Trolling motor: Electrical interferance with fishfinder

[teslaman]

Hello all,

I have my trolling motor connected to the same deep cycle battery as my fishfinder. The power leads for each device are ran along opposite sides of the boat, however there is common contact at the battery terminals. When I hit the pedal for the trolling motor, the inrush current causes a spike on the screen. Then during the operation of the trolling motor the fishfinder sees more fish than usual. A second but smaller spike occurs when the pedal is released.

I have rulled out cavitation and other forms of mechanical interferance due to the response time of the fishfinder screen and the time that the pedal is depressed. I think that this is entirely an electrical phenomenon caused by currents coming from the battery. Of course, I've been wrong many times before.

I've though about powering the fishfinder from my engine battery. I used another deep cycle w/ lots of cca for this purpose and it works great. However I fear two things:
1) That I may drain the battery so I cannot start the motor
2) While running the engine the rectifier will cause similar noise problems on the fishfinder. (this may be a scenario that I can live with since I mostly use the fishfinder when trolling)

Any ideas? (someone suggested placing a capacitor between the batt terminals to suppress the interferance). That may work since caps are an open circuit to dc and will soak up energy due to surges. The cap size will have to be strategically selected to be effective however.

 

[Barnacle_Bill]

Re: Trolling motor: Electrical interferance with fishfinder

I run 2 of them off my starting battery with no problem. They draw very little current so you don't have to worry about draining your battery while out.

 

[rndn]

Re: Trolling motor: Electrical interferance with fishfinder

I had the same issue and needed to run the fishfinder off of the starting battery. The fishfinder would work until I turned on the electric motor and it would not aquire accurate depth readings. As mentioned above the fishfinder uses very little electricity and will not run down your starting battery.

 

[Sig_Mech]

Re: Trolling motor: Electrical interferance with fishfinder

Teslaman, I agree with bhile on this one. By moving the power lead for your fishfinder to the starting battery you will protect the electronics in your fishfinder from constant voltage surges and voltage drops. The current draw for these units is so minimal that it will have little affect on the charge of the starting battery. I would also recommend that you use a marine starting battery and not a deep cycle for your main engine. Rather than try and explain why I'll just refer you to http://www.windsun.com/Batteries/Battery_FAQ.htm this guy does a great job of explaining all the pros and cons.

 

[teslaman]

Re: Trolling motor: Electrical interferance with fishfinder

Thanks for the help guys. I'll switch my fishfinder over to my engine battery.

Sig_Mech: I looked into using a deep cycle for starting before I purchased it including reviewing the website that you linked to. Here is what it says:

Using a deep cycle battery as a starting battery
There is generally no problem with this, providing that allowance is made for the lower cranking amps compared to a similar size starting battery. As a general rule, if you are going to use a true deep cycle battery (such as the Concorde) also as a starting battery, it should be oversized about 20% compared to the existing or recommended starting battery group size to get the same cranking amps. That is about the same as replacing a group 24 with a group 31. ....

So as long as you purchase a deep cycle with the proper capacity it is OK. This way, I can use my other deep cycle for starting if one goes out.

 

[WillyBWright]

Re: Trolling motor: Electrical interferance with fishfinder

For your motor, completely ignore any information whatsoever from battery manufacturers and adhere to the letter to the motor manufacturers mandates or you'll have problems. I see them all the time on EFI motors and Direct Injected motors. Deep Cycle and Dual Purpose batteries on these specifically lead to a myriad of problems. They do OK on smaller carbureted outboards ONLY! Twins-OK, Triples-maybe. Four and up-I wouldn't. The main problem is their ability to take voltage back in. They are deep-cycles at heart and charge slowly. They might not charge fast enough to keep up with the demand. One step forward, two steps back.