Information required re 12v refrigerator

[Waterways Jim]

Since I have had a problem with my engine (see this thread: http://www.canalworld.net/forums/index.php?showtopic=5645) I now have a problem with the fridge.

 

As the engine is not helping to keep batteries charged, my fridge is running the batteries down to a point where the fridge doesn't work properly.  I decided to get a 12v psu to power the fridge during this time and, as the fridge plugs into a 5amp roundpin socket, wired another 5amp roundpin socket, so that I could just move the plug across, with the other socket powered from the power supply which in turn is plugged into the hook-up circuit whilst moored up.

 

At first I had difficulty finding a 12v psu which could also deliver up to 10amps, so made do with a 5amp one.  This lasted for about a week and a half continuous use before it gave out.  By this time I had sourced a 12v regulated psu delivering up to 20amps and wired that in.  The fridge just woudn't have it and did not start up properly.  I borrowed another 20A 12v psu just to make sure and - same thing.

 

Now, I also have a solar panel which trickle charges the batteries during daylight hours.  This means that, after a while spent away from the boat, I can actually use the boat batteries to power the fridge - for a while anyway, and the fridge starts up with no problem.

 

My question is:  WHY DOES MY FRIDGE WORK WITH THE BATTERIES, BUT NOT WITH A 20A 12V REGULATED POWER SUPPLY?? Bearing in mind that it did work with a 5amp 12v psu for a bit, i.e. until that psu gave out from too much current being required of it.

[Yoda]

 

My question is: WHY DOES MY FRIDGE WORK WITH THE BATTERIES, BUT NOT WITH A 20A 12V REGULATED POWER SUPPLY?? Bearing in mind that it did work with a 5amp 12v psu for a bit, i.e. until that psu gave out from too much current being required of it.

 

Your 'fridge is knackered.

[Waterways Jim]

Your 'fridge is knackered.

 

Well, I thought that might be the case, but why does it work with the batteries - makes no noise (hardly) and works fine apparently.

[Yoda]

Well, I thought that might be the case, but why does it work with the batteries - makes no noise (hardly) and works fine apparently.

 

Sounds like the motor is drawing too much current.

[Waterways Jim]

Sounds like the motor is drawing too much current.

 

The psu I borrowed had an amp metre and a voltmetre on it and the current jumped to about 10A when it was trying to start up.  If the psu delivers 20amps, then that shouldn't be a problem.

[bottle]

Fridge works from batteries 12volt DC

 

Fridge does not work from psu,what voltage and type of current (AC OR DC) does it deliver.

 

I am clutching at straws and guessing, so will butt out if some one knows better.

[John Orentas]

Hi Jim.

 

The thing is how regulated is 'regulated', it obviously should work especially as everthing worked on a 5 amp unit for a while. As you may have gathered from other posts, though modern fridges are extremely efficient there is a price to be paid, that is they are a bit sniffy about the quality of Dc current they will work on.

 

Remember too that they need quite heavy cables to reduce volt drop in surge conditions. I would look for a better quality PSU with a high surge potential, that usually means it will be a heavy transformer type with a few capacitors thrown in.

 

Alternatively go about the job the other way and use a mains battery charger to keep the batteries charged.

[Sir Nibble]

go about the job the other way and use a mains battery charger to keep the batteries charged.

That looks sensible to me.

Edited September 26, 2006 by snibble

[Gary Peacock]

Shoreline do a PSU to do the job on their fridges.

 

 

 

 

A bit more detail HERE

[DHutch]

Alternatively go about the job the other way and use a mains battery charger to keep the batteries charged.

Yeah, i would deffonatly advise a mains charger, seeing as you have mains avilable, this would deffornatly be the most senstable way of running it, even if your engine did work!

- You will be doing your batterys a consderable amount of damage running them that flat as well.

 

Our battery charger will even function as a PSU, powering the lowvoltage system without any batterys present.

 

 

Otherwise i dont really know.

- You mention that you borrowed another 20a psu, was this the same brand/model as yours. If so, try getting hold of a totatly diffrent one.

- Alternatively, there is the shoreline one as well. What make is your fridge anyway? Is it s shoreline?

 

 

Daniel

Edited September 26, 2006 by dhutch

[Waterways Jim]

Thanx for all your posts whilst I've been working.

 

I did spend quite a lot on the psu, so am fairly sure that it's good. The other one I borrowed was not the same make.

 

I do use a charger to keep batteries charged, but can only use: 1) when I'm there 2) with the batteries disconnected at the positive side, so no power to rest of boat (water pump).

 

Will try using a charger to power the fridge.

 

I was also very careful about the cable I used and purchased good quality cable.

 

The quality of DC supply did occur to me, but I reckon that should be fine. I think it'd be an idea for me to fone Shoreline and ask their advice ('tis a Shoreline fridge of course).

 

Still working, so I'm off again for a while...

[Yoda]

Thanx for all your posts whilst I've been working.

 

The quality of DC supply did occur to me, but I reckon that should be fine. I think it'd be an idea for me to fone Shoreline and ask their advice ('tis a Shoreline fridge of course).

 

 

'fridge motors, the same as washing machine motors need a kick to get started. You might find that the capacitor has failed and therefore you are using a large amount of energy to get it started. As I wrote before, motors knackered.

[DHutch]

I presume your battery charger is crude car-charger?

- In which case, i would certainly not surgest trying to use it to power the fridge directly. This is usally not advisable with most chargers.

[John Orentas]

Yes Jim I meant that you should connect it up to charge the batteries (or just one battery) then run the fridge off the battery/s.

 

As Daniel says a battery charger is not clean enough to be used for any other purpose.

[Waterways Jim]

OK.

 

I have a recently bought a charger which says it can be used to power 12v appliances as well, but I will hold fire on using it for that, due to your comments.

I am almost always on the boat with my family, so charging batteries whilst there is difficult, as it is recommended that the positive terminal is disconnected (& cell tops opened), thus no battery power on the boat (main problem apart from fridge is water pump).

 

Would you say that it's OK to charge batteries without disconnecting the positive terminal and thus have battery power on boat whilst charging batteries?

 

Have not yet called Shoreline - life is a bit hectic PLUS my laptop has just given up.

[John Orentas]

OK.

 

I have a recently bought a charger which says it can be used to power 12v appliances as well, but I will hold fire on using it for that, due to your comments.

I am almost always on the boat with my family, so charging batteries whilst there is difficult, as it is recommended that the positive terminal is disconnected (& cell tops opened), thus no battery power on the boat (main problem apart from fridge is water pump).

 

Would you say that it's OK to charge batteries without disconnecting the positive terminal and thus have battery power on boat whilst charging batteries?

 

 

There should be no problem having a charger fitted and running appliances at the same time, the batteries will keep the voltage to a reasonable level. Using a charger for appliances without batteries fitted is another matter, the voltage will (must) float much higher than than the nominal 12 volts.

 

If your charger says different then perhaps it may be OK but I would be inclined to check the output voltage with a meter just the same. Yours must be a very big charger if they reccomend you open the caps, what is the current rating.

[Noah]

OK.

 

I have a recently bought a charger which says it can be used to power 12v appliances as well, but I will hold fire on using it for that, due to your comments.

I am almost always on the boat with my family, so charging batteries whilst there is difficult, as it is recommended that the positive terminal is disconnected (& cell tops opened), thus no battery power on the boat (main problem apart from fridge is water pump).

 

Would you say that it's OK to charge batteries without disconnecting the positive terminal and thus have battery power on boat whilst charging batteries?

 

Have not yet called Shoreline - life is a bit hectic PLUS my laptop has just given up.

I connect my car-type (the Horror!) battery charger directly to the boat's positive and negative battery terminals whilst they are still connected. Always plug in the charger afterwards as potential sparks near the batteries can really cause an exlosion. Make sure the crocodile clips are as secure as possible and cannot be disturbed. If they become disconected they might just make contact with each other (or the hull) and cause a spark. I sometimes charge overnight when I'm on the boat but would be concerned about leaving the boat unnatended with car charger connected, even if the makers describe it as "automatic." My boat, with fridge on, has very low electrical demands so the 8 Amp Gunson's charger can cope well. I cannot justify buying an expensive boat charger as I regularly cruise to charge.

 

Noah

[Waterways Jim]

Yours must be a very big charger if they reccomend you open the caps, what is the current rating.

 

I'm not with the boat/charger/etc at present so not sure John.  I'm sure I've always had that with car battery chargers - maybe not.

 

Thanx for your advice as well, Noah.

 

Another problem is that I have three ah101 batteries, not being sure which is which. I have an isolator switch which has 1, 2, BOTH or OFF positions, but the fridge remains connected in the OFF position.  This will possibly be a mystery to you guys, not being able to see the setup.  The 1st one is for the engine starter (position 1 on isolator) & beyond that I don't know.

[Waterways Jim]

I connected the battery charger and had a CARE FREE weekend at last (apart from engine probs of course)

Thanx again for all advice.

[DHutch]

Good to see you have it sorted, and thanks for posting back to tell us!

[alan_fincher]

I've come late to this post, and can see you have found a satisfactory work-around. So that's good news.

 

But just backtracking to the "why will a fridge work on the 12 volt batteries, but not on a 12 volts DC mains power pack" question.

 

Out of interest, how much cabling, and of what specification, do you have in each case between the fridge and the power source ?

 

12 volt fridges tend to need a very hefty cable, (and of course proportionally more so, depending how far the current needs to travel).

 

I'm just wondering if volts drop along the cable might have been anything to do with original problem. A well charged battery will give out appreciably more than 12 volts DC, whereas some kind of stabilised PSU designed to deliver 12 volts DC might do "exactly what it says on the can".

 

So if you were feeding the fridge through a cable experiencing some voltage drop, you might find there was still close enough to 12 volts when running from a battery of more than 12 volts. But if you used the same cabling, but your PSU were (by design) delivering a lower voltage, (i.e. something pretty close to 12 volts), then maybe was was received at the other end was by then low enough to cause the fridge problems.

 

I have just tested a 12Volt DC/25 Amp stabilised PSU that I used to use for amateur radio, and it indeed delivers pretty damn near 12.0 volts, even open circuit, rather than something close to 13 volts, as you might get from a lead acid battery.

 

So if you are using cabling that may experience volts drop, it might be worth measuring the volts at each end, whilst under load. I'm not sure if you'll be able to see exactly what happens during the short surge of power as it starts up though ?

 

Anyway, this was just a thought - but if you have everything connected together by short lengths of starter motor cable, then please ignore my ramblings.

 

Alan

[Waterways Jim]

Out of interest, how much cabling, and of what specification, do you have in each case between the fridge and the power source ?

12 volt fridges tend to need a very hefty cable, (and of course proportionally more so, depending how far the current needs to travel)

I was careful to purchase heavy enough cable under advice from knowledgable local friends, and then only used a short length of it to test, so it wasn't a voltage drop problem.

A well charged battery will give out appreciably more than 12 volts DC, whereas some kind of stabilised PSU designed to deliver 12 volts DC might do "exactly what it says on the can". So if you were feeding the fridge through a cable experiencing some voltage drop, you might find there was still close enough to 12 volts when running from a battery of more than 12 volts. But if you used the same cabling, but your PSU were (by design) delivering a lower voltage, (i.e. something pretty close to 12 volts), then maybe was was received at the other end was by then low enough to cause the fridge problems.

The PSU stated a 13.5v DC output, so I don't think that's it.

 

I believe I have worked out what's going on here:

The brushes in the fridge motor have worn down, so that when it starts up, it demands a high current surge which the PSUs couldn't deliver, but the batteries can.

This may be wrong, but it feels right to me (after discussing with those knowledgable friends referred to above). If anyone can shed further light or vigorously disagrees with my diagnosis, please feel free...

 

However, I can definitely live with the arrangement I now have.

[DHutch]

To (DC) fridges have brushes?

- Never quite known how they actally work.

 

An AC fridge uses a brushless two-phase motor. With fixed widings, turning a permanet magnet amature.

- The megnets are inside the cooling loop with the pump, while the windings our ouside the cooling loop. Which gets around the need for a gland between the 'motor' (outside) and the pump inside.

- They obtain the 2-phase supply by using a large capaicter to delay the phase.

 

 

Daniel

[Waterways Jim]

Woh - Seriously tekky stuff!

 

I really couldn't comment. My electrical engineer friend suggested that bit about the brushes. My own knowledge of fridges is zero, but I did train as an electronics technician in my youth - City & Guilds & all that. That training was much more electronics than power and we didn't touch phase theory, as that was "power".

However, I did learn about electric motors and all our work was in DC, save learning how to build a rectifier to turn AC into DC. So electric motors can definitely be DC, and as my fridge runs off DC, I assume that any motor in it must run from DC.

It seems that I need to become more informed about my DC fridge!

[Tony Brooks]

Since I have had a problem with my engine (see this thread: http://www.canalworld.net/forums/index.php?showtopic=5645) I now have a problem with the fridge.

 

As the engine is not helping to keep batteries charged, my fridge is running the batteries down to a point where the fridge doesn't work properly.  I decided to get a 12v psu to power the fridge during this time and, as the fridge plugs into a 5amp roundpin socket, wired another 5amp roundpin socket, so that I could just move the plug across, with the other socket powered from the power supply which in turn is plugged into the hook-up circuit whilst moored up.

 

At first I had difficulty finding a 12v psu which could also deliver up to 10amps, so made do with a 5amp one.  This lasted for about a week and a half continuous use before it gave out.  By this time I had sourced a 12v regulated psu delivering up to 20amps and wired that in.  The fridge just woudn't have it and did not start up properly.  I borrowed another 20A 12v psu just to make sure and - same thing.

 

Now, I also have a solar panel which trickle charges the batteries during daylight hours.  This means that, after a while spent away from the boat, I can actually use the boat batteries to power the fridge - for a while anyway, and the fridge starts up with no problem.

 

My question is:  WHY DOES MY FRIDGE WORK WITH THE BATTERIES, BUT NOT WITH A 20A 12V REGULATED POWER SUPPLY?? Bearing in mind that it did work with a 5amp 12v psu for a bit, i.e. until that psu gave out from too much current being required of it.

 

 

 

Point 1

 

These Danfos 12v compressor units (I am told by one of my scientist friends) have a starting device that for a microsecond or three put something close to a dead short across the battery. This is to get the urge required to spin the compressor against fluid pressure. Its all over so fast that a conventional meter will never see it and I think many scopes won't as well. This is no problem for a battery, but can be for cables and certainly will be for a power supply.

 

Point 2

 

If a motor (and some other equipment) does not start properly the current drawn through the stalled motoer is very high. (The goings on with the magnets and the spinning in a running motor limit the current flow, not the motors resistance) so if left stationary and turned on it will burn out.

 

 

Point 3

 

12v Fridge manufacturers do not like facing warantee claims for burnt out motors when it was the battery wot done it, so the engineers fit a voltage sniffing device to the control gear so it refuses to start if the voltage at the terminal is not high enough to push sufficient current through the motor to start it. Their manual says that if the fridge beeps or the red light comes on the voltage at the fridge is too low and this MIGHT be caused by flat batteries.

 

The marketing bods get the manual and tell us that they are real good sorts because if the thing beeps or flashes the red light it means it has turned itself off to prevent your batteries being flattened any more, so users think that all that can give the signal is falt batteries.

 

Point 4

 

If you RTFM it will tell you to wire the fridge in cable of 1mm sq for each meter between battery & starter (this size for both leads), so a 54ft boat with "convetional" layout shoudl use starter cable sized stuff to prevent voltdrop on the cables being large enough to trigger the low volts device when starting. (Voltage "used up" trying to push current through the cable)

 

Unfortunatley the fridge is rated at something like 2.8 amps (they do not mention then starting surge that I think may be well up in three figures) and 2mm sq cable has either 17 or 25 amps written on the drum - so it must be large enough, musn't it? Well it is if you are building down to a price or are a first rate joiner who knows about wood but the rest you have just picked up.

 

So a lot of people have "flat battery" problerms based on things they think the fridge is telling them, when in fact its undersized cables.

 

The rawson the power supply would not work and also failed is because it just could not cope with the very high starting surge so initially sufferd volt drop across itself and the burnt out. (the voltdrop gets dissipated as heat).

 

 

 

 

Tony Brooks