battery dimensioning basics....

[Refueler]

thanks but cables were replaced 3 months ago. Indeed 80Ah is very little but as said fully charged it should provide the needed 3 AMps for some hrs

Just to comment .... the 3A you mention ... most likely the average once temp is down ... but getting there Amps will be higher. Also as the power source - the battery voltage drops - the amps increase to maintain the Watts demand of the unit.

Lets say you engine running and alternator is pumping out 14.4v .. and you register 3A to fridge..... 43.2W (seems low !)

Stop engine ... battery voltage drops to 13.2v ... to maintain same wattage - now will be 3.2A

Battery reduces to 12.5v .... amps now 3.5A

OK - the increase in amps may not seem much ... but remember that change applies to all items on board ... amps increasing when voltage drops ..

 

[MontyMariner]

Also as the power source - the battery voltage drops - the amps increase to maintain the Watts demand of the unit.

That doesn't sound right to me, as Ohms Law says, the current drawn is governed by the Voltage divided by the load Impedance, as the Impedance is fixed, if the voltage drops the current drawn will reduce.
From what you suggest, say the battery voltages reduces to zero it will still supply sufficient current required by the load, and we know that isn't the case.
Or have I totally misunderstood what you are saying?

 

[Refueler]

That doesn't sound right to me, as Ohms Law says, the current drawn is governed by the Voltage divided by the load Impedance, as the Impedance is fixed, if the voltage drops the current drawn will reduce.
From what you suggest, say the battery voltages reduces to zero it will still supply sufficient current required by the load, and we know that isn't the case.
Or have I totally misunderstood what you are saying?

Yes you have misunderstood ..

Watts = Volts x Amps

Its a fundamental factor that I and other modellers encounter every day with our Electric powered models as example.

Your comment "say the battery voltages reduces to zero it will still supply sufficient current required by the load, and we know that isn't the case." fails to take into account limitations of the power source and that once voltage has dropped sufficiently - the power will basically be limited until it fails. It does not just ramp up amps ad infinitum..

My comment was to illustrate within the capability range of source.

 

[vas]

thanks but cables were replaced 3 months ago. Indeed 80Ah is very little but as said fully charged it should provide the needed 3 AMps for some hrs

if everything else is fine, cables (and even neg. bus bar!), length (which you've still not disclosed!), voltage on fridge when running, then I'd expect the battery not to be full, or dying. Simple.
there's no way a fully charged 80Ah batter will only run a fridge for a couple of hours and crap out, sorry.

 

[MontyMariner]

Yes you have misunderstood ..
Watts = Volts x Amps

I did wonder if you were going down the P=VI route
Yes, to maintain the level of power supplied if the voltage drops the current will have to increase but I only knew of constant voltage and constant current generators. I looked up constant power generator and found these Generators Home are small versions available?

 

[Refueler]

I did wonder if you were going down the P=VI route
Yes, to maintain the level of power supplied if the voltage drops the current will have to increase but I only knew of constant voltage and constant current generators. I looked up constant power generator and found these Generators Home are small versions available?

I was not even considering Generators ... I thought topic was more on batterys ?? If a motor requires X watts ... then the battery has to supply V x A = X watts

 

[MontyMariner]

I thought topic was more on batterys

So we are back to Ohms Law!

 

[vasant]

if everything else is fine, cables (and even neg. bus bar!), length (which you've still not disclosed!), voltage on fridge when running, then I'd expect the battery not to be full, or dying. Simple.
there's no way a fully charged 80Ah batter will only run a fridge for a couple of hours and crap out, sorry.

Sorry i thought i had asnwered,,,,, Length is 4meters. I dont think the battery is crap, i mostly consider the "smart cutoff" of the fridge not to be understood well.

 

[vas]

Sorry i thought i had asnwered,,,,, Length is 4meters. I dont think the battery is crap, i mostly consider the "smart cutoff" of the fridge not to be understood well.

unlikely the fridge is to blame, unless it's 30yo! btw how old is it, has it been regassed, when/ by whom?

Do another easy test:
Get your voltmeter + and - on the relevant terminals ON THE FRIDGE el. controller on the back (may have to pull the fridge out of the cabinet if all integral). IIRC it's the top two connectors. They are spade terminals either use crocodile clips or push the probes in somehow (mine stay in fine). Ignore what the voltmeter on the el.panel of the boat shows, it's irrelevant, you want to see what's happening at the fridge end!
Monitor voltage when idle, start fridge (with your full battery) and watch changes, monitor over the hour or so it will take to stop. Check what voltage it shows when stopped.
if it's not 10.something, there may be a chance someone fitted a resistor (as mentioned in a post above) to force the fridge to stop early but it's unlikely. Its a faff, did it once and then realised the issue was elsewhere and fixed it. If you have good access to the el.controller, you'll see the extra cable and resistor, it will be a non-standard assembly.

If it is indeed 10.something when fridge stops, then give attention to the battery and wiring/breaker.

Re battery being fine and fully charged, have a look at a thread earlier this week, or late last week, where it turned out batteries weren't that full (and one of the 6 was dead). Borrow a battery tester that shows CCA and voltage to make sure battery is OK. Check charging system if it's not fully charged when you think it is.

cheers

V.

 

[Refueler]

So we are back to Ohms Law!

Fine BUT ... take this in your stride :

Motor lets say is designed to output 100W ... say as in a compressor ... then that motor will draw 100W / Battery volts = Amps .... let say battery starts at 13V ... then 100 / 13 = 7.69A .... battery charge dropping and we chack at 12.5V .... 100 / 12.5 = 8A ...

Not a huge increase _ BUT the rate the battery is now dropping in charge is accelerating ... but that's another matter.

 

[PaulRainbow]

Fine BUT ... take this in your stride :

Motor lets say is designed to output 100W ... say as in a compressor ... then that motor will draw 100W / Battery volts = Amps .... let say battery starts at 13V ... then 100 / 13 = 7.69A .... battery charge dropping and we chack at 12.5V .... 100 / 12.5 = 8A ...

Not a huge increase _ BUT the rate the battery is now dropping in charge is accelerating ... but that's another matter.

That wouldn't happen with a resistive load like a motor. When the volts drop it's the amps that drop with it, resulting in a reduction of watts (Ohms law).

 

[B27]

That wouldn't happen with a resistive load like a motor. When the volts drop it's the amps that drop with it, resulting in a reduction of watts (Ohms law).

A motor is not a resistive load.
Except maybe when it's stalled.

A modern DC fridge is some kind of brushless motor, behaves like a switchmode power supply and may we draw constant power, depending on exactly how it's controlled.
So yes, the amps can go up if the volts go down.
The fridge on my boat runs either from the 12V, or from the mains using an adaptor which came with it, the adaptor gives out 24V.
On 12V it draws about 5A peak. The 24V supply is 3A. Average over 24 hours is about 12 watts once it's down to temperature.

My fridge came with a 12v cable about half a boatlength long, shortening that to what was needed made a difference.

 

[vas]

On 12V it draws about 5A peak. The 24V supply is 3A. Average over 24 hours is about 12 watts once it's down to temperature.

guess the 12 watts is a typo, but even 12Ah is going to be too ambitious over a day at 12V.
I assume 4A@12V on normal operation (ok 5A peak for the first few seconds when firing up)
with a duty cycle of say 20% that's going to be 4A*24h*0.2 = 19.2Ah
Granted my experience is from med summer (where you wont get 20% duty cycle!) could be doable on northern winters...

Re OP question, the above calculation explains why some of us argue that there's something seriously wrong if the fridge wont be able to get it's 20Ah out of an 80Ah fully charged battery and stops after running for an hour or so which is something like 4-5Ah if running constantly (and assuming it was running on shore power before, it wouldn't!)

V.

PS. just realised OP is also from Greece, so forget the 20% duty cycle, but still should be easily a day!

 

[B27]

No the 12W is not a typo, it's a measurement of about 270 to 300 watt hours per day, when I tried out the fridge at home before taking it to the boat. Measured with one of those plug-in power meters, which is accurate enough. No point trying to be mega-precise, as the operating conditions will vary with the weather and all that, it was in summer in a warm kitchen IIRC.
On the boat 25 to 30 Ah per day from a 12V battery seems more than enough, so in good weather a 50W panel accounts for a fair slice of the fridge's use. That allows for opening it everytime we make tea etc, putting some warm stuff in from time to time.

It's a bit 'finger in the air' but when the other side of the equation involves English Sunshine, it's not a precision game.

Even with our single 110Ah house battery, we're fine, the weather doesn't have to be specially good for the solar panel to extend the life of the battery to several days, and generally over an average week cruising a few engine hours add some bulk charge now and then.
The battery is several years old so probably isn't actually 110Ah any more!

It's not like being at home, we do have to be 'slightly mindful' that the battery capacity is finite, but it's good enough to allow us to eat pretty well on the boat. The biggest constraint is the physical capacity of our fridge.

Our fridge was about £200 from some random UK seller on ebay.
The insulation is pretty good, a few frozen things at the start of a trip keeps everything cooler than 3degC for the first day or so before it starts drawing power!
We do tend to switch it off while we sleep, so long as there's some stuff in it, like a litre of beer or water, it won't warm up much overnight.
It's not noisy as such, but not silent either, and it's close to my bunk.
Best £200 I've spent on a boat.

 

[Refueler]

I have 2x 80A/hr Lead acids make up my domestic bank .. and they are definitely end of life ! There is no way I can think of running my fridge on board ... until I get them changed for new.

I would also like to replace the solar panel on deck which looks pretty old ... a few small areas around the edge have lost their black ... controller still shows it working but I think its time to look at new ... and maybe a little bigger.

 

[B27]

I have 2x 80A/hr Lead acids make up my domestic bank .. and they are definitely end of life ! There is no way I can think of running my fridge on board ... until I get them changed for new.

I would also like to replace the solar panel on deck which looks pretty old ... a few small areas around the edge have lost their black ... controller still shows it working but I think its time to look at new ... and maybe a little bigger.

Depends what you call 'end of life'.
ISTR there's some IEEE standard saying 'end of life' is when it's down to 75% or something of the original capacity.
When a lot of batteries finally get discarded, they are down much lower.
I think the failure curve may accelerate as capacity falls, so running a fridge from an old battery might finish it off PDQ.

The capacity you need is an ill-defined compromise based on a few factors, like how often you run the motor, any solar etc, but also how much of a disaster it is if you run out of fridge power. If we can't run our fridge, we'll drink black coffee and go to the shop ashore for fresh food, live from tins, or sail somewhere with decent pub.
The fridge was £200, a new battery is £100, our fridge has saved us more than that with meals we've cooked afloat instead of going to the pub more often.
Diminishing returns on having more capacity as far as the fridge is concerned.
Not sure what we really want/need for the heater though!

 

[Baggywrinkle]

If you want to get a definitive measurement for any 12v consumer on board then one of these from amazon is worth its weight in gold ...

https://www.amazon.co.uk/dp/B093P64J13/


I connected one in line with my fridge and left it running for 7 days then divided the total power used by 7 to get the daily usage for my battery and solar budget calculations.

I also had a BD35 based fridge but the amount of stuff you have in it, the quality of the insulation, the ambient temperature, and your preferred fridge temperature will affect the power consumption - which is why I measured mine.

It came out at around 0,6 kWh per day which was the biggest consumer on board by far. This equates to around 50Ah a day - so your 80Ah battery seems a bit undersized to run a fridge permanently IMO.

 

[Refueler]

That's similar bit of kit to what we use to setup our E-motor models ... Wattmeter.

 

[Stemar]

Jazzcat had a couple of 60 AH batteries that powered the domestics and one starter, and everything worked fine. I got a fridge. My next job was to scrap the 60 AH batteries and replace them with three 110AH batteries. With a bit of sun, we're now good for a week before it's time to run the engines.

 

[B27]

Jazzcat had a couple of 60 AH batteries that powered the domestics and one starter, and everything worked fine. I got a fridge. My next job was to scrap the 60 AH batteries and replace them with three 110AH batteries. With a bit of sun, we're now good for a week before it's time to run the engines.

Do you tend to want to sit at anchor for a week without running the engine, or do you manage to sail long trips with no engine?

Once you have enough solar to fill the gaps between engine use, the need for lots of battery capacity can almost go away.
At least on a minimalist boat with no inverters and thrusters and all that.

But OTOH, batteries are not really expensive in the scheme of things, if an extra couple of 110Ah batteries gives you a better Summer holiday with less stress, it's a bargain. Might even pay for itself in saved mooring fees?