Inline Battery Fuse Amperage

[dtaylor7897]

I'm going to wire up a second battery this weekend. I have a 680 CCA main battery and a 775 CCA deep cycle AGM auxiliary battery. I have a 105 amp alternator and a 200 amp Stinger battery isolator. I've been looking at how I should wire the second battery with the isolator and I've seen a couple people wire an inline fuse between each battery and the isolator. But how many amps should the fuses be so that they're effective?

If it matters, my plan is to be able to run my radio (1000w amp), lights, phone charger, etc. when not running. 79 K20 Suburban 350.

Thanks

[bd]

May I ask why you settled on relay isolation in lieu of diode isolation?  Each method has unique advantages, depending on the subtleties of what you wish to accomplish.  In some cases it is advantageous to incorporate both.

Where do you intend to connect the alternator output lead and accessory load into the circuit?

[dtaylor7897]

Money. I picked up my isolator for $40. Much less expensive than many other options but still had good reviews and free shipping. I'm planning to connect the alternator output lead as well as the starter to the main battery, and all accessories (radio, lights, etc.) to the auxiliary battery.

[bd]

Just to be clear, you are not going to use the isolation relay as a tie-in point for any accessory load - only the interconnecting battery cables will be connected to the relay.  So, the isolation relay exists strictly to connect the D/C AGM for charging, meaning that you anticipate the maximum current flow between batteries will be charging current.  The D/C battery will never be used for cranking.  Is this correct?

[dtaylor7897]

That's correct.

[bd]

Energize the isolation relay through a 20-amp fuse via circuit 300 (the 12-gauge orange wire from the ignition switch).  Circuit 300 powers the 20-amp CHOKE fuse.  Circuit 300 supplies "ignition power" only in the RUN position, so the batteries will be isolated during CRANK and in the ACCESSORY position.  Configure the battery interconnecting cables for 150 amps (conservatively, 4-gauge copper cable).  Then, instead of using redundant fast-blow fuses, substitute 4" long, 8-gauge fusible links inserted at the cable connections to both batteries.  This configuration should fulfill all of your requirements and provide suitable electrical protection.  Of course, all attached accessories will need to be independently and appropriately fused.

[bd]

OEM 1979 Wiring Manual

[75gmck25]

I have the OEM dual  battery option in my '75 camper special, and it uses a relatively simple relay setup to isolate the battery.  It is designed to charge the 2nd battery while the truck is running, but allow you to run the camper while parked without drawing from the main battery.  You would have to upsize all the charge wiring to allow for your 105 amp alternator.

- 2nd battery is installed on driver's side.
- small battery relay is installed on fender well on same side (must be rated at about 60 amps, since OEM alternator was 63 amp)
- 10 gauge charge wire for 2nd battery runs from 2nd battery positive to the relay, and then over to the junction on the firewall behind the brake booster (which is already connected to the main battery and alternator).  I assume the original scheme may have had a fusible link in this wire, but my wiring ends at the relay (2nd battery and charge wire is long gone).
- activation wire for the relay comes out of the wiring near the fusebox (brown wire?) and it has power when the ignition is on.
- there is an additional power junction on the fender well near the 2nd battery, and it is is connected to the 2nd battery positive.  This serves as the junction/power point for the power running back to the camper.

When the ignition is on, the relay is powered and the 2nd battery is now connected to the main junction on the firewall using the 10 gauge wire.  This allows the 2nd battery to charge from the alternator (although it a fairly long path).

When the truck is turned off, the relay disconnects the camper power junction from the rest of the truck so that running the camper while parked will only draw on the 2nd battery.

Bruce