckoned running a diesel engine without a load (the load of an alternator is tiny compared to the load when an engine is powering a boat through the water) was the quickest way to wreck it. The engine runs cold, and the fuel doesn't burn as completely as when it runs hot, and the deposition of carbon and other substances gave the mechanic plenty to fix. Another problem with using an engine alternator to charge the batteries is that often the output voltage is limited to 13.5 - 14 volts or so - enough to put a useful amount of power into a depleted battery, but not enough to fully charge it, no matter how long you run the engine.
Windmills. A very long time ago, I was the first yachtie to install a Rutland wind charger on a boat. At the time, it supplied far more power than all the others around at the time, which were mostly trickle chargers that could top up a battery in a gale. I was a fan. However, the blades can be noisy (some you can hear right across an anchorage!) and when the bearings start to go, they are noisy too. I used to change the bearings a lot on my old Rutland. I believe their design has improved since then, but still, moving parts, noise, wear and tear. Solar panels have become far more efficient than they used to be and far cheaper. So now, it is just solar panels for me.
Just great, so long as you have some sunshine. And an unshaded place to put the panels. The doctor had his panels on a frame at the back of his boat. The wind generator was mounted on one side of frame. On the other side of the frame were various antennas. So there was almost never a time when the solar panels were in full sun. It's handy to have a couple of ammeters on your electrical panel which shows how much current is going in and how much current going out. If you haven't tried this already, you'll be amazed I think to shade a solar panel just a little and see how much that reduces the current going in to the batteries. The shadow of a rigging wire across one of my panels is enough to reduce the power output to nearly half. And to get full power from the panels, they need to ideally face the sun, or at least be orientated to within 30 degrees of directly facing the sun. Most solar panels on most boats spend a significant time shaded. If you insist on having mounted panels, you need to take account of this, and buy more panels to make up for the loss of power.
When I bought my boat, I intended to build a raised area at the back for a couple of solar panels. But I found out about the size of the shadow effect, and I also wondered how I could arrange things so that I could tilt the panels. I imagined my Atlantic crossing, with an array of horizontal panels. They wouldn't give full power till mid-late morning. Then they'd be shaded by the sails, and they'd give almost nothing. There's nowhere on a sail boat that doesn't have a shadow over it at some time. In the end, I decided to keep my panels mobile, and though sometimes it is a hassle moving them about I don't regret that decision.
I can put my panels anywhere on the boat. They have long leads that connect via MC4 sockets and plugs. I have a pair of sockets at the back of the boat and a pair inside an anchor locker that has a notch in it to accommodate the leads. The leads are very thick - the copper needs to have a cross-sectional area of 4 square millimetres. Otherwise too much power is lost as it is transmitted along the leads. Crossing the Atlantic, I tied my panels outside the guardrail on the port side, so that soon after sunrise, they were in full sun. Mid morning, I usually moved them onto a part of my roof that faced S-SW on that route. That was often enough exposure, which is good, because from mid-afternoon onwards almost all of the boat way in the shadow of the sails (in light wind anyway). I did have a spell of particularly cloudy weather mid-Atlantic, where try as I might, I didn't manage to fill my batteries each day. On those days, I went to extremes, and tied one panel to the outside of a pulpit on the port hull, and the other, well...
Currently I spend most of my days at anchor in the trade winds, and so never have to move my panels. They are both tied to a part of the starboard coachroof that isn't affected by any shadow till late afternoon, and then it is only a rigging wire a few feet away. I don't need a noisy generator, to run a diesel engine or have a noisy wind generator. True, I don't use much electricity. I can generate and store much more than I currently use. I'm considering using that spare capacity to run a fridge.
Another useful aspect of keeping my panels mobile is that I can store them indoors (against the bathroom wall, held in place by a towel rail) when the weather is rough, when I don't need them and they might get in the way or when I am in an area where they might be stolen.
So, I seem to have the electricity issue pretty much sorted out - bar the hassle of having to monitor my battery state and equalise them manually now and again - but I wouldn't recommend you do what I've done!
If I was to start over, I'd switch to lithium batteries. They're much more expensive, but much lighter - weight per Ah capacity, and they can accept charge much faster than lead/acid batteries, which means that if you have your solar panels charging and run the engine too, the lithium battery can just keep on absorbing pretty much all the power you can throw as it until it is full - no need to taper the current as the battery fills. This means that to some extent, the extra you spend on batteries you can save in have fewer solar panels. The panels you have will always run at close to 100% efficiency until the battery is full. This better ability to accept charge also allows you to either fit a much bigger alternator to your engine, or install a separate generator driven off the engine. The engine will then run under some load, and so won't run cold if you just use it for charging, and you will need to run it for much shorter periods. Anyway, here is a more fully formed opinion on the matter, from a fellow who has installed lithium batteries on his boat.