I have a 2006 GM hybrid truck; I have replaced the batteries and read 36v at the high voltage port. I have also replaced the battery under the hood. The three 12v batteries on the hybrid pack are all showing 12v each. I removed and individually charged them (revived all batteries but 1) 1 battery now is smaller 250ca it balances out to 36v at the business end + 12v under the hood. It doesn't even attempt to turn the engine. No errors shown on dash. No warnings from battery pack. I forced a warning by circumventing the 48v switch so I know its sending data to the ECU. The truck was functioning a few years ago but sat around for years due to a bad battery pack, the owner replaced them with new batteries (1 year ago) but 1 of the new old stock was 11v prevent it from working. This truck is in good condition other than this hybrid system and maybe old gas. I never saw it running so I have my doubts; maybe something else is causing it from turning over.

1- The square water cooled inverter upfront (under the hood - right hand side)... could it be causing it not to start? How does one check this? Can I force a bypass. How?
2- Since I have ZERO response when I turn the key and no errors I'm assuming some safety device is causing this. Is there a way to bypass low voltage switches on the truck to allow it to attempt to start?
3- Does the truck require that the main 12v up front also be 100% charged to start? I left the new battery connected with the bad battery for a few days after replacing (I didn't realize it was adding the 12v upfront to the total voltage on the battery pack - face palm GM). I charged it overnight last night and will try again this morning with the 1 small battery in the pack. I also I have the original battery revived 85% giving 12v this morning so I could put that back and try again.
4- What else can keep this truck from starting if all batteries are charged?
5- Anyplace I can measure voltage / ohms or check continuity? Any suggestions on how to test relays or other elements?

Suspicions. Could the total CCA be the issue? It would not make sense really, if a 48v system wants to see 48v is has that. It should attempt to crank the engine however sluggish. Right now I have two original Panasonic monster batteries and 1 smaller battery.

Please respond with technical info I have required experience.

Thanks in advance.

 

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With car off and ALL accessories off, open up any fuse boxes, and go one by one. Easiest way to find the drain. Put voltmeter on battery, and pull fuses until you see a massive drop on the voltmeter. Then whatever the fuse corresponds to is the parasite. At least narrows it down to an area