Adding a fridge to an existing 12v system

[Bsmiffffy]

Hi I currently have a 100w solar panel that feeds into a pwm controller then into 2 12v 110Ah batteries connected in parallel.
This feeds into a caravan 12v system and works fine but I now need to add a 90w fridge to the system. It will run on gas during the winter and 12v during the summer.

I am replacing the 100w panel with a 400w panel and replacing the controller with an MPPT controller feeding into the same 12v batteries.
The caravan live feed comes from battery 2 and the negative from battery 1 so one 220Ah 12v battery.

My question is how do I stop the fridge from draining both batteries. I need to charge both from the solar but I only want the fridge to use one of the 2 batteries and leave the other for lights etc.

Thank you in anticipation.

 

[johnb2713]

A couple of possibilities spring to mind:-
1) DC to DC charger for the caravan battery, solar charges the fridge battery and the DC to DC charger takes charge from the fridge / solar feed and charges the caravan battery.
2) Parallel the batteries and set a cut off voltage for the fridge at around the 75% mark.
3) Get an MPPT controller designed for two batteries (readily available and not expensive).

 

[Bsmiffffy]

A couple of possibilities spring to mind:-
1) DC to DC charger for the caravan battery, solar charges the fridge battery and the DC to DC charger takes charge from the fridge / solar feed and charges the caravan battery.
2) Parallel the batteries and set a cut off voltage for the fridge at around the 75% mark.
3) Get an MPPT controller designed for two batteries (readily available and not expensive).

Thank you johnb2713, how would I set a cut off at 75%

 

[Geoff1946]

You haven't said what sort of fridge, but a warning just in case:
If it's a typical Thetford 3way, (230volt/12volt/gas), absorption type then if I understand correctly the 12 volt mode is only intended to keep an already chilled fridge cool, whilst travelling, not offer a fully functional alternative to mains or gas operation.

 

[johnb2713]

You haven't said what sort of fridge, but a warning just in case:
If it's a typical Thetford 3way, (230volt/12volt/gas), absorption type then if I understand correctly the 12 volt mode is only intended to keep an already chilled fridge cool, whilst travelling, not offer a fully functional alternative to mains or gas operation.

I had a large Thetford 3 way fridge in my caravan. They are capable of running very successfully from 12v IF they get a good 12v supply. I towed with a LWB Sprinter, I wired from front to back with 25mm2 cable, the tow socket on the van had 4mm2 cable for the fridge supply. In the caravan again I wired from the plug to an under seat locker in 4mm2 and in the locker installed a DC to DC converter. The converter was adjusted to supply 13.8v at the fridge and the result was the separate freezer section would freeze down to -15 degC whilst on 12v supply. It was a lot of work but I wanted to prove a point and it worked no problem.

In terms of the OP here, if he can get a decent supply to the fridge, it should work dependent of course on how good the solar is.

 

[johnb2713]

Thank you johnb2713, how would I set a cut off at 75%

Voltage sensing relay that will open at a preset voltage which you adjust to suit.

 

[johnb2713]

Voltage sensing relay that will open at a preset voltage which you adjust to suit.

Set to open at 12.25v

 

[Geoff1946]

I had a large Thetford 3 way fridge in my caravan. They are capable of running very successfully from 12v IF they get a good 12v supply. I towed with a LWB Sprinter, I wired from front to back with 25mm2 cable, the tow socket on the van had 4mm2 cable for the fridge supply. In the caravan again I wired from the plug to an under seat locker in 4mm2 and in the locker installed a DC to DC converter. The converter was adjusted to supply 13.8v at the fridge and the result was the separate freezer section would freeze down to -15 degC whilst on 12v supply. It was a lot of work but I wanted to prove a point and it worked no problem.

In terms of the OP here, if he can get a decent supply to the fridge, it should work dependent of course on how good the solar is.

That's interesting. Perhaps the limitation I've read of is assuming the volt drop which "standard" wiring causes. I also note your idea of a DC_DC boost convertor. I had wondered about doing that to give the van battery a better charge whilst towing. With my routines I'm not really bothered about the fridge but I do use a motor mover at both ends of my journeys.
My other thought is that with your fridge set up there is no active thermostat.

 

[johnb2713]

That's interesting. Perhaps the limitation I've read of is assuming the volt drop which "standard" wiring causes. I also note your idea of a DC_DC boost convertor. I had wondered about doing that to give the van battery a better charge whilst towing. With my routines I'm not really bothered about the fridge but I do use a motor mover at both ends of my journeys.
My other thought is that with your fridge set up there is no active thermostat.

My last tug was a 4.4 TDV8 Range Rover and I installed a DC to DC converter for battery charging and it worked really well, I got a good 25 amps charging current when the battery was low, you do need to make sure the 7 or 13 pin plug / socket are of good quality though.