2008 Tahoe Hybrid 12v Battery Constantly Dying

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Dirtrunner702

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Hello,

I have a 2008 Tahoe Hybrid. All software up to date. 5-6 year old 300v battery pack and 2 year old 12v battery. No CEL or issues at all and regularly maintained.

About 4 weeks ago it would not start in the morning. Nothing had changed or had been changed up to that point.

Took it to the dealer and they Replaced the negative battery cable and new 12V battery, they did an airbag recall on it as well. Went to get it the following day and the service manager said it started but all all sorts of lights came on, traction control, air bags, etc.
They kept it and decided to replace the SDM-C module under driver seat. Came back to get it and all was good. Took it home.

2 days later it wont start, same as before. Turn the key and the CEL light turns on and eventually things light up but no crank.
12v Battery drained. No Crank

Replaced OEM battery with Optima yellow top ( Dealer said the battery they put in tested fine, but wont start the truck) and it drove PERFECT for 2 days

Proceeded to do a parasitic draw test every way that you can do one and found a big draw coming from Fuse 25 DSM (Driver Seat Module and Remote Control Door Lock Receiver which also does the Tire pressure monitoring sensors). Pulled the fuse 25 and draw goes from 70-112mA draw down to 50mA. Ive done these tests where you do not interrupt the power to the vehicle so as to not lose its state while connecting a multimeter.

Replaced the battery AGAIN with a brand-new Optima because the previous optima would not let the truck crank, even after charging it overnight. Drove around without the RCDLR and with the new Optima batt, then it started to show symptoms of a low charged 12v battery after 3 days.

Im back to square 1.

I am waiting for a new positive battery cable assembly to arrive as I am completely out of ideas and the battery terminal does feel hotter than normal after driving for a few tests making sure the electrical system is charging the battery while its on. No CEL, no service hybrid system errors or anything.

Does anyone know what the mA draw should be when the tahoe is fully asleep? I know 50mA is a little high but maybe thats normal for this year range tahoe?

TLDR: Everything perfect and one morning no crank. Took to dealer and they replaced negative battery cable assembly and 12v battery. Issue returned and they replaced SDM-C module. Got a new Optima battery and it also died after 2 days. Narrowed parasitic draw to RCDLR. Pull and it drops mA fluctuation from 70-120mA down to 50mA while asleep. Purchased ANOTHER Optima batt and 3 days later the battery died again even with RCDLR disconnected.

Ive Been pulling my hair out on this for 3 weeks. Thank you for reading and any input is HIGHLY appreciated. I love this thing and want to stay in the no car payment club but fear something else major may be wrong, perhaps in the hybrid RAP system. I dont know.
 
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BG1988

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is it even charging? like 14.5-14.8v on cold start?
 

j91z28d1

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yeah I wonder if it's charging while it's on? can you check the voltage at the battery while it's running?

battery drains are so ******* newer vehicles. it takes like 30mins for some modules to fully go to sleep in some vehicles. I've chased my tail for weeks over some newer Ford trucks.

I'd have to look up a spec, but 0.100amp doesn't sound like a crazy draw to kill a battery that quickly. especially a new yellow top.

I've had good luck with a DC amp clamp helping during t/s. you can get them pretty cheap off Amazon. don't really need to be fluke quality. just show you close enough. you can just clip over the battery cable after it's been sitting for a few hours to make sure everything is asleep and see what the drain is without disturbing anything

voltage drop test is the best way to check the cables. volt meter with on lead on the neg post of the battery, other lead on the block. it should be basically 0. if you have any voltage that shows its dropping voltage from a bad connection. do the same on the pos post to where ever the pos cable goes in these trucks. it's bit of a mess to follow haha.


only other guess and it's a complete guess is the brake module, it's mounted under the truck about the drivers seat area. few cases of guys having to replace them because water got up inside them. but I have no idea if that has power while the truck is off and it would probably throw a module code.
 

ivin74

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Check the alternator fuse, it might be bad and the alternator might not be charging the battery.


 

j91z28d1

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Check the alternator fuse, it might be bad and the alternator might not be charging the battery.


yeah these trucks are odd balls. they don't even have an alternator.

the electrical side is kinda crazy.
 

BG1988

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yeah these trucks are odd balls. they don't even have an alternator.

the electrical side is kinda crazy.
yeah the 200 amp fuse can still be bad

you can leave an 1amp load overnight and it would still be fine (i.e leaving interior lights on)
 

BG1988

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yeah I wonder if it's charging while it's on? can you check the voltage at the battery while it's running?

battery drains are so ******* newer vehicles. it takes like 30mins for some modules to fully go to sleep in some vehicles. I've chased my tail for weeks over some newer Ford trucks.

I'd have to look up a spec, but 0.100amp doesn't sound like a crazy draw to kill a battery that quickly. especially a new yellow top.

I've had good luck with a DC amp clamp helping during t/s. you can get them pretty cheap off Amazon. don't really need to be fluke quality. just show you close enough. you can just clip over the battery cable after it's been sitting for a few hours to make sure everything is asleep and see what the drain is without disturbing anything

voltage drop test is the best way to check the cables. volt meter with on lead on the neg post of the battery, other lead on the block. it should be basically 0. if you have any voltage that shows its dropping voltage from a bad connection. do the same on the pos post to where ever the pos cable goes in these trucks. it's bit of a mess to follow haha.
one cable goes to the fusebox and the other two goes to the AUX fusebox (next to the radiator )
One for the MEGAFUSE and other for the AUXfuse block...
inspection for corrosion might be necessary
 

j91z28d1

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a tsb pops up. 08-06-03-008A

if you can find it it give a few things to check.

besides the normal make sure it's charging the battery while on, says onstar should be 3-20mA but can stay on standby for 48h and pull 250mA if it trying to get a call.

check the front and back hvac motors are shutting off at key off and some cars have afterblow enabled. it runs the blowers to dry the ac evaporators about 30mins after driving by running them for a while.


check for under hood relays struck on when key off, examples given cooling fan relay

a rear door lock relay sticking concern. that one says if you find that one sticking you gotta replace IP fuse box. the relay is internal and can't be replaced separately.

and circuit 1240 (left door) 1340 (right door) they run thru they left and right B pillar and might short, if so there's a 25amp fuse that should be hot to the touch. but it doesn't give a fuse number haha.

this bulletin is horribly written. not very specific but if you Google the number you can probably find and read it. other than that I see it can set a dtc
B1527
if the state of charge is 30% lower than last time the engine was running or the 12v battery drain is greater than 2 amps.

the bcm doesn't seem to care until it kills the battery by 30% from the last time your drove it, or the drain is over 2 amps. I'll attach the current drain chart. I'm not 100% sure this is hybrid specific but I did put the vehicle type and year in. but I do find this software sometimes brings up form general info if it doesn't have anything else to display haha.
 

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Dirtrunner702

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Thank you everyone! I will be doing all this today and tomorrow. I’ll see if I can find something to report back. I truly appreciate the input and time
 
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Dirtrunner702

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Hey all, Saturday night update. Replaced the 200amp fuse and made no change.

Confirmed the system is charging the 12v battery. At fresh crank it’s 14.7 volts and just fluctuates over 5-10 mins down to 13 and back up to 14 and such. Getting good voltage while ice runs and when in auto stop.

Craaazy thing happened. I was just wiggling everything that I can to see if I get anything strange and then bam! I wiggled the heater core hoses a bit and the AC compressor shuts off… almost like a short. The hvac controls locked up and the air bag light flashed a few times. I can reproduce by wiggling the hoses again.

So I have turned the truck off and put my multimeter in series without interrupting power to the system and will monitor for a while. I will take off the air intake and air box tomorrow and check all wiring down there. I hope I’m onto something here
 

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