TP2 Aux Battery

[davepl]

My truck has the TP2 Aux Battery option and there's a (currently empty) extra tray. I don't see a switch, and I'm wondering if someone could describe the operation and how they're used.

On my boat, which has dual batteries, I installed a relay block so both charge at all times and then you select which to draw from. But since this is a "Custom Camper" I wonder if it only gets charged and then is used for the camper. Or does it also participate in starting?

[ETsC10]

Dave, not sure myself but maybe the gurus in electrical can be of help.

[john]

Heres a pic of the aux batt wiring. Probably the easiest is just use a relay for 2nd batt.

[davepl]

Thanks. Found that page in my assembly manual as well.

From what I can tell, the relay connects the aux battery to the main battery whenever IGN has power. So, while the truck is running the batteries are connected, in parallel, and the aux battery can be recharged by the alternator as well. Is this right?

The part that has me a little confused is that it's only 10 gauge wire. Since is ignition is hot during cranking (which I assume it must be or a normal HEI setup couldn't be used), what if your main battery was low and the aux battery was fully charged?

It seems to me that in this scenario the AUX battery would be connected during cranking and you'd be feeding 500 amps or whatever through the 10 gauge wire over to the main battery and down to the starter.

So I think I must be wrong about one of my assumptions: (1) that IGN is hot during cranking and the AUX battery participates in starting, or (2) that a 10 gauge wire is too small for cranking the starter.

Can someone fill me in?

Thanks!
Dave

[fixit-p]

In the above configuration the aux battery is not intended for starting. The 10 ga wire you refer to is supplied by a 14 ga fusible link from the aux battery positive post which would protect the 10 ga wire from burning up in the event of a short circuit or as your concerned with starter draw. If you want the aux battery for starting it needs to be wired parallel with the battery cables with same size cables, you will loose the isolation of the 2 batteries unless you use an aftermarket isolator that can handle big loads. Or if needed for starting you can simply use jumper cables between the 2 batteries.

[davepl]

That's why I'm confused. In the schematic, look at the 10P (10 gauge pink) wire. When its hot, the relay coil will be energized, connecting the two 10R (10 gauge red) connections together.

One of them goes to the "aux" junction block, the other to the "main" junction block. So as far as I can see, if the "IGN UNFUSED" terminal is hot while cranking, the batteries are indeed connected in parallel during cranking.

Not satisfied with guessing, I used a meter (and then a spare headlight) to confirm that the "IGN UNFUSED" is hot during ON and START, and of course not during OFF.

I then hooked the light up to the AUX junction block, and the light was lit during ON and START as I theorized. So from the wiring schematic and a few tests, the second battery IS engaged during cranking.

I'd like to be wrong on this, because it doesn't make sense. The 14 BRN fusible link should blow in this setup (I don't actually have a low main battery and charged AUX to try blowing it).

I wish someone could tell me where and why I'm wrong, 'cus it doesn't make sense at the moment.

[fixit-p]

You are not wrong, but again that system is not designed for starting. Typically used for camper/motor home applications where the second battery powers the camper and the solenoid isolates it from the main battery preventing it from going dead while allowing it to charge with the engine running. You can always wire it so the relay drops out while cranking by connecting it to the aux side of the ignition switch.