Charging boat batteries

There's more to low voltage electrical systems than you might think. It's very important to get a proper balance between charging and storage.

As boats become more electrically dependent, the challenge of satisfying an increasing rate of consumption becomes more and more pressing. But, when we talk of boats, we don’t really mean all boats. Let’s face it, when not in the marina, hooked up to the pontoon, most powerboats have whacking great engines that – via the good offices of their alternators – will satisfy even the most extravagant electrical thirst. Then there are large sailboats. Above 45ft (13.7m) or so there’s usually room for a diesel generator to be tucked away somewhere without intruding too awkwardly on other amenities.

No, the most defenseless of the boating clan are small to mid-size sailboats, who pay a relatively heavy price for carrying what many of us would consider essential gear. Whereas the electrical drain of, say, a radar would count for very little against a sixty-footer’s total consumption, it would be a major burden for a boat half the length.
So, this article is really dedicated to smaller sailboats, though hopefully there is relevance for all.

Avoid the audit

The conventional wisdom is undeniably true. When it comes to electricity there’s a triangular relationship between generation, storage and consumption. A well designed electrical setup will balance those three considerations to make sure that the first two will satisfy the third.

Popular advice is to deal with that third issue first. This involves conducting a detailed audit of your electrical consumption. You list all of the onboard appliances, estimating their daily usage in hours – both at sea and at anchor – and totting up the total current drawn in amp-hours. An amp-hour (Ah), as the term suggests, combines time and consumption to give a meaningful definition of how much is being consumed. If, for instance, an appliance draws 5 amps for 1 hour it therefore consumes 5Ah; another draws 1 amp for 5 hours and, again, you have 5Ah. It’s a simple concept but often confuses.

Of course, to follow this exercise involves a lot of guesswork, since we can only very roughly estimate what is actually being consumed. Refrigerators work harder when it’s hot outside. Ditto autopilots when it’s rough – or sometimes not at all with a human hand on the helm. While you might spend your evenings reading under a single light, visiting friends will be greeted with the whole saloon brightly lit – the gathering probably also increasing the load on that fridge as cold libations are dispensed. And so on. The truth is that accurate predictions are unattainable. At best, we can only get a rough approximation of how much electricity we need.
The good news is that rough is good enough. We can take a more relaxed view of consumption by focusing on those other two issues – generation and storage.

But first we must understand ‘charge acceptance rates’ – by which we mean the maximum rate of charge a battery can take at any time. These vary with the type of battery, its capacity and, lastly, its state of charge. Before we proceed, it may be worth defining the various battery types, bearing in mind that all discussed here fall into the ‘lead acid’ category, meaning that they have lead plates in a sulphuric acid electrolyte. I am also excluding the starter battery type which has very thin plates and is unsuitable for the service role. So, here goes;

  • Flooded batteries. The most familiar and least expensive type. These have a liquid electrolyte and most have to be topped up with de-ionised water every so often, via screw caps to each cell.
  • Sealed gel batteries. In these the electrolyte has been made thixotropic which, together with the sealed casing, makes them spill proof. No topping up is possible.
  • Absorbed glass mat (AGM) batteries. Again these are maintenance free and spill proof but the electrolyte is liquid, held by capillary action in layers of glass mat between each plate. Expensive but efficient.

Regardless of type – though some are tougher than others – battery life is shortened by deep discharges, and it’s generally accepted that a good trade-off between usefulness and longevity it to limit the depth of discharge to 50% of the fully charged capacity. The photo that heads this page shows Shindig's battery to be 83% charged with an input charging voltage of 2.4A – mainly from the solar panels since it's a near calm as I write. Better than nothing though!

Now, a conventional lead-acid battery will accept a hefty current when partially discharged, but the rate of charge tails off as the chemical changes unfold and resistance builds. It has been determined that, when cycled between 50% and 80% of being fully charged, liquid lead acid batteries have average acceptance rates of about 25% of their nominal capacity.

To put this in context, let’s adopt a mid-size sailboat as an example. It’s electrical system was specified by the builders: a 65A alternator serves a 220Ah flooded lead acid battery. Although the alternator is rated at 65A, that’s only achieved at full speed and running reasonably cool (its output falls with temperature). So, at cruising revs on a steamy day it’s better to assume 50A maximum to be on the safe side.

Now, the 50% to 80% span of this boat’s battery capacity ranges from 100Ah to 176Ah – amounting to 76Ah in all. With a charge acceptance rate of 55A (220 x 25%) our 50A alternator would take just over 1h 22m to bring it back to 80% charge. Which would be quite reasonable if you only consumed 76Ah per day.

Unfortunately, few modern sailboats achieve such frugality. A typical daily consumption for a comprehensively gizmoed mid-size sailing cruiser works out at around 140Ah at sea (and perhaps 110Ah at anchor). To restore that 140Ah would mean the engine has to run 2h 48min every day – not an attractive prospect for a sailboat.

The temptation is to add another battery in parallel, thus doubling the capacity. This clearly provides more storage and has the added benefit of increasing the charge acceptance rate to 110A (440 x 25%). Unfortunately, the alternator is still only capable of producing 50A so won’t be able to take advantage of this.

So, what if we had stuck to the original battery and fitted a more powerful alternator – say, one with twice the real output: 100A? Now we have plenty of charge current but the battery’s acceptance rate remains at 55A so there's very little to be gained. All that extra power is wasted.

If this seems like the end of the trail, don’t lose heart. Whereas our conventional lead-acid battery has a miserly charge acceptance rate, gel types do much better at about 50% of rated capacity and AGMs will accept up to a staggering 100% (if cycled between 50% and 80%). If either were combined with the 100A alternator they would be recharged in less than half the time of the old battery. Similarly, the replenishment time for the 140Ah daily burden imposed by our yacht at sea would also be slashed.

Before we move on, it might be helpful to imagine a system where a 200A (again real output) alternator serves a 400Ah AGM bank – far from impracticable if there’s space to accommodate them. With a 100% charge acceptance rate, the AGMs will take any charge current thrown at them, with lots to spare from other sources – a subject we touch on in Power from the planet. Thus equipped, our specimen yacht could mop up its daily 140Ah consumption with just over 40 minutes of engine power.

Audit the various electrical loads if you must, but you may find it more fruitful simply to engineer the best possible charging method you can fit to your boat and then deal with consumption as it comes. The best systems are those that match the various components with regard to charge acceptance rates. If space and weight aren’t a problem, an over-sized alternator charging a large conventional lead acid bank will do nicely. For more compact installations, gel or AGM batteries could be the answer.

Finally, resist any impulse to split large domestic banks in two. Remember: it’s the size of the bank that increases acceptance rates, not the number of batteries. And also bear in mind that repeatedly charging up to only 80% capacity is not the best regime for batteries. The closer you get to 100% the better.



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