I (Haven’t) Got The Power

January 10, 2014 at 9:17 pm

One of the most annoying problems I’ve found so far with Duet is with her 12v power consumption.  My previous experience has always been on boats with inboard diesel engines, with an alternator to charge the battery, and a skipper who started the engine if we were making less than 5 knots, so it was never really an issue…  To be honest, the first time the Duet’s battery went flat, I’m ashamed to admit it was a bit of a surprise!

She came with one 70Ah leisure battery, a small (peeling) solar panel riveted to the lazarette lid (that I’m not sure is connected) and there is also a cable on the outboard that you can plug into the deck which I assume is for battery charging from the outboard. I’m not sure if that works either.

At the end of last season it became apparent that the 70Ah battery was just not holding a charge, and after leaving it at Halfords for a diagnostic test and test charge they told me it was “kaput” (their words were perhaps a little stronger).

So, it’s pretty apparent that we need a bit of a review.  Especially if we’re intending anything more than day sails next season… So I’ve bought some books, and done some research and figured out that the general theory is to 1) calculate how much ‘power’ you’re going to need in a day. 2) Times it by 3 to calculate how much you need to store in your battery bank, and then 3) figure out how you’re going to replace the charge once you’ve used it. Well, that’s the theory anyway…

So I’m going to try and keep this really simple.  Here goes:

1)  Now I’m no electrical engineer, and have no experience of 12v systems so it took a bit of time to get to grips with these Amps, Watts and Volts… but basically we have items that consume power and this is measured in Amps. So the powered item itself will have a wattage and the higher the watts, the more power is consumed (like a 100W bulb is brighter than a 25W bulb).  So to calculate the Amps you simply divide the Watts by the Voltage (eg 12v though it’s actually more like 12.75v on a charged battery).  (Or like I did, you can look at the manufacturer’s specifications for your kit and also look on the internet at other people’s calculations – but proabably don’t get too hung up on those.)

Once you have the Amps, you take a guess at how long (in hours) you might use this item for and simply multiply to give you the Amp Hours (Ah).  The best thing to do is to split it into ‘At Sea’ consumption for on passage, and ‘At Anchor’ consumption for when you’re at anchor (or in harbour with no hook-up) and calculate for a ‘worst case scenario’. If you’re in a marina with power, you can obviously hook up to the mains (and have no need for the anchor light).

So Duet is not exactly very ‘technologically advanced’. Part of the draw of sailing for me is to get away from technology so I have paper charts, and do DR, and have no AIS (though I do have a fixed GPS as backup, and a 3G iPad for emails). I also love the cosy glow of a paraffin lamp and am happy to live without hot running water (we have the kettle).  I don’t really miss having a fridge; things seem keep cool enough in the lower lockers to last a couple of days even in hot weather. The other benefits of the KISS principle are (should be) reduced cost and maintenance: batteries are expensive, stuff breaks.

I’ve calculated for a worst case scenario of an overnight passage on longer cruises, with the Tiller pilot (I mainly single hand so this is really important) and instruments in use for 20 hours, and the nav lights on for 8 hours.

12v boat power consumption calculator

(Ways of reducing this consumption even further might be to get a wind vane to replace the Tiller pilot, or replacing bulbs with low draw LEDs. But these aren’t necessarily the most cost effective… And comparing my 50Ah to many on the cruising forums (with their washing machines, microwaves and air compressors for diving) this is already quite low!)

2) So from the above table we have a power requirement between 6.5 and 50 Ah per day.  As you should never draw more than 30% of the capacity from a battery bank as it reduces the life of your battery (note link between 1st and 3rd paragraphs above), we times this figure by 3 to give the size of the battery bank we need for a day.  So that’s 150Ah for Duet… We will need 2 x 75Ah capacity (size) batteries.

3) Finally we have the not insignificant matter of recharging the batteries.  A common method for weekend KISS sailors (and what I did last season) is just to remove the battery, take it home and charge it from the mains before bringing it back next time. However, for longer periods on the boat or larger battery banks it gets more complicated.  If you have shore power with an AC socket on it you *can* hook up the battery charger on the boat (you shouldn’t, but many do).

The other options are harnessing energy generated from the inboard or outboard engine with an alternator, and/or solar power and/or a wind generator.

I have my eye on a Tohatsu 6hp outboard for Duet (more on that in another post) which has a 60W charging capacity.  In practice this will give 5Ah (there’s an easy calculator here – make sure it’s DC and you have the volts set at 12).  So this will replace 5 amps every hour it’s run.  If we do use 50 Amps (worst case scenario) we would have to run the outboard for 10 hours to recharge.  So it’s probably not going to be enough then… 

Duet has a 10W solar panel (not sure if it works but a new one is about £70).  In theory this should give 0.8 Ah and consensus seems to be that on a good summers day in the UK you might get 6 hours sunshine.  So this might replace 5 amps in a day (bit more complex than that but you get the idea). Double that if you have 20W panels and so on but still not really that meaningful til you get to about 90W (45Ah for 6 hours of sunshine). 10 or 20W is OK for trickle charging though, if you don’t want to do night sails every night and have a bit of time for it to slowly top the batteries back up. And they need a regulator so that they don’t drain the batteries when there’s no sun.

Finally there’s wind power. Turbines seem quite expensive and the only one really anywhere within reach of my budget is the Rutland 504 which is marketed for trickle charging (there was a good article in PBO).  It is 25W so will replace 2.1 amps per hour, though of course unlike solar it will run 24 hours a day (if there is more than 5kts wind) so it *could* replace 48 Ah in 24 hours. Much more hopeful, though it won’t be of any use sailing downwind or in light airs.  It is pretty compact though, which is good on a small boat. Again it needs a regulator so that there’s no battery drain when it’s not working.

So, in an ideal world I’d want to look at hooking the outboard up, probably getting a Rutland 504 and perhaps also getting a 20W moveable solar panel for sunny days when there is no wind (and for trickle charging).

Oh, and if we wanted to go longer between charges we could increase capacity in our batteries.

So, that’s the theory anyway… It does seem to be a bit of a contentious issue, and there are other factors that can come into play.  For example if you have long runs of cable, or you have any corrosion on the cabling then some amps will be wasted.

I think the only way is to try it… and get a good battery meter… and if the batteries get low, turn some stuff off.