Lithium related question - Fuel gauge not working

[Hyco Bass]

Reaching out to those who may have experienced this. Ref: 115 optimax, Nitro Z6 2013

Note: Everything worked on the boat prior to this exercise.

I recently purchased a 100AH lithium battery to use to power "everything but the engine and trolling motor". I wanted to take the electrical load off the starting battery and have a virtually inexhaustible power supply for my graphs, pumps, radio etc. House battery as it were. In short, I moved the "wire pair going to the fuse box" on my lead acid starting battery to the lithium battery, leaving only the large gauge engine wires and the charger wires on the lead acid battery. Two pairs of wires were moved to the lithium. One that feeds the fuse box and the other that keeps the constant power on the radio. The charger wires for the lithium are just a quick connect for a Noco 10 and not hooked up to the lead acid.

When I tried the arrangement in the garage it appeared everything worked as it should including the gauge lights. I took the boat out to the lake and I used about 15% of the lithium battery in one day of fishing . What I did notice however was that my fuel gauge was not reading on the dash when I was out there. Gauge needle was not at zero but well below zero with 3/4's of a tank of fuel. Kind of an important gauge to stop working.

Got on the internet when I came home and found lots of good information on how fuel gauges are wired (and how to troubleshoot). Sender, gauge, wires.....ground. I slept on the information and decided to try the "do no harm" troubleshooting first that would eliminate 99% of the possible failures. I hooked everything back to the Lead acid battery. Fuel gauge works and read 3/4 tank when I turned the key on. I turned the key off and the gauge fell to zero (normal). Next,I put only the fuse panel wiring back on the lithium and turned the key on. Fuel gauge dropped from "0" to well below zero.

So, sender works, gauge works, lead acid wiring set-up works....This leaves something in the wiring path when the fuse panel wiring moved.

My suspicion based on the fuel gauge wiring schematics is the there is a ground being used in the fuse panel (lithium(-)) to the fuel gauge and not from the (-) wire off the engine leads.

Yes, I can leave everything on the lead acid. Yes, I can put is a second fuse panel and move all my electronics to that. No, I don't want a hybrid setup (parallel lead acid and lithium).

Any help would be appreciated on a safe and simple solution to keeping my fuel gauge working while using the lithium.

[Bill Reynolds]

Connect the negative of the electronics battery to the negative of the motor battery. You did not mention it but I suspect the bow trim switch would not work either.

[Hyco Bass]

Will try. Will also check the bow switch.

[Hyco Bass]

Bow switch works ok with the lithium set-up as is.

I put a jumper between the (-) on the lead acid and the lithium. Walla-----joyful noises. The fuel gauge now works with the lithium as the house battery. Many thanks Mr. Bill Reynolds.

[Bill Reynolds]

Bow switch works ok with the lithium set-up as is.

I put a jumper between the (-) on the lead acid and the lithium. Walla-----joyful noises. The fuel gauge now works with the lithium as the house battery. Many thanks Mr. Bill Reynolds.

You are welcome. Glad this worked out.