[Dr_Snapid]

If you already have have an isolation relay why not just trigger that when starting? Why are you using a second relay?

[britvit]

The fact Mrs Britvit drives the car is one good reason - she would never remember to flick the switch! If the aux battery in my set up gets connected to the starter it may blow the fuse in the link cable. I could just run one relay to open on start but I would like to have some control in case my main battery goes flat and I want to charge that first.
I have been having a look around, I might give one of these a go - http://www.reuk.co.uk/buy-30A-SCHOTTKY-DIODE.htm
Only .3v drop across the diode.

[Fatzook]

Is this done yet?

[ZookFan]

Yes

[Blakey]

britvit wrote:

The fact Mrs Britvit drives the car is one good reason - she would never remember to flick the switch! If the aux battery in my set up gets connected to the starter it may blow the fuse in the link cable. I could just run one relay to open on start but I would like to have some control in case my main battery goes flat and I want to charge that first.
I have been having a look around, I might give one of these a go - http://www.reuk.co.uk/buy-30A-SCHOTTKY-DIODE.htm
Only .3v drop across the diode.

most diodes unless its a zener diode or LED only have around .3 - .8 forward voltage drop

and your better off putting in a larger diode, as 30A could easily pass between the batteries at some point destroying your diode quite quickly.

and putting in a larger diode wont make it any different to a smaller one, just its able to handle more current

[fordem]

Most diodes would be silicon diodes (you're not going to find germanium diodes rated to handle more than a few miiliamps, if you find any at all), your typical silicon diode will have a voltage drop around 0.6V and that's enough to reduce your charge rate significantly.