This is especially true on a small yacht like Arabella. Weight and space restrictions dictated a modest battery bank. This would have been fine if all I’d wanted to do was run a log and echo-sounder, and to switch on the nav lights occasionally, as previous owners had been content to do. Unfortunately, I’d changed all that in the course of the refit. Arabella was now a great deal more power-hungry than she used to be.
Deciding how to satisfy this increased power consumption necessitated working out a proper plan. I’ve set out the thinking in detail here. It’s a bit lengthy, but I hope it will help others going through a similar thought process. At the end of it all, I came up with my “24 hour +” rule – namely that I knew I could extract at least 24 hours out of the battery, and that could be extended with rationing and/or by running the outboard engine.
(Disclaimer: I came to the issue of battery management as a complete tyro. Hours of reading and online research, coupled with talking to various suppliers, have educated me to a point where I can state what follows with reasonable certainty. As always with things that you read on the web, however, take my opinions as a starting point only before double-checking with a reliable source.)
Arabella’s battery management plan broke down into a three-step process:
- carrying out a power consumption audit
- selecting a battery with suitable capacity
- deciding how the battery could be charged
(A) Power Consumption Audit
Before I could make any decisions about battery capacity and charging for Arabella, I had to work out how much power I was likely to consume in a variety of situations. I could of course disregard power consumption when Arabella was tied up in a marina and hooked up to shorepower, as the Studer Innotec MP12/12 charger (rated at 12A) would be constantly charging and maintaining the battery as required, and in addition I could use the 240v power socket installed in the galley to run a variety of 240v appliances.
Above: Studer Innotec MP12 series battery charger (240v)The real-life situations that I had to think about therefore were:
- day sailing;
- night sailing; and
- anchoring.
Every source of information that I looked at informed me that I couldn't rely on any general rule of thumb here. I would need to conduct a thorough audit of the amount consumed by each electrical item (including electronics) on board in each of the above situations.
A trawl through the web and some book-reading produced a variety of worksheets, some more user-friendly than others, from which I drew up my own version. An example can be found in this useful article by Nigel Calder. My version can be downloaded by clicking on the link a few paragraphs further down - it's in Microsoft Word format and can therefore be recycled for your own needs.
Conceptually, the power audit was simple. It involved:
- finding out the power consumption of Arabella's electrical items. I had to list all the 12V electrical items in use on a "typical day" (more on which shortly), and find out how many Watts (or Amps) they each consumed. Usually this is on the appliance or in its handbook. If you can only find a figure for Amps, simply multiply this by 12 (volts), to convert it to Watts.
- calculating the daily total Watt-hour (or Amp-hour) requirement for all the listed items. I had to estimate how many hours I would use each item, then multiply each item's wattage (or ampage) by the hours that I would use it for in a "typical day".
- adding all the totals together to get the final daily total Watt-hours (or Amp-hours) consumed.
Simple really. Except that it actually took much longer than I expected, for two reasons. First, not all suppliers actually provide details of the power consumption of their products, and even if they do, they do so somewhere obscure and hard-to-find (hint: the manufacturer's website is usually where you will end up) and in an inconsistent manner, with some items rated in Watts, others in Amps. Second, what is a "typical day" on a small yacht - how do I know how many hours I will be sailing for? Contrast a six hour trip down the Solent with, say, a fifteeen hour Round The Island Race. There's a bit of a difference.
The first issue required simply determination and a calculator. Where necessary I pestered manaufacturers for power consumption figures by phone or email. With one notable exception, whose products I will not be buying again, I got prompt and helpful replies. To resolve the problem of having all of the data jumbled up in Watts and Amps, I chose to stick with Amps. Others may be more inclined to work in Watts, but in my case I chose to go the other way simply because my (eventual) battery would be rated in Amp-hours. This meant employing all those formulae that I learned in school and promptly forgot, namely:-
Watts = Amps x Volts
Amps = Watts / Volts
Volts = Watts / Amps
So, for example, if I knew that an item consumed 2.4 watts per hour and was rated 12V (a fairly safe assumption for British marine equipment running on a 12V system, but one that will have some purists up in arms), then it consumed 2.4/12 or 0.2 amps per hour (0.2Ah).
The second issue – analysing the “typical day” – was more difficult. In the end I found it easier to do away with the whole day thing. Instead I used building blocks. I worked out the likely consumption for 6 hours’ day sailing and, separately, for 6 hours’ night sailing (when additional items, such as running lights, would be in use). It wasn’t a precise art, because one couldn’t say, for example, for how long the VHF would be in listening mode and for how long it would be used in transmit mode – each of which uses very different amounts of power. The same went for the tillerpilot, which would spend some time in standby.
I could assume some reasonable maximum usages for each six hour block though, which I then tabulated, printed and laminated for inclusion in Arabella’s on-board reference manual’ for ease of reference. Those wishing to see all the details can view the tables by clicking this link or download them from my Downloads area, but in summary I came up with the following ‘average’ consumption figures based on worst case (maximum realistic) power consumption – this assumed for example that the tillerpilot was in constant use, not merely on standby:
- 1 hour’s day sailing: 2 Ah (or 12 Ah in a typical six hour period)
- 1 hour’s night sailing: 3 Ah (or 18 Ah in a typical six hour period)
(I didn’t do a full table for consumption at anchor or on a mooring, but assuming the very worst case of having all the interior lights on, and an anchor light as well, I came up with 2.5 Ah for each hour. This wasn’t too realistic, however, as I wouldn’t use all the interior lights at once and in any event I carry a Freeplay wind-up lantern (an overhang from my camping days) for use in such situations. So I basically didn’t waste too much time on planning for the load at anchor).
Before moving on, what this exercise did demonstrate to me – very clearly – was just how profligate some onboard items are in terms of power consumption. In particular, the decision to go for LEDs across the board for nav and interior lights began to look like one of my more intelligent decisions.
(B) Battery choice and capacity
Vehicle batteries are not suitable for use on yachts, simply because they are not designed for deep discharge – once the engine has started, all the power demands (such as headlights) are simultaneously offset by the input from the car’s alternator. Repeated deep discharges will kill a vehicle battery in no time.
Marine batteries in contrast are designed for deep discharge cycles, that is, they can be be repeatedly discharged and recharged over quite a long lifespan. There is a limit, though, beyond which draining a deep discharge battery is unhealthy for it. The limit is somewhat disputed, but most people would agree that discharging below 50% of nominal capacity will shorten the life of a marine battery to an extent, and going below 70% is no-no. If the 50% limit is observed, then all things being equal, a modern marine battery should be able to withstand several thousand cycles of discharge and recharging. (Incidentally, there is a very helpful 12 volt primer here, including a reference table showing battery state of charge against voltage under load).
It made sense, therefore, to go for the largest capacity battery I could. After all, if I could safely use up to 50% of its capacity before needing to recharge it, then at the discharge rates described above I could likely survive a good deal more than six hours, day or night, without bothering to use the outboard or rig up the solar panels. However, that theory only held true up to a point. Batteries are heavy, and the more capacity they have, the bigger and heavier they become. And heavy isn’t good on a small yacht like Arabella, so there is a realistic maximum driven by weight and the ability to shoehorn the battery into its home under the starboard quarterberth.
In the end, the best I could manage was a 115Ah liquid battery. That would give me up to 57.5Ah of usable capacity (up to 50% drain). So now I knew how much capacity I had. How was I going to maintain or replace it? On to the final stage of the plan…
(C) Recharging
The next thing to work out was how, and how quickly, all that power consumption was going to be replaced by recharging the battery. That entailed working out (a) by what means power would be generated and (b) how much power would be generated, and over what timescale. It seemed fairly clear to me from the outset that recharging at sea was likely to be a losing battle, but the important thing was to work out the net drain, i.e. how quickly I would run out of electrical capacity despite recharging as I went. Or, put another way, to what extent recharging could extend Arabella’s time away from a 240V socket by stretching out the nominal capacity of whatever battery was ultimately installed.
A number of factors came into play here. First, it would be wise to leave the marine with a full battery – so fitting shorepower and a decent battery charger was a no-brainer. Second, when it came to charging out at sea or at anchor, I needed to balance what I might quite like against what I already had. What I already had was a Tohatsu 6HP outboard with a charging circuit. What I would like was solar power. So that was to be my starting point.
Tohatsu 6HP outboard
This shoves out 5A per hour maximum, and is therefore a significant contributor to recharging the battery - at a price. It burns petrol. Specifically, it drinks 2.5 litres per hour at maximum revs. So the electrical capacity it generates is limited by fuel capacity (and it’s unlikely in most circumstances that I ever have more than 7 or 8 hours’ fuel on board, for reasons of safety and space).
Solar panel
I’m a big fan of solar, having for some time used a nifty little SunLinq 12W panel that folds up into a small, lightweight pack and comes with a whole range of cables and connectors included. The downside is that small solar panels like the Sunlinq don’t pump out all that much charge, especially on a typical English day, and you can’t drape too many of them over a boat the size of Arabella.
Nevertheless, I wanted to make use of solar power. It’s not cheap to install, but I just love the way you get power for free thereafter. After a fair amount of research, I alighted on Solara semi-flexible panels, supplied by Barden UK. These were the same panels that were installed by Barden on Ellen MacArthur’s yacht, Kingfisher. They could be fixed with adhesive or screwed and, due to the synthetic surface, would shape around slight curves, such as Arabella’s coachroof. Critically, given how small the coach roof was, these panels could also be walked on with no fear of broken glass or damage.
Above: Solara 120M solar panelThe space I could spare on Arabella’s coachroof – forward of the main hatch – would permit the installation of a 120M panel rated at 34W. That might seem as though it would put in a huge amount of charge, but the reality is somewhat different. Recalling the formulae above, the nominal 34W rating has to be divided by the panel’s rated voltage (which is 16.5V, not 12V) which results in a mere 2A in every hour. Moreover, most reputable solar panel suppliers recommend that you assume that on a typical English summer’s day, the panel may generate (from dawn until dusk) only 4 hours equivalent of charge. They then suggest you discount that figure by 20% to allow for battery resistance and loss in the cabling.
On that basis, the 120M panel would provide only 6.6Ah of charge on a sunny summer’s day. Interestingly, this (entirely theoretical) charge rating appears not to be massively out of step with the real-life results obtained by Practical Boat Owner magazine in a recent solar panel group test (PBO 466, October 2005). Based on an average over three days, PBO obtained 8Ah/day from a Solara 120M. A spot reading taken with the panel partially shaded gave 5Ah/day. PBO found that even a very slight, hazy overcast on an otherwise bright day, would reduce output by up to 25%, suggesting an output of 6.0Ah/day for the Solara 120M in such conditions. Accordingly, the 6.6Ah/day based on the theoretical calculation seems like a conservative averge and I am inclined to rely on it.
On a practical level, the Solara panel was – unlike many others - thin enough to permit Arabella’s main hatch to slide over without fouling it. And, as a final reassurance that I was on the right track, Phibius had recently fitted the identical panel to Arabella’s sister ship.
(D) Putting it all together
I now had two sources of charging when away from shorepower. I could generate 5 amps every hour by burning petrol – of course 2-3 amps would be consumed by the onboard systems while I did so. Alternatively, I could generate up to 2 amps per hour from solar power but subject to a conservative limit of 6.6 Ah per day.
On a six hour midsummer day-sail, therefore, I could maybe generate 2.5 Ah from using the engine at the beginning and end of the sail, and a further 6.6 Ah (average daily maximum) from using the solar panel all day, giving me a maximum of 9 Ah. Since my estimated consumption @ 2 amps over that 6 hour day sail was 12 Ah, I would ‘drain’ power to the extent of 3 Ah (or so) on that sail. Insignificant.
But at night, the drain would be far worse, especially since the solar panels would not be in use, and I would likely already be at sea (hence the engine would not be used to get in and out of port) so I had to assume a full 18 Ah drain in each 6 hour period.
The real problem, however, would arise on an extended passage. Again assuming summer sailing, if I started out in daylight and assumed an exact 12 hour cycle between day and night sailing, after 24 hours – in other words on the morning of day 2 - I would have consumed 60Ah and, at best, replaced 6.6Ah of that from the solar panel. I might then (just) make it through the rest of day 2 in daylight before hitting the 50% battery consumption limit. The battery would still give me some more hours of power, of course, but after exceeding the 50% limit, I would be damaging it. I would need to run the engine for some hours by dusk on day 2, in order to build up any kind of reserve to get me through the night ahead.
Hence my “24+ hour” power rule. How much “+” I could ever extract would depend upon:
- the amount of direct sunshine received by the solar panel;
- how much I wanted to run the outboard engine (allowing for restricted fuel capacity); and
- cheating: what could I switch off? Assuming I plugged in my trusty Walkers Trailing Log then, if I was well out to sea and clear of hazards, I could turn off all the nav instruments and helm myself or try to use the Tiller-tamer, relying solely on occasional GPS plots from the handheld, backed up by DR. That would save some serious Ah.
