Battery charging issue

[EvergreenZ71]

I’ve searched through the forum and didn’t find my exact situation though I think I know the solution and am looking for some thoughts before I do it.

2009 Tahoe LTZ 6.2L with dual battery (OEM style - call it RV or PPV) installed by dealership. Both batteries new w/ dual battery installation (Interstate AGM). New engine in August (the one time the bank pushed the aftermarket service contract I actually got my money out of).

Over last month the primary battery has been struggling, progressively more difficult starts and actually had to jump it from the dual battery (OEM dual battery set up on these have no support for starting). Tested battery & alternator at O’Reillys which said battery is good but needs a charge, but alternator was only putting out about 12 volts with the headlights on using their battery tester unit. However, dash voltage shows at over 14 while running and the replacement accessory usb port I installed that has a volt meter matches. I know both of those are more likely to be off than the O’Reillys tool, but I’m thinking that it’s more likely to be a bad ground than a bad alternator, probably not well seated from the engine installation (who also had the oil filter work loose). The engine was done at a dealership, but not my preferred one who had installed the dual battery system. Battery is about 2 years old and charged fine using an external charger.

So my short question: can a bad ground limit the voltage going to the battery and thus prevent it from recharging during driving while allowing it to provide power to start?

My planned solution is installing the Big 3, though I’m a little reluctant to install an addition while there’s a potential for an unknown problem.

 

[BG1988]

check the resistance on the cables with a mutimeter

 

[Doubeleive]

check the cables as mentioned above.
go to harbor freight or any autoparts store and grab a cheapy multimeter. they are handy to have around anyway.
when you first start the vehicle you should have a solid 13.4v or more at the battery for the first minute or so.
if not then something isn't right, check voltage at the alternator output and I would check the cable from the alternator output to the battery positive and check the battery ground

 

[swathdiver]

A weak battery will drag the other down. I would trust the car's instruments over the box store. These GMT900s were not designed to charge AGMs, they work fine but will not last as long as they could have. Check all the connections that come off of the rear battery.

 

[EvergreenZ71]

Was it 2000 or the 2003 models that changed?
My old 02 did great with its first pair of Optima AGM batteries … 7-8 years. The replacements lasted about 3 each; just enough to leave warranty, but that was when Optima’s changed their production. These Interstate seem to be doing fine. They take a recharge on the external charger just fine, but not from the truck though I’m still feeling it’s a wiring issue, not the battery yet.

 

[intheburbs]

Bad ground is a 2-way street. If it's bad enough to not allow charging, then it also won't allow for proper current flow when starting.

Regarding AGM batteries - I have two GMT900 trucks, both I installed dual batteries with factory parts to duplicate the TP2 SSV option. When I did the install in each truck, I bought new AGM batteries. Did the Suburban in 2014, the Denali in 2016. I just replaced the last of the original 4, they lasted 5, 6, 7, and 8 years. And no, I didn't replace them in pairs.

 

[EvergreenZ71]

Bad ground is a 2-way street. If it's bad enough to not allow charging, then it also won't allow for proper current flow when starting.

Regarding AGM batteries - I have two GMT900 trucks, both I installed dual batteries with factory parts to duplicate the TP2 SSV option. When I did the install in each truck, I bought new AGM batteries. Did the Suburban in 2014, the Denali in 2016. I just replaced the last of the original 4, they lasted 5, 6, 7, and 8 years. And no, I didn't replace them in pairs.

That’s why this has been bugging me for weeks. Battery checks out fine. No headlight dimming. Battery charges fine on external charger and even 45min on the freeway doesn’t charge the primary. Had to use jumper cables from the 2nd battery once after a freeway drive… but it started up and drove fine.

I am considering an upgraded alternator, but I’m still leaning towards it being a ground problem.

Edit: the only electrical additions so far is a replacement 2010 OEM navigation stereo, replaced accessory outlets with USB, and ran USB outlets to the glovebox & overhead center console off the dash accessory outlet wiring.

 

[Geotrash]

That’s why this has been bugging me for weeks. Battery checks out fine. No headlight dimming. Battery charges fine on external charger and even 45min on the freeway doesn’t charge the primary. Had to use jumper cables from the 2nd battery once after a freeway drive… but it started up and drove fine.

I am considering an upgraded alternator, but I’m still leaning towards it being a ground problem.

Edit: the only electrical additions so far is a replacement 2010 OEM navigation stereo, replaced accessory outlets with USB, and ran USB outlets to the glovebox & overhead center console off the dash accessory outlet wiring.

For what it's worth, I replace my alternators as a maintenance item. At or around 100K, I install a band new (not rebuilt) unit when I do the serpentine belt and pulleys. I keep the old one as a spare in the garage. The one time I did not do that was shortly after I bought my 2007 with 196K on the clock. It had an alternator that looked clean and had a reman sticker on it, so I left it alone. About a year later, the alternator quit as I was pulling in the driveway after running errands getting ready for a cross country drive in it. I hate breakdowns so I maintain all of our cars like a helicopter - I replace parts after time in service before they fail, whenever possible. Right now I'm saving for a new transmission and TC for the 2007 in my car fund account. With 230K on it, I'm already on borrowed time.

 

[EvergreenZ71]

So sent it in to the shop (new one as my former trusted guy retired and closed his shop) and they found the battery voltage sensor had failed, but also reported a blown 175 amp fuse.

Question is, how was it running and starting with that large of a fuse blown? My understanding of the OEM dual battery system in these is that the second battery doesn’t support the starter (which is supported by my experience as a was able to self jump from it) so shouldn’t it being blown have killed the battery’s connection?

 

[intheburbs]

@Evergreen71

The way my trucks behave is that if the primary is dead, using the key does not start the truck, but it can be remote started using the fob. That assumes there's enough juice left in the battery to energize the isolator. If the primary is completely dead, jumper cables are needed.

The only 175-amp fuse on my trucks is the primary megafuse. If that blew, it would sever the only electrical connection between both batteries and the truck, immobilizeing it.

The only thing I can think of is that your truck has a second 175-amp megafuse for the secondary battery. On mine it's 125-amp. If that blows, it simply isolates the secondary, and you'd never notice unless something went wrong with the primary