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2008 2500HD Dual Battery Question


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#1 robertbick

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Posted 27 December 2011 - 11:51 AM

Hello all. I have an 08 2500 with the factory dual battery option. I have noticed that the driver side battery is always at or above 12.8V (100%) but the passenger side battery is usually 12.1-12.3V (50-70%). This voltage is checked after the truck has been sitting for a few days and up to a week.

My truck is not used much and sits in the garage up to a week or two between uses. Could it be because any accessories like the clock that draw power when the truck is off only draw from that one battery? If that is the case, do you think I should switch the batteries around so now the other one will get more of a workout?
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#2 dewfpo

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Posted 27 December 2011 - 02:59 PM

I have the TP2 Dual battery option on my '05 and the primary battery is only used to start the engine andr maintain the clock and ECM memory etc.. The second battery is only connected after the engine is running and that is so it can be charged by the alternator along with the main battery. Both batteries are the same size and capacity (from the factory) so you can swap them around if you want. I still have the original two Delco's in my '05 built in '04 but I do put a maintainer on them once or twice a year to desulphate them.

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#3 Diesail

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Posted 27 December 2011 - 05:23 PM

^^X2, my understand is the second battery does nothing unless you are using it for 12v feed to a trailer. So your primary battery is doing all the work and thus is getting weak. If you don't have a trailer that you are feeding with it then swap them.

#4 robertbick

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Posted 28 December 2011 - 08:56 AM

Which one is the primary? The one in the very front drivers side or the one towards the rear on the passenger side?
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#5 Diesail

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Posted 28 December 2011 - 09:33 AM

Which one is the primary? The one in the very front drivers side or the one towards the rear on the passenger side?


Primary battery is the one on the passenger side.

#6 robertbick

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Posted 28 December 2011 - 10:56 AM

Which one is the primary? The one in the very front drivers side or the one towards the rear on the passenger side?

Primary battery is the one on the passenger side.


Thanks. I also have a snow plow I had installed at the dealer when I purchased my truck back in 2008. I wonder why the powered the plow from the primary battery instead of the secondary battery?

I just ordered a BatteryMinder that I will put on that battery when the truck sits in the garage.
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#7 markw8

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Posted 28 December 2011 - 05:02 PM

On my 2000 the primary was the driver, front.

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#8 Diesail

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Posted 28 December 2011 - 05:09 PM

On my 2000 the primary was the driver, front.

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#9 robertbick

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Posted 28 December 2011 - 06:01 PM

On my 2000 the primary was the driver, front. Mark


That's how I thought it would be. I'm just curious why the passenger side rear battery always has a lower voltage.
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#10 markw8

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Posted 29 December 2011 - 11:50 AM

On my 2000 the primary was the driver, front. Mark


That's how I thought it would be. I'm just curious why the passenger side rear battery always has a lower voltage.


I can't answer why it's lower but if they are still the same, the second battery is only on line with the truck running. It helps power the entire truck, not just a trailer. If you have a trailer pluged in with the key off it draws from the starting battery.

All that said, I've read enough to think this might all be different now. I currently have a 3500 D/A that both batteries are always in use.

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#11 Diesail

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Posted 29 December 2011 - 01:43 PM

The primary battery, passenger side, is lower because it is powering the truck. Even with the truck off there is a load to power the computers and such. So the battery has a load 24/7. The Auxiliary Battery only power what ever accessories have been connected to it and is connected by a relay when the truck is running to charge it. Since the OP has nothing connected to it, it is simply sitting there doing nothing expect being charged when the truck is running. Thus the cause of the voltate being lower on the primary battery and the voltage of the auxiliary battery being a 12.8 volts or a full charge.

Mark, in the diesel there is no "auxiliary" battery. Both are connected in parallel (making one large battery) and both power the truck all the time. The diesel has a much larger starting load requirement and thus needs both batteries to start.

#12 robertbick

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Posted 29 December 2011 - 07:48 PM

The primary battery, passenger side, is lower because it is powering the truck. Even with the truck off there is a load to power the computers and such. So the battery has a load 24/7. The Auxiliary Battery only power what ever accessories have been connected to it and is connected by a relay when the truck is running to charge it. Since the OP has nothing connected to it, it is simply sitting there doing nothing expect being charged when the truck is running. Thus the cause of the voltate being lower on the primary battery and the voltage of the auxiliary battery being a 12.8 volts or a full charge. Mark, in the diesel there is no "auxiliary" battery. Both are connected in parallel (making one large battery) and both power the truck all the time. The diesel has a much larger starting load requirement and thus needs both batteries to start.


This makes a lot of sense. Thanks! I would have thought the dealer would have powerted the plow from the Aux battery. Do you think I should change the plow power leads to the aux battery?
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#13 markw8

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Posted 29 December 2011 - 10:55 PM

The primary battery, passenger side, is lower because it is powering the truck. Even with the truck off there is a load to power the computers and such. So the battery has a load 24/7. The Auxiliary Battery only power what ever accessories have been connected to it and is connected by a relay when the truck is running to charge it. Since the OP has nothing connected to it, it is simply sitting there doing nothing expect being charged when the truck is running. Thus the cause of the voltate being lower on the primary battery and the voltage of the auxiliary battery being a 12.8 volts or a full charge.

Mark, in the diesel there is no "auxiliary" battery. Both are connected in parallel (making one large battery) and both power the truck all the time. The diesel has a much larger starting load requirement and thus needs both batteries to start.


I'm aware of this as was in my post. When did the passenger side become the starting battery on the gassers, or am I loosing my mind when I say drivers side? My aux. bat on my 2000 went bad on the way back from Alaska. I'm as sure as I can be it was pass. side. I disconnected and taped the cable and replaced it when I got home.

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#14 markw8

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Posted 29 December 2011 - 11:29 PM

I just looked in my 08 FSM and see the primary is indeed the passerger side. Can someone, Diesail maybe, set me strait about my 2000 being the opposite?

Mark

#15 dewfpo

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Posted 29 December 2011 - 11:48 PM

I just looked in my 08 FSM and see the primary is indeed the passerger side. Can someone, Diesail maybe, set me strait about my 2000 being the opposite?

Mark


I believe the primary switched from the front, drivers side to the rear, passengers side with the GMT900 trucks. My '05 has the primary on the driver's side.

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