The last time out, my boat motor cranked fine until late in the day. Lowrance HDS 8/5 got black. Turned them off and on again to restore bright screen. 5 minutes later, repeat. Big motor didn't crank. Moved leads to TM batteries, no problem. Found bilge pump left on. I thought that was the issue.

Got home and fully charged batteries. Next trip, boat cranked fine the first time, but not the second. Lowrance displays got very dim again. Took battery to get tested yesterday and it tested good. Strange.

Got back home tested voltage at the battery terminal, power lead of HDS 8/5 and displayed voltage on HDS display. Generally, the power at the HDS lead was 0.5 to 0.75 of a volt less than at the battery terminal and the voltage on the HDS display was 0.75 to 1.0 volt less than seen at the power cord of the HDS. All in all, this represents about a 1.5 to 2.0 volt difference between the battery terminals and what the HDS displays as its input voltage.

Is this normal? Currently, the HDS displays, both console and bow are wired through switch panels, not directly to the battery. Should these be wired directly to the battery? Could the battery be bad, even though it tested good? My boat has live quite well in this exact configuration for well over a year. Any input is greatly appreciated.

You may need to install a larger ground lead up to your console and branch into that for your graphs. I would have your battery tested again, sounds like its got a bad cell to not hold up for the day.

From what I've seen, not uncommon for the voltage display on the HDS unit to read low -- I've had the same issue with mine.

On mine, there are dedicated 14 AWG power wires from the battery to a terminal strip under the console. I can measure voltage with a digital voltmeter and see voltage at the terminal strip within .1 of what it reads at the battery terminals. The HDS display will show .3 - .4 volts less.

So - personally, I wouldn't trust the accuracy of the HDS voltage display, at least not as a trouble-shooting tool to make changes.

In my case there are dedicated power wires (not boat wiring) for the HDS units because I confirmed that there was significant voltage drop (less) in the boat wiring. I never found the exact culprit, but there are many switches, junctions, splices and wire lenghts in the boat wiring between the battery and the HDS unit -- any/all of these can be adding resistance to the circuit causing voltage loss.

C.

I agree with the others. Some voltage drop to your display is normal. If the engine won't crank and the display goes dim, you probably have a battery issue...you can check the voltage with a meter while that's happening to get a better idea.

I thought I had a battery issue too until it tested good. Granted, I took it to Wal-Mart, so I might have a false positive.

As what was mentioned above switches, splices, fuses will cause a voltage drop. But also corrosion! Check all contacts and be sure they are clean. You will have some voltage drop (the more switches, etc, corrosion in that wire between power source and unit will cause more of a drop.

Honestly, I completely ignore the results of parts store/Wal-mart battery tests, unless the battery fails. The equipment just isn't accurate enough. Starting an engine is the best load test you can put on your battery.

A check of voltage when the problem occurs will give you a better answer.

Complete agree however, this is exactly what I'm trying to avoid. I don't want to be dead in the water when my main motor won't crank and then try to test the battery voltage.

I think I'll err on the side of caution and just replace the battery. I suspect the battery isn't holding a charge under load.

Thanks for all you guys' help.

Keep jumper cables on board.

^^This. Trust only the voltmeter, the built-in HDS volt display just isn't calibrated to be accurate. Sounds like it's your crank battery.

I disagree, a true load tester will tell you just how good any battery is, then hydrometer in each cell if you can.
napa, oriellys and others have load testers and will check it for free.