Monday, Apr 01, 2013 at 14:28
Hi Graeme,
Three way fridges have the big advantage of being able to run from gas. They are actually heat powered. You can’t run from gas while on the move though, as for safety (and legal) reasons the gas should be turned off while travelling. In any case the flame would almost certainly be blown out if you tried to use gas when moving. To keep the fridge running while travelling, it has an electric element to provide the heat. These fridges aren’t nearly as efficient as compressor fridges such as Engel and Waeco, whose motors cycle on and off, and averaged over an hour or so will typically draw 1 or 2 amps. A three way fridge running on 12V will run continuously and draw about 12 amps!
This raises problems. Not only is this burning energy very fast, but there are resistive losses that reduce the voltage available at the “consumer” end of the supply cable. These losses are directly proportional to the current flow (amps).
If you were to use a single supply cable to the trailer to feed both the fridge and the trailer’s battery, two problems arise. 1) Under these conditions the fridge and the battery are electrically connected to that single supply, and inevitably they are therefore tied to each other. Consequently, when you stop and turn the engine off, the trailer battery will continue to power the fridge – you must find a way to automatically disconnect them once the engine is stopped. 2) Even using very heavy wiring there will be voltage loss in the wiring resulting from the high fridge current. Battery charging is very voltage sensitive. It isn’t easy to fully charge your trailer battery using the full voltage available from the alternator, but with a fridge hanging on the same line, there will not be enough voltage available to provide proper charging to the battery.
So, to stop making words (!) and address your question about adopting Bill’s suggestions:
The 3 way fridge is the problem. The aim is to use the alternator to power it while the engine is running. The alternator will be feeding the battery/s at this time too and consequently they are connected to the fridge. The batteries should never be called on to power the fridge though, and you need some way of automatically disconnecting them from each other when the engine stops. This switching is most easily done in the engine bay.
“can I assume that the fridge will draw equally from both batteries and both batteries will still take some charging from the tug?” Yes and no (does that help?!) With the engine running, the fridge and both batteries are fed by the alternator. (The alternator voltage is a bit higher than the battery voltage, and current (amps) will always flow from a higher voltage to a lower one.) If you were to use Bill’s system AND connect the 3 way fridge to the same line, there would be a small voltage loss in the wiring from alternator to the Thumper, then a further loss between the Thumper and the trailer. Without the 3way fridge, and assuming heavy cables, these losses would be negligible, both batteries would receive some charge (though probably not equally.) This arrangement would also provide extra power to run the Waeco when stationary – the alternator will be powering it when the engine is running.
I 100% agree with Bill’s final paragraph – a dc-dc charger in the trailer is very desirable to ensure good charging. The D250S dual provides the switching, manages charging, and also provides an MPPT solar controller – that’s all the good stuff in one box!
One last thing – you may find
Electricity for Camping a useful read.
Cheers
John.
| J and V
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