TheSamba.com :: View topic - Question about Blue Sea auto charging relay & battery selector

Thu Apr 19, 2018 1:34 pm

I am installing the Blue Sea auto charging relay & auto battery selector switch and wonder if I have to connect the main 1 AWG cable coming from the alternator to the battery to the Blue Sea switch or leave it connected to the battery and only connect the red cable going to the dashboard breaker box to the switch and than back to the dashboard.When doing so, I would not have a 100% battery disconnect.

Fri Apr 20, 2018 12:07 am

I don't know which disconnect switch you have - they make several styles. You don't want to run the alternator without a battery connected to it, ever. It is especially bad to disconnect while running, this can cause a voltage spike that will destroy the regulator. Some battery disconnect switches (like some Perko) have a "Field" switch built into them to remove power to the alternator field winding should you switch "off". Kind of hard to implement this with a stock VW alternator. These switches were originally sold for marine applications where you wanted to kill power to the whole boat when you left it, the "Field" switch was in case you got careless while the engine was running.

I would run the alternator output to the main battery, then two wires from the positive terminal of the battery: one to the battery isolator/combiner "#1 BATT terminal, the other would be the DC to the vehicle: Starter cable and #8 red wire to the dash fuse block. Aux battery side of the isolator do what you want based on the current draw. Since alternator can put out 45 amps (give or take) I'd use at minimum #8 wire from isolator to aux battery. If you are planning on using aux batt as backup start batt through combiner switch, use at least #4.

Do you want to put a disconnect switch in main batt circuit? Your choice. For theft prevention, it's an idea. If you can figure out a good physical layout that works, and make sure and use heavy enough cable, protected against chafe, go ahead. Remember there is no fuse in these battery lines (unless you install one) so if the positive cable insulation gets chafed through you're going to have a real hot spot somewhere. I'd go with #2 or larger if you're carrying starter current through it any distance. If just dash power through the disconnect (starter remaining connected to batt directly), the original wire to the fuse block is #8, don't go smaller but if adding much length you might want to go larger.

Use finely stranded automotive or marine-grade wire, not stuff off of a roll from Home Depot.

FWIW, in my Westy, I just put a main disconnect switch in the AUX batt line, figure the stock hookup worked OK for the vehicle itself but it was likely I might leave something on in the AUX side of things and run the battery down.

Fri Apr 20, 2018 5:56 am

If you put a battery cutoff switch on either the main or aux batteries do it across the negative cabling, basically ground strap to cutoff then another ground strap from the cutoff to the chassis. That way if there is any problem with the cable touching 'ground' there is no smoke let out of the system.

Fri Apr 20, 2018 8:30 am

Here is what the manufacture suggest but I was hoping that I do not need use 2AWG to the switch.

Fri Apr 20, 2018 8:42 am

Thats more like the way I want to do it. :

and the rest as Dampcamper described....

Fri Apr 20, 2018 8:45 am

That cutoff switch is carrying the entire electrical load thru it. You need to use a large enough 'wire' to handle that load, especially when starting the engine.