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12V Switching Ideas

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2ezy

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Hi everyone, I'm a newbie and this is my first post, so please forgive if I stumble.
I'm trying to devise a circuit to do the following:-
I have a 12V (only) portable fridge/freezer mounted in my tow vehicle. It can be run from either the car battery when the engine is running or from an auxiliary 120AH deep cycle battery when parked. The deep cycle battery is solar charged from a 130W panel mounted on the roof of the tow vehicle. I've set it up this way because we often camp for many days away from power so we can't depend on a traditional "dual battery management" system to keep the auxiliary battery topped up while driving.
At present I'm using a simple SPDT relay to affect the switching, driven by whether the ignition is on or off. All well and good except - when the ignition is turned on the relay is energised and switchover is 'instant'. However, when the ignition is switched off, there is a momentary pause before the auxiliary switches in. This causes the fridge compressor to stop and then a second later, restart. This isn't healthy for fridge compressors. I need a simple seamless switch from one source to the other. Can I 'buffer' the existing switchover? I've heard about diode option switching but that is beyond me. I have a basic understanding of electronic components and can use a multimeter and soldering iron. Can anyone help?

Mark
 
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