Auxiliary Battery (tp2)

djv1951 · September 21, 2008

I have a 2004 2500HD Silverado with 41,000 miles on it. I have the Auxiliary battery option (TP2) on it. Can you explain to me how it works? I've read several posts in the HD fourm and there seems to be some confusion how the system works. I have even asked 3 GM tecs at 3 different GM dealerships and all three differ. Does the starting system use both bateries to start the engine?, do both battries charge at the same time?, If I replace the main battery should I replace the aux. at the same time? Thanks, Dave

merlin5577 · September 22, 2008

I have a 2004 2500HD Silverado with 41,000 miles on it. I have the Auxiliary battery option (TP2) on it. Can you explain to me how it works? I've read several posts in the HD fourm and there seems to be some confusion how the system works. I have even asked 3 GM tecs at 3 different GM dealerships and all three differ. Does the starting system use both bateries to start the engine?, do both battries charge at the same time?, If I replace the main battery should I replace the aux. at the same time? Thanks, Dave

It works good! Mike is right though. Always replace them in pairs.

djv1951 · September 22, 2008

I have a 2004 2500HD Silverado with 41,000 miles on it. I have the Auxiliary battery option (TP2) on it. Can you explain to me how it works? I've read several posts in the HD fourm and there seems to be some confusion how the system works. I have even asked 3 GM tecs at 3 different GM dealerships and all three differ. Does the starting system use both bateries to start the engine?, do both battries charge at the same time?, If I replace the main battery should I replace the aux. at the same time? Thanks, Dave

It works good! Mike is right though. Always replace them in pairs.

Thanks, that helps alot and is what one of the GM tec told me. The vin # is 1GCHK29G74E385140

5Alive · October 29, 2008

SparksTech...

You said:

The auxillary battery is active anytime the ignition switch is in the RUN position with TP2, it is not involved in the circuit when the switch is in the CRANK position.

So you are saying that the TP2 auxilliary relay coil, which is normally open, closes (becomes energized) when the key is turned to RUN, and then opens again (becomes de-energized) when the key continues to turn to START, and then closes again when the key is relaxed back to RUN ?

So and this relay jitterbug happens every start cycle? Where the relay goes from Open in ACC to Closed in RUN to Open in START to Closed in RUN?

5Alive · October 29, 2008

In doing a search to try and find out more about this, I also ran across this post from BBSilverado (whatever happened to him?)

With the isolated dual battery setup (TP2) every time a heavy load is applied to the main battery the isolation relay will open the connection if the main voltage falls below 11.0 volts, thats is why your aux. battery will not start the truck if the main battery is dead unless you use jumpers cables and jump your main from your aux. battery.

Is this true?

minibike132 · November 1, 2008

SparksTech...

You said:

The auxillary battery is active anytime the ignition switch is in the RUN position with TP2, it is not involved in the circuit when the switch is in the CRANK position.

So you are saying that the TP2 auxilliary relay coil, which is normally open, closes (becomes energized) when the key is turned to RUN, and then opens again (becomes de-energized) when the key continues to turn to START, and then closes again when the key is relaxed back to RUN ?

So and this relay jitterbug happens every start cycle? Where the relay goes from Open in ACC to Closed in RUN to Open in START to Closed in RUN?

According to the diagram the only time the relay is energized is in the run position so yes your "jitterbug" cycle is correct. It isn't a problem for the relay, many relays are on off on off at a higher rate of frequency.

minibike132 · November 1, 2008

In doing a search to try and find out more about this, I also ran across this post from BBSilverado (whatever happened to him?)

With the isolated dual battery setup (TP2) every time a heavy load is applied to the main battery the isolation relay will open the connection if the main voltage falls below 11.0 volts, thats is why your aux. battery will not start the truck if the main battery is dead unless you use jumpers cables and jump your main from your aux. battery.

Is this true?

There is nothing in the diagram of this vehicle to indicate that. It isn't all that smart. The key gets turned to run and the 4wd fuse is given power which delivers power to the control side of the relay. If power is taken away from that fuse, which happens in crank or off or accessory, then the control side of the relay is off. That is all of the brains to the circuit on this truck.

minibike132 · September 22, 2008

There can be alot of confusion because there are a few different ways to wire dual batteries up. There are also options on individual vehicles after the batteries that make a difference. Without knowing exactly what you have (TP2 isn't the only code that makes a difference in how it all works so a vin would help) I can only give some basic answers to your questions.

Does the starting system use both bateries to start the engine?-------No not on your vehicle
do both battries charge at the same time?-------------Yes on your vehicle
If I replace the main battery should I replace the aux. at the same time?------Yes, always in my opinion

Ok well I can do better than basic but this may not be the whole story. The auxillary battery is active anytime the ignition switch is in the run position with TP2, it is not involved in the circuit when the switch is in the crank position. Charging with the alternator only takes place with the vehicle running and switch in the run position so it is going to be charging at the same time as battery 1. That doesn't mean it's charging at the same rate, that depends on battery condition. If one is not as healthy as the other than charge rates are going to vary side to side. That brings us to the last question about replacement, there is debate here, but my suggestion is to maintain system integrity I always replace them at the same time with the same type of battery and I try to make sure the manufacturing date is close on both.