@Datatek re the continued charging, that's why I'm thinking if the controller only charges over around 13v input it would isolate the 2 batteries and stop the second battery pulling power until the engine is running and the alternator is up to speed. Otherwise like you say an isolation relay.

Any particular reason for you recommending that controller only I have 2 of the ones I posted as I like the voltage display?
I have not checked but I think that the controller relies on the potential difference between the source (solar panel or car) and the battery to be charged. So if the battery attached to the controller is down at 12.2 volts for example and the car battery at switch off is 12.8 or more, it may continue to charge. Easy enough to check.
The one I use is cheap and simple with just LED's to indicate what is going on. Displays may not like extreme cold. I have seven of them, so price was important.
 
Which would use more power.

Agreed but if it's running off a 900amp battery it should be neglidgable.

The spec from the one I have looks like trigger voltages are adjustable

12V voltage system (if it is a 24V voltage system, data x2):
Float charge: 13.7V (default, adjustable)
Discharge stop:10.7V (default, adjustable)
Discharge reconnect:12.6V (default, adjustable)
 
Agreed but if it's running off a 900amp battery it should be neglidgable.

The spec from the one I have looks like trigger voltages are adjustable

12V voltage system (if it is a 24V voltage system, data x2):
Float charge: 13.7V (default, adjustable)
Discharge stop:10.7V (default, adjustable)
Discharge reconnect:12.6V (default, adjustable)
That sounds good for your application.
Discharge stop:10.7V (default, adjustable) I would set that to 12 volts if you do not want to knacker the battery PDQ
 
That sounds good for your application.
Discharge stop:10.7V (default, adjustable) I would set that to 12 volts if you do not want to knacker the battery PDQ

Sound like a plan, need to rig it up to test what happens before the install.
 
Any idea if 16awg will do 20amp please? looking on web says 15amp but ready made cables come with a 15 and 20 amp fuse.

Cheers
 
Took longer than expected but dual battery conversion now complete'ish

Battery 2.jpg


Battery 1.jpg



3D printed a new housing so I could move the light and use the existing cut out.


Batt panel 1.jpg


Batt panel 2.jpg
 

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