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12V Switching Ideas

I have a 12V 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 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 automatically 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 relay de-energises and trips to the auxiliary battery. This causes the fridge compressor to stop and then a second later, restart. Most unhealthy for fridge compressors. I need a simple seamless switch from one source to the other. Can I 'buffer' the existing switchover? Or is there a better way to achieve what I need? Can anyone help?

Cheers Mark

Reply to
2EZY
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What I have done is to make a circuit up that provides a ground for the switching relay once the oil pressure light has gone out, so that power is held off until the engine is running. This will also stop the deep cycle being connected to the main battery. This same circuit is also used for the grounds of relays for other stuff (air comp etc.) that need the engine running for use.

Cheers ............ Rheilly P

Reply to
Rheilly Phoull

Thanks Rheilly. I was thinking about a blocking diode to protect against my deep cycle battery ever being called on to provide starting current. Thanks for your suggestion, but I'm trying to make the switchover faster not slower by having to wait for the oil pressure light to go out.

Mark

Reply to
2EZY

I suspect you have a wire to the the normally open relay contact from the ignition (or accessory) circuit. run that vire instead from engine battery (via a suitable fuse) or from the wire that supplies the horn and dome light circuits...

the turn off delay is not long enough to drain your battery.

Reply to
Jasen Betts

I would be lazy and just put a diode from the battery side of the relay switch to the compressor side. When the switch is set to battery the diode is shorted and does nothing. When the switch is on the car side the diode will be slightly reversed and won't conduct (because you will tie the returns) When the switch is switching the diode will provide enough power for the compressor.

Reply to
MisterE

Run it from the aux battery all the time, then have a voltage sensative relay (VSR) charging it from the vehicle. This is the standard way of doing it, and it ensures that your starting battery always gets charged first.

Reply to
The Real Andy

the way you have the system wired is wrong what brand and model solar reg you using?

Reply to
Paul Mannis

Paul it is a Morningstar Sunsaver 10. It is wired according to the manual that came with it, at least I reckon that it is. A forum member, Mike, has been good enough to work with me in setting up a simple "Diode Auction" circuit using a pair of Schottky diodes. Basically running the fridge permanently from the auxiliary battery and switching the charging source, whichever is available and putting out the higher voltage. I've finished the construction and will test this afternoon or tomorrow. Thank you for your suggestions. Mark

Reply to
2EZY

Not the best idea. VSR is better option.

Paul it is a Morningstar Sunsaver 10. It is wired according to the manual that came with it, at least I reckon that it is. A forum member, Mike, has been good enough to work with me in setting up a simple "Diode Auction" circuit using a pair of Schottky diodes. Basically running the fridge permanently from the auxiliary battery and switching the charging source, whichever is available and putting out the higher voltage. I've finished the construction and will test this afternoon or tomorrow. Thank you for your suggestions. Mark

Reply to
Paul Mannis

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