Narrow boat fridge

[Nemysys]

10 minutes ago, Alan de Enfield said:

A 1kw Immersion heater for an hour ?

Assuming 100% efficiency, which I know is not the case.

batteries in parallel, so 110ah * 2 = 220ah at 12v

 

watts=amps*volts

1000w=240v*4.2a

 

110ah battery/4.2ah= 26 hours

220ah battery (as they are in parallel)/4.2ah= 52 hours

 

now take efficiency into account, guess at 50%, 13 hours / 26 hours for a 1kw load

 

ypu cannot run a battery flat, so take 50% off of this, 6.5 hours / 13 hours.

 

 

please if anybody knows better and can correct me, let me know.

 

 

[cuthound]

Peukert's Law will come into effect, which basically says that the higher load you put on a lead acid battery, the lower its available capacity will become.

 

https://en.m.wikipedia.org/wiki/Peukert's_law

[TheBiscuits]

From experience, a lot of us fudge the numbers to allow for inefficiencies in a battery - inverter - mains appliance setup.

 

I always work it out at 10V for convenience and a safety factor.

 

1000W / 10V => 100A

 

If you have 2 perfect new 110Ah batteries and only discharge to 50% (for better battery life) you have 110 Ah available to use.

 

If you run a 100A load from that battery bank, you would be lucky to get an hour before it was below 50%.

 

[Alan de Enfield]

3 minutes ago, Nemysys said:

Assuming 100% efficiency, which I know is not the case.

batteries in parallel, so 110ah * 2 = 220ah at 12v

 

watts=amps*volts

1000w=240v*4.2a

 

110ah battery/4.2ah= 26 hours

220ah battery (as they are in parallel)/4.2ah= 52 hours

 

now take efficiency into account, guess at 50%, 13 hours / 26 hours for a 1kw load

 

ypu cannot run a battery flat, so take 50% off of this, 6.5 hours / 13 hours.

 

 

please if anybody knows better and can correct me, let me know.

 

 

You have missed out the inverter part of the equation.

 

Converting 12v DC to 230v DC (when taken losses into account) is basically a ratio of 10:1

 

A 1000w load when taken from the batteries vis the inverter will put a demand of 100 amps on the batteries.

 

Assuming your batteries still have 'full capacity' (unlikely) and not wanting to take them below 50% SoC then you have 100Ah available.

 

Hence the 1 hour I quoted.

 

1000w is 1000w irrespective of the voltage so 

 

W = A x V

 

A = W/V

 

A = 1000 / 12 = 83.3 Amps

take into account 10-15% losses and round it off and you have ~ 100amps.

 

So irrespective of powering a 12v 1000 watt inverter, or a 230v 1000watt inverter the answer is (virtually) the same.

[Tony Brooks]

9 minutes ago, Nemysys said:

Assuming 100% efficiency, which I know is not the case.

batteries in parallel, so 110ah * 2 = 220ah at 12v

 

watts=amps*volts

1000w=240v*4.2a

 

110ah battery/4.2ah= 26 hours

220ah battery (as they are in parallel)/4.2ah= 52 hours

 

now take efficiency into account, guess at 50%, 13 hours / 26 hours for a 1kw load

 

ypu cannot run a battery flat, so take 50% off of this, 6.5 hours / 13 hours.

 

 

please if anybody knows better and can correct me, let me know.

 

 

I think something is odd here.

 

Yes 1kW @ mains voltage = 4.2 amps but you are getting it from an inverter. For ease of working we normally assume an inverter efficiency of 80% so the calculation is 1000 watts @ 10 Volts (10 being 80% of 12) so 1000/10 = 100 amps

 

The immersion will draw around 100 amps from the battery.

 

Assuming the batteries have full capacity and are 100% charged, which will not be the case the 220Ah batteries will last 220/100 = 2.2 hours. It will then be dead flat and probably damaged shortening its life. Peukert says such a high current draw will reduce the two hours.

 

As we do not want to ruin the batteries and optimise their life we normally only discharge to 50% so that less than 2 hours becomes less than 1 hour.

[TheBiscuits]

... I told you a lot of us do it that way.  Very few of us read the full thread before posting though! 

 

[Alan de Enfield]

Well, that's 3 of us that have explained his error - hopefully that should be enough.

[TheBiscuits]

Just now, Alan de Enfield said:

Well, that's 3 of us that have explained his error - hopefully that should be enough.

Yes, and three slightly different explanations all showing workings that come to the same conclusion.  I'm just waiting for @WotEver to finish typing his version of the same thing ...

[mrsmelly]

38 minutes ago, Nemysys said:

We have a pure sine wave inverter which we can use with the engine off. Draining batteries.

 

‘The 240v while the engine is on, is a completely different system, so I presume some sort of travel power? We have at least 2 alternators (maybe 3), (I’m not mechanical, I understand electrics more), I thought one of the alternators was producing 240v anyway?. The box I mentioned is about the size of an a4 bit of paper, blue and white, with a black and red button on it. On and off. It does dip the engine revs when turned on. I will post a picture soon. But is completely separate to the inverter.

The box is a travel power. If you have a crap inverter it will indeed drain the batteries. A good inverter will take a tiny amount to power it when on standby. If your inverter is a low draw your best bet is a mains fridge, available for peanuts and leave inverter on 24/7 365 like myself and others do.

[system 4-50]

Keep the gas fridge. 

[WotEver]

1 hour ago, TheBiscuits said:

I'm just waiting for @WotEver to finish typing his version of the same thing ...

I very nearly did... then I read the rest of the thread and saved my fingers the exercise

[Nemysys]

2 hours ago, system 4-50 said:

Keep the gas fridge. 

Thanks, that sounds the best option.

Thanks everyone for your comments, I appreciate it.

[TheBiscuits]

2 hours ago, system 4-50 said:

Keep the gas fridge. 

 

1 minute ago, Nemysys said:

Thanks, that sounds the best option.

Thanks everyone for your comments, I appreciate it.

 

That would have been excellent advice when you couldn't get solar panels and propane was a fiver a refill.  At thirty quid a refill for gas and with solar very cheap it may no longer be true, but that depends on your usage pattern of the boat and the fridge.  

 

If you do keep the three-way fridge, I'd say run it hard on 240V off the travelpower and only use the gas when stopped.  If you are doing a refit anyway I'd suggest a mains fridge and a couple more batteries.

 

 

[Chewbacka]

11 hours ago, TheBiscuits said:

 

 

That would have been excellent advice when you couldn't get solar panels and propane was a fiver a refill.  At thirty quid a refill for gas and with solar very cheap it may no longer be true, but that depends on your usage pattern of the boat and the fridge.  

 

If you do keep the three-way fridge, I'd say run it hard on 240V off the travelpower and only use the gas when stopped.  If you are doing a refit anyway I'd suggest a mains fridge and a couple more batteries.

 

 

Depends on your boat usage.  If in the summer you have say 300W of solar and in the winter you either don't use the boat much or are on shore power a lot I would certainly go with the above advice, however if you plan to use the boat a lot in winter without weekly access to shore power  or lots of cruising then you will have the big problem of keeping your batteries charged, and one of the biggest not optional power consumers is the fridge. 

[Nemysys]

23 hours ago, TheBiscuits said:

 

 

That would have been excellent advice when you couldn't get solar panels and propane was a fiver a refill.  At thirty quid a refill for gas and with solar very cheap it may no longer be true, but that depends on your usage pattern of the boat and the fridge.  

 

If you do keep the three-way fridge, I'd say run it hard on 240V off the travelpower and only use the gas when stopped.  If you are doing a refit anyway I'd suggest a mains fridge and a couple more batteries.

 

 

Thanks, we only use the boat for recreational use during the summer. We primarily use it on 240v when the engine is running and then go to gas when stationary / overnight. 

[David Mack]

15 hours ago, Chewbacka said:

 however if you plan to use the boat a lot in winter without weekly access to shore power  or lots of cruising then you will have the big problem of keeping your batteries charged, and one of the biggest not optional power consumers is the fridge. 

 

If you are boating in winter, just keep the relevant food items outside, and the fridge power consumption is entirely optional.

[MtB]

4 minutes ago, David Mack said:

 

If you are boating in winter, just keep the relevant food items outside, and the fridge power consumption is entirely optional.

 

I don't buy that. Fridges are typically 3-5C inside but in winter, the outside daytime temp is often up in double figures. A recipe for food poisoning.

 

 

[Johny London]

I've tried a box outside - it seems like a good idea when there is a cold snap, but actually it's a real pita - if you leave the boat and the sun comes out unexpectedly, you worry about stuff spoiling. Also just having to keep opening the boat doors to go and get stuff every five minutes and loosing all your heat   Definitely solve the problem by adding more power in some way and keeping the fridge on - it is a bastion of civilisation.

[Tony Brooks]

11 hours ago, Mike the Boilerman said:

 

I don't buy that. Fridges are typically 3-5C inside but in winter, the outside daytime temp is often up in double figures. A recipe for food poisoning.

 

 

 

I don't buy that either. For years perishable food was kept on a stone slab in a larder and people made sure they shopped appropriately, used perishable food first, and could tell when food was suspect. I don't remember much food poisoning. Of course if people have lost those skills it is a different matter.

[MtB]

18 minutes ago, Tony Brooks said:

I don't remember much food poisoning. Of course if people have lost those skills it is a different matter.

 

Which they have.

 

The bit I don't buy is the assertion by David that keeping the food outside is equivalent to keeping it in the fridge. It isn't.

 

Those food-sniffing skills need to be polished up and used well if you plan to do that!

 

 

[bizzard]

My gas fridge goes off about November until about April. I made a bottomless box in the front well where it stands on the cold steel floor. Never been a problem, never been ill because of it. Most folk do a shop every couple of days so even summers can be fine without a fridge too, if careful.   Years ago not many folk had a fridge at all in houses never mind boats,, perhaps a larder that kept things a bit cooler and there were no sell by dates as a guide then either, narthen!!      Co Op divi number 365748.

Edited June 14, 2019 by bizzard

[Alan de Enfield]

6 minutes ago, bizzard said:

Co Op divi number 365748.

Ours was 174446

 

Used to call in the Co-Op on the way home from School - everything went onto the 'divi'.

[Johny London]

Yeh, we didn't have a fridge when I was a kid. I think your bottomless box makes good sense - I did wonder about making a "larder" in a similar way (is it how some do a wine cellar?) by having a hatch in the galley and storing stuff on the cold steel base plate. Thus not having to go outside every five minutes.

[TheBiscuits]

1 minute ago, Johny London said:

Yeh, we didn't have a fridge when I was a kid. I think your bottomless box makes good sense - I did wonder about making a "larder" in a similar way (is it how some do a wine cellar?) by having a hatch in the galley and storing stuff on the cold steel base plate. Thus not having to go outside every five minutes.

Ours is usually between 4 and 6 degrees for most of the winter.  Once the water cools down it stays that way for a long time.

 

If it ever freezes 3 feet down we will have more to worry about than spoilt milk!

 

In summer it's usually between 12 and 15 degrees so perfect for beer ...

[bizzard]

7 minutes ago, Johny London said:

Yeh, we didn't have a fridge when I was a kid. I think your bottomless box makes good sense - I did wonder about making a "larder" in a similar way (is it how some do a wine cellar?) by having a hatch in the galley and storing stuff on the cold steel base plate. Thus not having to go outside every five minutes.

Best if the wooden bottomless box and lid is lined with aluminium sheet, with a few small vent holes near the bottom.

Edited June 14, 2019 by bizzard