Power requirements sanity check

[PhillM]

I am trying to work out my power requirements and what I need to do to put enough back in to be able to keep going without needing to go into harbour for a shore power charge. I know I could go LED but I don’t have that and am not going to be changing for this season. My power usage is:

1x25W tri light – about 2 amps
Various instruments inc radio that take 1 amp
Perhaps transmit on radio daily – assume 4 amps
(We use oil lights and torches for internal lights – so no need for electricity)

So, assuming that we are sailing for 24 hours (24 amp for instruments), use the radio once (4 amp) and have the tri light on for 8 hours (16 amp) we are using 36 amp each day.

I have a new 110 amp leisure battery and a new 40 amp alternator on a new Beta 14. I am assuming that I would run the engine for one hour a day.

So am I right in thinking that I’d be using 36 amp and adding up to 40 amp back in by runing the engine – so could effectively run the boat for as many days as I have diesel in the tank?

 

[daveyw]

+ the amps it takes to start the engine

 

[PhillM]

+ the amps it takes to start the engine

Sorry, should have said, I have a dedicated engine starter battery as well. I had assumed that I would give that a few minuties after starting the engine to get that topped up before switching to charge the leisure battery.

Now you mention it, I guess I need to work out how much it takes out of my starter battery and how long to charge that each time.

 

[johnalison]

For the sake of sanity, I find it necessary to keep amps and amp-hours separate in my mind. I think your arithmetic may be right but you may be optimistic in thinking that the alternator is putting it's rated output into the battery. I doubt if you need to take radio transmission into account unless you are very long-winded.

 

[VicS]

Sorry, should have said, I have a dedicated engine starter battery as well. I had assumed that I would give that a few minuties after starting the engine to get that topped up before switching to charge the leisure battery.

Now you mention it, I guess I need to work out how much it takes out of my starter battery and how long to charge that each time.

You need to distiguish in your mind the differenec between amps ..... the actual current flowing at any instant in time and .... amphours ... a measure of the quantity of electrical energy used over a period of time


You need to make an estimate, in amphours, of the daily drain on your house battery.

For each item multiply the current taken ( in amps ) by the number of hours in a day that it is in use. That will give you figure in amphours per day for each item.

Add all these amphour figures together obtain a daily total.

You will need to put that much plus a bit extra back into the battery per day.

 

[Momac]

If you have used 40 amp hours .
You will not recharge the battery by runnng the engine with a 40 amp alternator for one hour.
The charge current at first may be high but it will soon drop off.
I have the same capacity of alternator on a motorboat. With one starter and one leisure battery. If we motor three hours it seems sufficient to recharge the battery.

 

[pvb]

So, assuming that we are sailing for 24 hours (24 amp for instruments), use the radio once (4 amp) and have the tri light on for 8 hours (16 amp) we are using 36 amp each day.

I have a new 110 amp leisure battery and a new 40 amp alternator on a new Beta 14. I am assuming that I would run the engine for one hour a day.

So am I right in thinking that I’d be using 36 amp and adding up to 40 amp back in by runing the engine – so could effectively run the boat for as many days as I have diesel in the tank?

No, not right, I'm afraid. Batteries don't accept as much current as the alternator might be able to supply. With a single 110Ah battery, your charging current might initially be 25A or so, but would rapidly decline. You'd need to run the engine for several hours to replace 36Ah.

Larger battery banks accept charge more readily, so adding a second house battery (or a third) would help. It would also help to use a "smart" regulator (Adverc, Sterling, etc) which will boost the charge voltage and therefore increase the charging current into the battery.

 

[Mistroma]

Using the information in your first post, it appears that you will use 36Ah in 24 hours from a 110Ah battery and will recharge using a 40A alternator. I'm assuming that you don't have a smart charger and the leisure battery is just a sealed maint. free battery (not AGM).

1) If you take out 36Ah you would need to put back about 43Ah.
2) The manufacturers will problably suggest limiting charge for your battery to 15 - 20 Amps for longevity
3) The battery will restrict the charge anyway. Likely to be around 15A initially and falling after that quite quickly
4) The battery will charge quickly to 80%-85% level and then take a very long time to get back to 100%

If you start at 100% charge then 36Ah usage would take it down to 67%. I imagine that you might get back up to 85% after 2-3 hours motoring.
On subsequent days you will probably take the battery down to 50% (which represents around 36Ah usage starting from 85%) and charge back to 85% again. That would probably take 4-5 hours.

These figures use a lot of guesswork and simplification but you get the idea. It takes a lot longer to put the last 15% back into a flooded lead-acid battery and you will usually end up at around 85% level. Smart regulators are meant to help by boosting the charge rate. Even then, you'd be unlikely to keep the battery at 100% with only 1 hour on engine, more like 3-4.

 

[TQA]

Adding a smart regulator which will put more back into the battery will help but only up to about 80% capacity. Google 'bulk charge' I hr engine running about 25 to 30 amp/hrs with a smart reg. 10 to 20 amp/hrs with a std auto one.

Change to a LED bulb for your tri. 0.2 amp draw X 8hrs = 1.6 amp hours This is REALLY worth doing.

 

[PhillM]

Thanks all, so reviewing my maths (it was Sunday after a decent lunch!) and ignoring radio chat, I need about 40 amp hour each day.

But 1 hour on engine is not going to give me that back into the Battery.

So assuming I want to run for 5 days, I am going to be using 200 amp hours.

I dont have room for solar and I dont want to add Wind. So what does the forum think I need by way of battery bank and what is a reasonable engine time, bearing in mind the whole idea is to go sailing and not motoring!

 

[Stemar]

Change to a LED bulb for your tri. 0.2 amp draw X 8hrs = 1.6 amp hours This is REALLY worth doing.

+1 for that - you'll cut your consumption by 1/3 or more.

You say you haven't got room for solar panels. I thought the same until I decided I really needed some and actually had no problem finding the space for 40w of panels on my 24 footer. They have the big advantage that you'll be starting each weekend with fully charged batteries, as well as providing a good chunk of your requirements day by day - I expect about 10amp hours per day from mine.

One thing that might help you to keep on top of your battery state is a battery monitor. You could pay £100 or more for a clever one, or just get a cheap digital voltmeter and keep an eye on the resting voltage, an hour or so after charging and with only a small load. At 12.7v it's near enough full; if it drops below 12.5v, you'll want to get some charge in some time soon; at 12.5v, do it now or you may start to damage the battery. (that's oversimplifying, but you get the idea)

 

[prv]

You say you haven't got room for solar panels. I thought the same until I decided I really needed some and actually had no problem finding the space for 40w of panels on my 24 footer.

Phill has a pretty-looking wooden boat, though, and it would be a pity aesthetically to drape panels all over it.

Pete

 

[pvb]

Thanks all, so reviewing my maths (it was Sunday after a decent lunch!) and ignoring radio chat, I need about 40 amp hour each day.

But 1 hour on engine is not going to give me that back into the Battery.

So assuming I want to run for 5 days, I am going to be using 200 amp hours.

I dont have room for solar and I dont want to add Wind. So what does the forum think I need by way of battery bank and what is a reasonable engine time, bearing in mind the whole idea is to go sailing and not motoring!

There really isn't an easy answer. Obviously, reducing the consumption will help, and putting an LED tricolour light on would be an idea. If you're relying only on the engine for charging, you need to add a "smart" regulator, fit a larger alternator (say 70A), and greatly increase the battery capacity. A larger battery bank will soak up charge much more readily, so you'll be able to minimise engine run time. I'd suggest you need at least 4 x 110Ah batteries wired in parallel for your domestic bank. Remember too that the alternator won't give much output unless it's spinning quickly, so it's no good just having the engine idling, you need to put it in gear and motor along quite quickly. Ideally, it would be worth having a proper battery monitor (not just a voltmeter) to see the state of the batteries.

 

[johnalison]

If you want to keep going indefinitely, the above is what you need to do. We have had much less capacity and never had much of a problem. One reason is that there is often little wind at night and we tend to end up motoring some of the way.

Our current set-up is about 200Ah capacity on the service side and a 36w solar panel. Our drain is about the same as OP but we have a fridge, set on low, and no LEDs. The longest we ever do is two days without stopping and never had a problem. I check voltage on my Raymarine instrument after allowing for a fixed error of 0.2v. It helps to only sail in the summer.

 

[Pye_End]

With a similar demand, and same engine/alternator, but with a 40W panel, smart charger and more battery capacity (one engine starter and 2*80) I needed to run the engine for maybe an hour to an hour and a half a day at about 1,000 rpm. Didn't run in gear is it was not in the rules, but if cruising it is probably better. Used very little diesel. Not convinced I got very much out of the panel, so you probably need similar.

Best to have a dedicated engine starter battery, and then you can dig a bit deeper with the house bank. The advantage of this is that it is quicker to charge batteries that are bit more discharged than trying to force in the top say 25%. Without a dedicated engine battery you will need to be fairly careful so that you can still start the engine.

 

[William_H]

As said your demand should be calculated in amp hours over 24 hrs. So your instruments if on for 24 hrs will take 24AH.
Your mast head light say 2 amps for 6 hrs in summer is 12 AH. Your radio will take about .3 amp on receive so about 8 AH if on 24hrs. On transmit will take 4 amps for 30 seconds will be about 120th of an AH so disregard. So about 44AH by my estimation.
As said charge from the engine alternator will depend on battery size and depth of discharge.
You might start with around 20 amp charge but this will quickly diminish to more like 10A. What you really need as a minimum is an amp meter to show what charge you are getting at any stage in the engine run. Or more to the point when to stop the engine because the charge is not worth the noise. You might find 2 engine runs better than one. In any case recharge at the end of the day to take into account any motoring of the boat during the day. Definitely a smart charge controller might reduce engine run for charging times by 40 or 50 %.
In any case I think the engine driven alternator is your best recharge option. Compared to cost and inconvenience of solar and wind. Solar is good for long periods of inactivity of the boat keeping batteries up. good luck olewill

In any case

 

[PhillM]

Thanks all,

Looks like there are a few things I can do to get the most range.

It looks like the order of cost is:
Change to LED tri-light
Add a 2nd 110 amp leisure battery
Battery monitor
Smart charger

As for motoring, it sounds like we need to run the battery for a couple of hours a day. Do it at night and it will cut my consumption because the lights will be run directly from the engine for some of the time.

As for backup, I am thinking about some of those battery operated lights. The idea being that once I know how many batteries its takes to keep them going all night, I can x that by the most days that we will be away. Oil lights also seem like a reasonable backup. We carry oil for inside lights, so why not have a couple of side and a stern on board?

 

[Rivers & creeks]

The extra battery or two has a big imapct in terms of engine use - you may find that for two days you barely use the engine at all and the third day there's no wind and you motor all day. With two or three house batteries that's fine on your useage and the alternator will be working much more effciently rather than trying to squeeze a few amps in each day.

 

[blackbeard]

+1 for an extra battery (or batteries - another two if you can find room). Extra battery capacity needs to be at the top of your list, not halfway down it.

 

[doug748]

I don't know what your cruising plans are but your power requirements are very basic (good thing) and I think you will be ok.

Turn off your cockpit instruments, when not required, and you have cut your requirement by up to 2/3. You may well find that, an average, your masthead light in on less than a couple of hours a day.