Power management

[nathanlee]

Hi All,

Thanks to my previous thread about solar/wind, and all your replies, I've taken the advice and decided to go for solar. However, I'm a bit stuck with the figures and wondered if somebody could help me out since my electrical theory is very, very rusty indeed. Also to check that what I've come up with is a reasonable estimate.

My chart plotter and instruments consume 491.5ma, so presuming I'll be sailing for 12 hours that's 5.9amps I've eaten from the battery.

Now, for the following 10 hours I'll either have the tricolour or, more likely, the anchor light, which is 25w, so 2amps, or 20amps in total over the 10 hours.

I've also included 4 hours of cabin light, totaling 2.6amps.

So rounding up a bit, I'm going to be using circa 31amps in a 24 period.

Firstly, does that look correct? And, how much can I expect to get back from a 20 or 30w solar cell, given standard British sunlight conditions?

I have an 85ah battery, so if I presume that's about 70% efficient, let's say it can deliver 60ah.

Given my consumption, that gives me a realistic 2 days of that sort of power consumption, without putting any juice at all into the battery. Is that correct?

Also, what rating of solar cell would people advise given the above figures. I need to, because of funds, go as small as possible, and I am willing to accept some imbalance in input/output, thereby extending the "range" of the battery, even though it can't be sustained over a long period - if that makes sense? if i can get 4/5 days out of it without the need for a shore power top-up, I'd be happy.

All the best,

Nathan

 

[johnalison]

You will get precise answers soon from people who can count, but I would say your service battery is nothing like big enough and you need to double it to be comfortable. I think your figures are roughly right, though I don't know if you've include VHF, probably another 1/3 amp and it looks as if you won't be getting cold beer as you've presumably not got a fridge. The most you'll want to take out is about a half of the battery capacity, but I try to limit mine to less though I have 3x 65amp hours.

I tend to find it confusing when "amps" are used to mean "amp hours", though I know the meaning is usually clear. Using a 25 watt light for an anchor-light is over-generous, and you could save a massive amount by using something like 10 or less as it only has to be visible for a mile. Although I have a measthead white, I normally use a tiny anchorlight made for the purpose which switches itself off. An LED would be even better I imagine.

 

[Dave100456]

Your calcs seem roughly correct for consumption. I would work on the PV panel working for no more than 5 hrs day (depends on characteristics, weather, panel temp, etc) and will have its rated wattage at possibly 17volts - so a 30 watt panel may give less than 2 amps thus daily, 10amphr. Check the panel's spec.
No doubt you'll get other opinions but the specs are always best case scenarios and it's worth building in "efficiency" factor. I think you'll be pushed to get two days use with no input into that battery.

 

[tobble]

ok first point to be pedantic, if you draw ~500ma for 12 hours, you'll use 5.9 ah (amp-hours), which is (sort of) a measure of energy, whereas amps are (sort of)a measure of power... get this right and you'll confuse yourself less ;0)

so... 31 ah per 24 hours sound about right for what you are suggesting.

however, you will seriously reduce the life of your battery if you take out more than 50% of the quoted capacity ona regular basis. If you look at the specs for good quality batteries, you will find a lifespan in number of cycles quoted for a set of given discharge depths. for example from This website "Cycle Life: 2000+ cycles to 20% DOD; 1000 cycles to 50% DOD; 800 cycles to 80% DOD." (DOD=depth of discharge)

So I would think about at least doubling the size of you battery bank, which should pay for itself in not having to replace the batteries so often, if that makes sense... also that give you a bigger margin for emergencies.

as far as I can tell from your other thread, you just need a set up to keep you cruising for a few days at a time; so my next point would bewhat sort of engine charging have you got (if any!)?

 

[elton]

It looks like you could make a huge power saving by replacing all your lamps with LEDs. Not cheap, but they'll last (virtually) forever.

 

[pvb]

[ QUOTE ]
I have an 85ah battery, so if I presume that's about 70% efficient, let's say it can deliver 60ah.

[/ QUOTE ]Sorry, but your battery won't deliver anywhere near that. Unless your battery is fairly new and fully charged, you're likely to only get about 25Ah usefully before the voltage drops too low.

A solar panel will help, but output in the UK will most probably be much lower than you'd expect from the panel's specification. If you're looking for several days' autonomy away from shore power, adding a couple of 100Ah batteries might be a very cost-effective alternative.

 

[nathanlee]

[ QUOTE ]

as far as I can tell from your other thread, you just need a set up to keep you cruising for a few days at a time; so my next point would bewhat sort of engine charging have you got (if any!)?

[/ QUOTE ]

None :|

30w solar panel is stretching my budget too, so it may have to be a 20w. If I spend 35 quid on one of those led bulb replacements, that's going to cut my anchor light usage down to 3.5w, which will help an awful lot, but yes I can see I'm going to be running into trouble here.

The VHF is handheld, so I didn't include it in the calcs as it will last the distance and I have a backup battery pack just in case.

Increasing the bank capacity is a tricky one since the Corribee doesn't have an awful lot of room. Although I could perhaps mount a smaller battery behind the current 85ah, giving me a total of perhaps 135ah-ish.

So, 20w solar cell, plus extra 45ah battery, minus 25w anchor light... what's the verdict?

Hopefully I will add a rutland 504 to provide some evening charge at some point, but it's simply well beyond my budget at the moment.

 

[nathanlee]

please ignore that shameful error. 130ah /forums/images/graemlins/smile.gif

 

[samwise]

If you have the Nigel Calder technical and maintenance manual it will take you through the process of assessing your power demands and installing a system to meet it.

I think you do need another battery. If space is a problem, you could look at the slimline Red Flash batteries and keep one of those as a dedicated engine start battery but it still means that the domestic unit will be working pretty hard.
Small solar panels are OK for topping up batteries while on a mooring or in a marina when unplugged from shorepower, but I think you are being ambitious to rely upon a small array keeping you in amps in the UK.

If it was me, I'd look first at (a) installing a second battery and (b) improve the charging system with a Sterling or Adverc arrangement or upgrading the alternator.Not very green, but more likely to deliver the result you are looking for.

 

[nigelmercier]

[ QUOTE ]
So rounding up a bit, I'm going to be using circa 31amps in a 24 period.

[/ QUOTE ]

See reply above regarding A and AH.

I would suggest that your figure of 31AH is a bit optimistic, but the rule of thumb for battery sizing is to take your requirement (in your case daily) and multiply it by 4.

 

[jimmcgee]

"If I spend 35 quid on one of those led bulb replacements"

find your local pound shop , £1 for a 9 white led torch, light your boat up for under a tenner

 

[William_H]

You need to do 2 different calculations. The first to see if your battery will supply what you need for your 2 days away.
The second is to see if your solar will replenish that power over what period.
Your 20W solar panel will give 1 amp if oriented to the sun midday.
It will give that for perhaps 6 hours. (a variable quantity of course) So 6AH per day or 30AH over a week between weekends. Or in practice about 20 or25 AH of discharge (inefficiencies).

I think in actual sailing you will find you can make savings from your first calculation of power drain. Especially if you can reduce drain of lights and perhaps sail with chart plotter off.

I reckon if you just try it all out. Perhaps keep a diary of actual use before you rush to buy more batteries or solar power. ollewill

 

[Blueboatman]

That I think is the most sensible advice so far, Nathan.

For what it is worth I, with a similar boat and one similar 85ah battery, managed with an18w panel, the occasional 4amp boost when running the engine, and sod-all help from a diminutive windgenerator which I ended up binning cos it was not worth the weight, noise,vibration, entanglement with ropes and sweet f-all output..
Anchor light you can make your own from a jamjar and the courtesy lamp from insoide a car- tiny saussage bulb uses .2 amp - hang it from the boom and plug it in to a cigar lighter socket. It may actually give you better illumination than something stuck up the mast using twice as much power..

I too had 'serious' navigation lamps that used 2.5 amps. Great for entering port but, ahem,not necessarily always on at sea,ahem. Each to their own..

A single fluorescent cabin light will give you good working and reading light for a miserly .5 amp per hour.

Navigation instruments ? Compass lamp uses f-all. Gps you can turn on every couple of hours if you want.
One 85a/h battery and one 18w solar panel did me fine, for years and years (and a nice oil lamp). The battery lasted 6 years before replacement..

By the way it is more efficient to just plug the solar panel into the cigar lighter and disconnect at night rather than rely on a blocking diode to stop it discharging the battery at night-the diode actually reduces the useful output of the panel !

Thinking about it some more. Have you had a quote or two to install a charging circuit for the outboard motor?
And the suggeston to use rechargeable batteries and led torches is excellent, they can be topped up from your hardworking cigerlighter socket using an inexpensive Maplins transformer and a battery holder, aneasy DIY project..

 

[chriscorr]

[ QUOTE ]
If you have the Nigel Calder technical and maintenance manual it will take you through the process of assessing your power demands and installing a system to meet it.

[/ QUOTE ]

Do you mean the Boatowner's Mechanical and Electrical Manual or the Boatowner's Practical and Technical Cruising Manual? I'm sorting out similar problems and would like to have a good book...