Weird Voltage readings F150 w/ aux charge lead

[capt_toddtt]

Over the weekend I installed the aux charging lead from my motor (F150) to a separate house battery. I recently purchased this boat used and the previous owner had the charging lead in the bag of stuff he gave me with the boat. He never got around to hooking it up so I did. It seems to work great but I am getting an unusual transient voltage reading while cranking the motor that I do not understand.

First here is a brief overview of how I have it hooked up.

2 batteries – one starting (AGM) and one deep cycle house (soon to be replaced by an AGM).

Wired to a Blue Seas dual circuit plus battery switch which is set up as shown below. The two negative terminals are hooked together (and grounded with the motor).



I have hooked up the aux charge lead the house battery and supplied the bus bar under the console from the house battery. Wire in this way, with this switch I believe I have completely isolated the house loads from the starting battery circuit. However…when I turn the key and crank to motor I noticed the voltage display on my Garmin chart plotter drops down. The boat also has a Lowrance bottom machine and it show the same reaction. Both those machine are wired into that bus bar supplied by the house battery. This is happening with the switch set to the on position not the combine batteries position.

Any idea why that could be occurring? My guess is something on that bus bar goes to the motor and it is what the Garmin is sensing but I'm not sure. I also put a handheld voltmeter directly on the bus bar while cranking and it is showing up on it.

Everything seems to work just fine and the batteries are immediately topped off by the charging system but I don't care for electrical mysteries and more than anything don't want to hurt anything by accident.

Thanks in advance!

Todd

[99yam40]

Not sure what is happening but it sounds like there is a connection between the batteries or the buss is tied to the wrong battery
Monitor the voltage at the battery posts to see if only one battery's voltage is dropping while cranking motor or both

[capt_toddtt]

Thanks I will check that Saturday and post the results.

[Bluefish]

AGM battery what’s that if it’s a seal battery Yamaha does not recommend them something to do with overheating you may want to search that out first

[capt_toddtt]

Thanks Bluefish. I did do extensive research on the AGM battery for the Yamaha. The non-recommended in the manual is nonsense for the most part. There is a bunch of confusion out there about these batteries but after reading and reading and reading I believe they are a very safe choice and a much better battery specially for the marine application. the biggest concern with an AGM battery is that it can handle a big inrush of charging current from the charging source due to its more efficient design. This means that potentially your motor's charging system could be working at maximum if it senses a dead AGM battery. The key word here is dead battery. Under normal usage there should be no problem at all and many folks on this forum have been using them for quite some time with their Yamahas.
I do appreciate the heads up though!

[capt_toddtt]

99Yam40,
I did as you suggested and there is zero voltage drop on the house battery (when measured directly on the battery) while cranking. So I believe this means that there is something on my bus bar that goes to the engine circuit somehow. My next plan of attack is disconnecting the items from the bus bar in a process of elimination to find the culprit. Thanks to your suggestion though I believe I am getting closer to solving this. Does this sound like a reasonable plan?
Thanks!
Todd

[99yam40]

Disconnect the cranking battery at the posts and then see if the gauges still come on, would be the first thing I would try. Just to make sure the buss is powered by the house battery and not the cranking one

[Finisher]

Some or most of Garmins equipment turns on or off using a ground. Nav equipment usually uses a ground as a switch to turn on something remotely like a course cpu or gps ant. The drops in voltage are not great but your units can operate in a range from 10-32v. so it shouldn't harm them and if it goes too low they will shut themselves off. AGM's are crazy money for a battery you will still prob. replace in 4 years.

[capt_toddtt]

First I'd like to say thanks for all the help so far. Unfortunately I still have this mystery on my hands. Here is what I have done for tests so far.

While cranking I measured both batteries at their posts with a digital multimeter. House battery is steady and the cranking battery drops down as expected.

Disconnected cranking battery and turned key on to see if the gauges would come on - they did not.

With both batteries disconnected I used an ohm meter to check for connectivity between the 1 and 2 batteries on the switch. There should not be any and there wasn't.

I reconnected to starting battery and left the house disconnected - checked for voltage at the bus bar - there was none.

I am getting pretty stumped here. I did notice something else today while taking readings. With both batteries hooked back up and everything in the "normal" settings I noticed that when I turn the ignition key to its on (not crank) position there was a small but measurable drop in voltage at the bus bar. I tried it over and over again and there was no doubt - when I turned the key to on the multimeter picked up a few tenths drop every time. I guess I all next take a closer look at exactly what is connected to the ignition key terminals.

The only connections between these circuits that I know about are the house and starting negative terminals are tied together. Of course both batteries are charged from the Yamaha but one uses the current flowing back up the starting cables, and the other uses the aux charging lead which is supposed to be isolated by the engine.

I am open to any other suggestion and again thanks so much of the help!

Thanks for the info Finisher. I'm not too worried about hurting the Garmin but I am concerned that something else on the engine might get toasted.

[99yam40]

Disconnected cranking battery and turned key on to see if the gauges would come on - they did not.

I reconnected to starting battery and left the house disconnected - checked for voltage at the bus bar - there was none.

I am not sure how you disconnected the batteries when doing these tests (disconnected both cables or just the positive), or how you were testing the voltages (where you placed the meter leads) but one of the batteries should be feeding the bus bar.
Maybe the way Neg(-)grounds are tied to motor and then to the negative bus that the gauges are hooked up to ( or does it even have a neg bus)and/or the way you were testing is the problem.

Where did you hook up the aux charge wire to the battery? At the battery side of the isolation switch?

[capt_toddtt]

Problem solved!
Just wanted to update you what I found. After doing every diagnostic test I and others could think of, I finally had to just go through my 12 volt electrical bus wire by wire until I found the culprit(s). After pulling every fuse on the hot side one by one, I found nothing. Even with all fuses out the bus was being drawn down upon key turn and cranking the motor. I also noticed erratic behavior from my trim LCD gauge (something I just noticed). After this I realized it had to be a problem with a ground. I made a simple jumper wire out of some alligator clips and a wire. Using this I carefully tested each ground until I found two that reacted very differently than any of the other grounds wires. These two grounds had 12 volts flowing through them (completed circuit) as soon as the key was turned to on. There are about a dozen ground wires on my bus and these two were to only ones that behaved like this. I rerouted those 2 grounds to a new ground only bus bar that I installed which is grounded directly to the starting battery. Success!! Now when I start the motor my lights don't dim, my electronics are perfect and their volt meter displays do not flinch. I am a happy guy! I took the time to identify and label every singe wire I touched so now I know where each wire goes. Lots of work but well worth it in the end.

Thank you to all of you for helping with suggestions. This forum is great!

Todd

[99yam40]

what did these 2 bad ground wires come from/ go to?
and did they get their 12 volt + from the start battery or the aux?
If from the aux why did you tie to the start battery neg?

[capt_toddtt]

The two wires in question definitely got their 12+ from the start battery circuit. To test this I completely disconnected the aux battery (ground and pos). That should have made the bus bar dead. When I tested all grounds those two showed a completed circuit upon turning the ignition key to on. I do not know exactly what circuit they go to but I am pretty sure they involved the engine gauges because I was seeing strange behavior from my LCD trim/tilt part of the gauge. When I trimmed the motor up or down the LCD bars would go the opposite at first and when I stopped trimming the gauge would settle down to where it should be. After isolating those two ground wires that has completely gone away and the gauge works perfect. As far as why I tied those two grounds to the aux battery - I didn't. The boat came with only one battery. I added the second and the switch. With only one battery this ground problem did not exist and all the grounds going to the bus were fine (one battery one ground). When I added the second battery I unknowingly create this problem because I moved the bus ground to the new aux battery to isolate it. This way every load on the bus should have gotten its 12V +/- from the aux battery only. What I didn't know is that two of the circuits grounded on the bus still got their 12V + from the starting circuit. Once I identified and removed those two circuits from the bus every single symptom was resolved.

[99yam40]

Sounds like you have a handle on it.

What all is powered from the aux battery if gauges and T&T are feeding from start? Lights ,radio, fishfinder?

Maybe a good idea to have 2 neg buss and make sure what feeds from each