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IamBrowny

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Hello All,
My Daughter is an apprentice carpenter and we have fit her van out with insulation, covered the walls with painted MDF and made some storage boxes, that she can put a mattress on for camping.
We have run 240v LED string lights in the roof panel and now want to power those lights and also set up a dual battery system to run a fridge and lights while camping.
Has anyone set up a dual battery system before and do you have a how to guide for that?
Can I power the string lights from the car power, or do I need to run these of an inverter attached to the dual battery system?

Many thanks for any help I get.
Cheers,
Browny
 
There are many ways of configuring a second battery but the simplest is with a voltage-sensing relay that connects the domestic battery to the vehicle main positive to charge it, whenever the voltage reaching the relay is high enough to indicate that the engine alternator is running. When the voltage drops to indicate that the alternator is not charging, the relay opens, preventing domestic loads discharging the vehicle battery.

This relay might suit: Durite voltage sensing split-charge relay and the site linked to has other goodies that might help you.

The main circuit (veh battery to relay in, relay out to dom battery, and dom battery -ve to chassis) need to be heavy cables with solidly made connections (10mm² and up is good). The domestic battery should be a deep-cycle 'leisure' type. Outgoing circuits must be fused.

What kind of fridge do you want to power? Remember that most gas / electric camping fridges and thermoelectric coolers are only designed to run on 12V when the engine is running as they are very heavy constant loads that will soon discharge even quite a large battery. 12V compressor fridges are expensive! 230V compressor fridges will run on an inverter, but the inverter needs to be very generously sized.

Are the lights natively 12V with a 230V power supply? If so, you can ditch the power supply and run directly off the battery, avoiding the losses in the inverter. Note that any 230V wiring installed in the vehicle should conform to BS7671 and the appropriate configuration of earthed neutral made at the inverter if the protection scheme requires.
 
230V compressor fridges will run on an inverter, but the inverter needs to be very generously sized.

even so, if powering the inverter from battery, it will soon drain battery. 2A @ 240V = 40 A @ 12V.
(+ inverter losses ).
 
It's not quite that bad these days, they're more like 100W which, running for perhaps 25% of the time, equates to 2-3A continuous load. Conventional '3-way' caravan fridges with ammonia absorption coolers, designed to run on gas when stationary but 12V on the move, also typically take 100W but do so constantly (the 12V element is often not thermostatically controlled) so is actually a much heavier load.

I recently replaced the 12V compressor fridge on the boat. It's a purpose-made marine 98-litre under-counter type that looks like a standard domestic fridge but uses a 12V VSD compressor with adaptive speed control. The previous unit was R134a and turned in impressive economy, taking 12-15Ah/day in average weather or 2A constant when chilling drinks from ambient. The present one is R600a which should be as efficient or more so, but without having run a logger on it I'm not yet convinced. The freezer is just a basic 230V 40-litre domestic one, but I've taken to letting that run on the inverter when away from dock as it really doesn't seem to use all that much juice either. The compressor only starts once in a blue moon but it sits at -22°C perfectly well. We're careful not to open it more than necessary.
 

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