Compressor Fridge, 12v, 240v and a charger - One for the Boffins!

[Captain Yoghurtpot]

Ok, so here we go.
Please forgive if this is a basic misunderstanding, but I am completely out of my depth.
We have a 2 year old Dometic 12v compressor Fridge, a new 100a Lead Acid leisure battery and a 10 amp battery charger and a 240v mains consumer unit with trip switch.
With the leisure battery at 12.7v, the compressor fridge when switched on starts and runs to cold after about half an hour but takes the leisure battery down to about 12.4v. If I leave the battery to get any lower than 12.4v the fridge then stops working.
If I switch on the main 10a charger to charge the 12v leisure battery and also switch on the compressor fridge, the fridge will run again for about half an hour, and then stops working, but is again cold.
My question is:
Is the compressor fridge,(a dedicated 12v unit), capable of being run on 240v via ehu IF the 10a charger is charging the battery at the same time, OR is the fridge only running still on 12v despite the battery showing a charge rate of 13.2v (ie the consumption of the 12v for the fridge is greater than the level and speed of replenishment from the 10a charger.)
Or, is it just not possible to run a 12v fridge this way via 240v and a charger, and if so, is there any way it can be so.
I am thinking that when on site I may want to run the fridge constantly , but as I do not have lithium or solar, this can only be done via ehu.
I believe the 12v compressor fridge is predominantly , and favoured, for off-grid use.
I will read the responses with interest and get back with probably more questions, but I am getting a bit frustrated with what I would perhaps think is a logical path, but then this is electrics and I am no Einstein.
Many Thanks.

 

[mitzimad]

my son had a waco compressor fridge and it came with a transformer direct from hook up

 

[oldiesontour]

If you run your fridge and mains charger the charger will be putting in at least twice what the fridge is using

 

[Captain Yoghurtpot]

my son had a waco compressor fridge and it came with a transformer direct from hook up

That's interesting. I am wondering if that is what I may need.
Thanks for the reply.

 

[Jabstatt]

Is the fridge cutting out because it is down to temperature?
The duty cycle should maybe be under fifty percent.

Is the charger really capable of ten amps, can you measure it?

Some conflicting evidence here.

On the face of it, you should be ok.

I would charge up on ehu. NO loads at all, leave it several hours.
Whilst on ehu turn on fridge.
What happens?
Don’t open the fridge every ten minutes to check it.
Can you measure dc current to fridge?

Tony

^{_{Subscribers  do not see these advertisements}}

 

[chesterfield hooligan]

Surly the fridge cuts out when down to temperature which gives the battery time to recover. Ours goes on and of through the night as required.

 

[Captain Yoghurtpot]

If you run your fridge and mains charger the charger will be putting in at least twice what the fridge is using

I appreciate that, but am wondering if that might be too much power, because after the fridge has stopped with a voltage level at around 13.2v having been connected to the charger, if I leave it for a bit to a resting voltage at around 12.6/7v, it can again restart.
Weird!

Surly the fridge cuts out when down to temperature which gives the battery time to recover. Ours goes on and of through the night as required.

Actually Brian, I think you are probably right!
But you know me.
I could worry for England.

 

[hja]

We had a compressor fridge with an ordinary 90 or 100ah battery. On ehu, battery charged, fridge worked, cycled on & off depending on its temp. Sometimes we remembered to put fridge on night time setting at night, which turns it off until about 5am, but more often we forgot. And actually not opening the fridge overnight meant that it turned off any way. We only had a problem when we were not on hook up when the battery was insufficient and the voltage dropped too low. We did have (still have) a 120w solar panel but the particular occurrence was when it was pouring with rain, so no solar. We have lithium now, so no problems.

 

[Captain Yoghurtpot]

Is the fridge cutting out because it is down to temperature?
The duty cycle should maybe be under fifty percent.

Is the charger really capable of ten amps, can you measure it?

Some conflicting evidence here.

On the face of it, you should be ok.

I would charge up on ehu. NO loads at all, leave it several hours.
Whilst on ehu turn on fridge.
What happens?
Don’t open the fridge every ten minutes to check it.
Can you measure dc current to fridge?

Tony

Thanks Tony.
Lot to think about there.
But, I may be over complicating things as never considered that fridge could have reached temprature and shut off to conserve power, ready to start again when needed to maintain temprature and give the battery a chance.
I shall have a ponder.

 

[chesterfield hooligan]

Our compressor cool box has a low battery level cut out I think it's set at 11.8v but we don't get that low even with the disel heating on, lowest level normally is 12.5v perhaps 12.4v odd time.

^{_{Subscribers  do not see these advertisements}}

 

[treblid]

Our Thetford 12V compressor fridge draws around 3.5A during the ON cooling cycle. Your 10A charger will provide more than enough to run the fridge during its ON cycle and still put the remaining power as charge into the battery. When the fridge is in it's OFF cycle, the battery will receive the full current from the charger (minus any other loads such as lighting).

When on battery only, the fridge will run it's ON and OFF cycles for duration the leisure battery can safely power it depending on your battery capacity and condition. Most compressor fridges have a low voltage cut out to protect the battery from excessive discharge. The fridge start up surge current maybe taking the voltage below this point especially if the battery is old and loosing capacity.

 

[Captain Yoghurtpot]

Our compressor cool box has a low battery level cut out I think it's set at 11.8v but we don't get that low even with the disel heating on, lowest level normally is 12.5v perhaps 12.4v odd time.

Wondering if our fridge has an adjustable cut out level re voltage but 12.4v would be about right I think as that's about when it cuts out.

 

[Captain Yoghurtpot]

We had a compressor fridge with an ordinary 90 or 100ah battery. On ehu, battery charged, fridge worked, cycled on & off depending on its temp. Sometimes we remembered to put fridge on night time setting at night, which turns it off until about 5am, but more often we forgot. And actually not opening the fridge overnight meant that it turned off any way. We only had a problem when we were not on hook up when the battery was insufficient and the voltage dropped too low. We did have (still have) a 120w solar panel but the particular occurrence was when it was pouring with rain, so no solar. We have lithium now, so no problems.

Thanks for the comments.
Am starting to get an idea now of how exactly a compressor fridge works.
On our last van, the fridge was 3 way, so completely different and ran on 240v like a normal domestic fridge, and only ran on 12v when driving, and on gas only occasionally.

 

[Captain Yoghurtpot]

Our Thetford 12V compressor fridge draws around 3.5A during the ON cooling cycle. Your 10A charger will provide more than enough to run the fridge during its ON cycle and still put the remaining power as charge into the battery. When the fridge is in it's OFF cycle, the battery will receive the full current from the charger (minus any other loads such as lighting).

When on battery only, the fridge will run it's ON and OFF cycles for duration the leisure battery can safely power it depending on your battery capacity and condition. Most compressor fridges have a low voltage cut out to protect the battery from excessive discharge. The fridge start up surge current maybe taking the voltage below this point especially if the battery is old and loosing capacity.

Interesting.
Battery is brand new. But I take your point entirely.
Second battery MIGHT be a solution, and I think I have space.
Understood that completely.
Thanks, as it all made logical sense, as have a lot of the replies.

 

[Richard n Ann]

Wondering if our fridge has an adjustable cut out level re voltage but 12.4v would be about right I think as that's about when it cuts out.

The voltage at the fridge could well be lower dependant on cable size but 12.4v is too high for the fridge to switch off due to low battery. 12.4v with several amps load could be a fully charged battery.
More likely it's switching off on the thermostat

^{_{Subscribers  do not see these advertisements}}

 

[meanders]

Yes a compressor fridge only works when the inside temperature rises and the thermostat then calls for more cooling. If you listen at home to your fridge or freezer, you will only hear the motor ( which is driving the actual compressor) running occasionally. Usually easier to hear them starting then stopping.

 

[treblid]

Battery is brand new. But I take your point entirely

12.4V does sound a bit too high a voltage for a fridge to do a low voltage cut out. If your battery is new, then there could be over reasons for the voltage to be lower at the actual fridge when the fridge turns ON (surge current). I would check for any poor connections, fuse holders, etc. in the 12V fridge supply.

 

[Captain Yoghurtpot]

12.4V does sound a bit too high a voltage for a fridge to do a low voltage cut out. If your battery is new, then there could be over reasons for the voltage to be lower at the actual fridge when the fridge turns ON (surge current). I would check for any poor connections, fuse holders, etc. in the 12V fridge supply.

Will do.
And thanks for the advice.

 

[Captain Yoghurtpot]

The voltage at the fridge could well be lower dependant on cable size but 12.4v is too high for the fridge to switch off due to low battery. 12.4v with several amps louad could be a fully charged battery.
More likely it's switching off on the thermostat

By thermostat Richard n Ann, do you mean that the fridge is at the set temprature, and thus switching off.
If so, then I understand.

 

[Richard n Ann]

Yes

^{_{Subscribers  do not see these advertisements}}

 

[Captain Yoghurtpot]

The voltage at the fridge could well be lower dependant on cable size but 12.4v is too high for the fridge to switch off due to low battery. 12.4v with several amps load could be a fully charged battery.
More likely it's switching off on the thermostat

And, after some investigating I am now running on battery only on the fridge, starting at 12.6v and have been on a constant 12.2v for the past 30 mins.
Fridge is now cold but I am going to see when exactly the fridge cuts out, and at that point what the state of the battery is in, ie at what voltage is showing, and how long it takes for the battery to recover to around 12.6/7v again without any ehu charging.
Think I am making progress and as they say, every day is a skool day!
Thanks for the reply Richard n Ann.

 

[Captain Yoghurtpot]

Update at 16.48pm.
Have just switched off fridge as it is nice and cold and battery immediately went up to 12.4v, from a 12.6v starting position and initial fridge switch on ,when it dropped to 12.2v.
So it would seem that my battery IS capable alone of powering the fridge without the need to use ehu, other than to replenish the leisure battery alone when needed.
And have just checked battery voltage now at 16.53pm and it's sitting now at 12.5v!!!!!
I am, I think, a Happy Camper, but it is so frustrating when you don't fully understand some things.
I am so glad I am part of this forum as the levels of advice and encouragement are priceless.
Thanks to all who responded with suggestions, ideas and comment.
Much appreciated.