running 12v fridge while travelling

Submitted: Monday, Oct 21, 2013 at 20:35
ThreadID: 104815 Views:9113 Replies:4 FollowUps:7
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can i run an electrical lead straight from my alternator to 12v fridge in caravan.or should i run it to my 100w agm battery then to the fridge thru anderson plugs.i was hoping the former was possible then i wouldn't have to worry about running battery down if i forget to unplug.and what size fuse would i need.can anyone please help me.
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Reply By: Ross M - Monday, Oct 21, 2013 at 20:57

Monday, Oct 21, 2013 at 20:57
ricky vw
There are many ways you can connect a fridge. If you are worried about flattening the AGM because you forget, then if it is connected to the alternator it will flatten the start battery quicker and then NO START.
Some sort of relay/solenoid which is voltage controlled is a sound idea. Called VSR's Voltage Sensitive Relay, which pull in after the start batt has had a bit of charge and it disconnects the two if the voltage gets down to a set level and therefore protecting your start battery.

You don't say what type of fridge it is: 3 way power hungry item or compressor fridge which us about 1/4 of the 3way fridges.
Your needs and wiring cables and any auto safety items will depend on the type of fridge to a large extent.

Fuses? They need to protect the wiring if a short happens to the negative. ie, both ends of the charge cable for safety.
A bit more info and others will have solutions/suggestions.

Get ready for the avalanche.
AnswerID: 520135

Follow Up By: ricky vw - Monday, Oct 21, 2013 at 21:11

Monday, Oct 21, 2013 at 21:11
thanks for that Ross.its a power hungry one but i just need it for traveling just to keep it reasonably cool then on to gas or 240v.i'm not very experienced and are just new to this traveling biz.
FollowupID: 800541

Reply By: steved58 - Monday, Oct 21, 2013 at 22:06

Monday, Oct 21, 2013 at 22:06
yes you can take a 12 volt feed to your fridge in the caravan but from the start battery not straight from the alternator you will need 3b&s cable to avoid voltage drop and inefficient cooling and a 50 amp fuse from the battery Then to prevent running the battery flat a fridge switch can be fitted when the caravan stops the fridge switches off after a small delay to prevent switching off at every stop It then starts again when the vehicle is moving again This is the simplist way to get a 12 volt fridge running in the caravan for travelling only then on 240 volt or gas when stopped There are many other ways eg solenoids dc to dc chargers some better some not But this is probably the cheapest and easiest also use Anderson plugs to connect to the caravan fridge not the standard trailer plug as it wont take the current draw causing inefficient cooling and an overheated plug
click on linkcaravan fridge cut off switch
this is s link to one supplier for the fridge switch
good luck
AnswerID: 520143

Follow Up By: snapper49 - Monday, Oct 21, 2013 at 22:21

Monday, Oct 21, 2013 at 22:21
Only way to do it
Well written
FollowupID: 800546

Follow Up By: steved58 - Monday, Oct 21, 2013 at 22:27

Monday, Oct 21, 2013 at 22:27
Sorry above I said a 50 amp fuse at the battery for this setup a 20 amp fuse is probably ok I was thinking dual batteries the instructions with the fridge switch will recommend a fuse size the 3 b&s cable also was for dual battery setups but the heavier the cable the better the fridge will run ie less voltage drop I am presuming you 100 amp agm is in the caravan if so take the feed from the car start battery direct to the fridge it will be more efficient this way and don't forget to use Anderson plugs to connect caravan fridge to car start battery
once again good luck
FollowupID: 800548

Follow Up By: Geoff in SA - Tuesday, Oct 22, 2013 at 07:24

Tuesday, Oct 22, 2013 at 07:24
As Steve said
well written.

Heavier cable will be better anyways. Volt drop in vehicles is a real problem. People forget its the current draw that dictates the cable size.

And remember to use a good quality crimp lug and tool for all connections as well.
The fridge switch is a good item.
I have one installed as an alarm system on my van.
Move the van and the alarm goes off.
It has a secret switch to turn on and off when we are in it or towing.

FollowupID: 800565

Follow Up By: Ross M - Tuesday, Oct 22, 2013 at 10:12

Tuesday, Oct 22, 2013 at 10:12
Steve 58
Why not the back of the alternator????

That is the point where the highest voltage is, before any VD is experienced and so gives the maximum ability to supply a heater type fridge with the highest level of energy which is what it needs to create KOOL.
Every 0.1 of a volt drop will cause less at the fridge, so if connected to the battery main terminals it will be less.
The negative line can also be direct to the alternator case for best results.

Driving light connections made this way also work far better than when connected to the battery. Every little bit helps with them too.
The alternator is the SOURCE, the battery is not.
FollowupID: 800578

Follow Up By: steved58 - Tuesday, Oct 22, 2013 at 16:07

Tuesday, Oct 22, 2013 at 16:07
Your right however I find it easier to go straight from the battery its usually much more acessable and works really well if I had voltage drop problems I would look at going direct from the alternater
Cheers steve
FollowupID: 800615

Follow Up By: Member - LeighW - Wednesday, Oct 23, 2013 at 12:14

Wednesday, Oct 23, 2013 at 12:14
It is not wise to run directly from the alternator, most alternators these days use remote voltage sensing, they sense their output voltage from either the battery terminal or the main fuse box.

In some vehicles there can be a fair length of cable between the alternator and the reference point, this can lead to a significant voltage drop between the alternator and the its reference point ie in some vehicles with the battery terminal voltage at 14.2V it not uncommon to see a voltage of 15.5V or higher at the alternator output terminal.

The 12V elements in the fridges are temperamental at the best of times and running one on 15+ volts is not a good idea.


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FollowupID: 800658

Reply By: Member - John and Val - Tuesday, Oct 22, 2013 at 07:54

Tuesday, Oct 22, 2013 at 07:54

There's good advice above.

I would add one suggestion. Provide the fridge with its own wiring - both positive and negative - from the engine bay to the fridge. (And include a fridge switch in that line as suggested above.)

Your fridge draws about 12-15 amps continuously when on 12 volts and consequently there will be significant voltage drop in the wiring. If you were charging the auxilliary battery through the same line, this voltage drop would seriously reduce the charging available to your aux battery.

I assume that you already have a voltage sensitive relay and fuse under the bonnet. I'd connect the fridge wiring after that relay and fuse, where the present aux battery wiring takes off. Note that it is important to provide some means of switching the fridge circuit off when stationary, as otherwise the fridge will be permanently connected to the aux battery - a fridge switch would be ideal, or a relay (headlight relay would be good) activated by the ignition switch.


J and V
"Not everything that can be counted counts, and not everything that counts can be counted."
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AnswerID: 520158

Reply By: ricky vw - Tuesday, Oct 22, 2013 at 19:46

Tuesday, Oct 22, 2013 at 19:46
thanks everyone for your is just what i wanted and all understandable.sometimes it can get confusing (threads).can't wait to hit the road in feb.Ricky.
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