OK to power isolator during cranking? (Dual Batteries)

[tprofeta]

Recently set up my dual batteries - have a deep cycle in the truck bed connected in parallel with the main battery. Both are fused at 200 amps, connected with 4 gauge wire, and using a Pac 200 battery isolator.

I'm powering the isolator through the fuse box by tapping into the fuel pump fuse. This causes the isolator to power up in the ON position as well as through the cranking phase. Is this okay? I keep reading mixed opinions as to whether the isolator should be on or off during cranking (people worried that a depleted house battery would affect the startup of the main battery if connected).

Thoughts?

 

[toymaster]

I'd be worried if the cranking battery was low and the aux battery was charged due to the low amperage fuses. Don't know what car you have but small cars take ~400 amps while a larger truck take ~1000 amps to crank. If that scenario happened you would blow a fuse if not the isolator.

If you have a newer car most turn off aux circuits during cranking, if this is the case just find another suitable circuit to use. If you have an 'older' car I would place a manual switch to flip on to charge the house battery.

 

[bicyclist]

I agree with toymaster.

 

[tprofeta]

Thanks for the feedback. The reason I hooked it up that way originally is because the directions state to connect the isolator to a "12v true ignition power source from the ignition harness that will keep power through the cranking cycle of the starter". I can easily change this to a switched source since I used an add-a-circuit.

 

[DiploStrat]

Bit of an Old Wives' Tale

Never worried about it. I use an intelligent relay. Usually, the camper battery is down at engine start and thus the batteries are not combined. When you start the engine, it pulls down the starter battery for a moment and thus they won't combine until the starter battery is back above the combine voltage - usually set to about 13v.

Even with a key controlled relay, it is not likely to be much of an issue. Read more here: http://www.smartgauge.co.uk/nosurge2.html

 

[Vanaroo]

One side note is that I wonder if some of the huge amperage figures might be inrush current figures. Those don't typically last long enough to affect a fuse. I'm sure the inrush current on my Ford V10 is probably quite a bit higher than 200 amps, but the starter will happily not blow a 200 amp fuse. There is more detail here if anyone is interested:

http://www.pbase.com/mainecruising/battery_fusing

 

[Joe917]

Regardless of the isolator you are over fused or under wired, take your pick.
4 gauge wire should be fused at 135 amps.
If you want 200 amps you need 1 gauge wire.

http://www.powerstream.com/Wire_Size.htm

 

[IdaSHO]

I have mine to pull from both house bank and truck bank if needed via a continuous duty relay and switch on the dash.

Helps that starter spin the big diesel wicked fast even below freezing :sombrero:



Just have to size the wiring and relay accordingly.

 

[tprofeta]

Regardless of the isolator you are over fused or under wired, take your pick.
4 gauge wire should be fused at 135 amps.
If you want 200 amps you need 1 gauge wire.

http://www.powerstream.com/Wire_Size.htm

Ah, thanks for catching that.

Hmm, okay, so assuming I stay with the 4awg that runs from the starting battery to the house battery in the bed of the truck and replace the fuses to 125, is that enough to still jump start a depleted starting battery from a charged house battery?

My starter is rated at 1.4kw (116 amps), so I would think yes?

 

[anickode]

I run my isolator solenoid off the retained accessory power circuit... it cuts out while cranking. My aux wiring is only set up for 100 amps and I didn't want to risk too heavy a draw from the aux battery.