Voltage drop from House Battery

I have a "new to me" 2019 242 Coup. I purchased it in Sept of this year and it's had some electrical gremlins that's causing some DC issues. Boat was upgraded to the lithium package by Pocket Yachts after removing the generator (has remnants of the generator plumbing and electrical capped off). Problem - I keep noticing the voltage is much less on the Garmin than at the shunt. When using House Batteries only, at times, the voltage gets below 9.0 and the unit shuts off. The balfer (sp?) shunt gauge is still reading 13.4 volts. I measured voltage at both 1 & 2 fuse panels and the input voltage to the panels reads the same as the garmin indicated voltage. I measured voltage at the House Rotary Switch and it duplicates the shunt at 13.4V. When I turn on the engine & parallel rotary switch the voltage begins to increase to over 12V. Any ideas on what's causing the voltage drop from the House rotary switch to the fuse panels? I have seen 12V on the garmin plenty of times and then I see it drop off even when the engine is running. I thought it was a garmin issue until I confirmed the input voltage. The more I look the more confused and frustrated I get. This may be totally unrelated but on the back of the House Rotary Switch there is a smaller gauge cable labeled Fuse 3 attached to the same terminal as the larger cable labeled House. When measuring voltage on the Fuse 3 cable it measured 2.5V. Not sure what that means? It may be left over from the lithium conversion. I've lived with this by turning the parallel switch on but it's time to get it fixed.

Sounds like a ground problem. I'd start with the main ground to the house buss. You could do a quick check with a long bit of 10gage wire run straight to the negative side of the battery buss. If problem goes away it proves that it's a ground issue. Then just start checking terminations.

Hi Dan, thanks for the input. Sorry but my knowledge is a bit limited when T/S electronics. Are you recommending jumping from the main ground buss, running wire through the cabin back to the fuse panel buss? I'm not sure where the house or battery buss is?

Ernest said:

Hi Dan, thanks for the input. Sorry but my knowledge is a bit limited when T/S electronics. Are you recommending jumping from the main ground buss, running wire through the cabin back to the fuse panel buss? I'm not sure where the house or battery buss is?

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Yes I mean the main ground buss. However now I re-read your post what you describe sounds like a high resistance connection but could be on either side of the circuit. Also you specifically mention voltage on the Garmin but are you simply using that to reference voltage on the house buss?

At any rate if it's the whole buss once you round up a long enough wire you can use it to check positive and negative side. The idea is that you use the long wire to jump over the circuit directly from source to load. If the problem goes away then there is a high resistance connection somewhere between. You can accomplish the same thing by walking down the circuit and checking voltage on both sides of every connection while there's a load on the circuit. If you find a connection with voltage drop you've found a problem. I tend to troubleshoot by narrowing down where the problem is then going point to point. Some people do the opposite. Either way will get you there.