Question about batteries and depth finder

[MississippiBoy]

My birthday is coming up on January 6 (if anybody wants to send me a present, let me know and I'll PM you my address ), and the wife has decided to give me a depth finder for my little boat. Nothing fancy, maybe a $200 or so unit, but that beats not having anything at all, right?
The boat only has one battery, but I have heard that it's not a good idea to have the trolling motor and the depth/fish finder hooked to the same battery because of interference issues. I think I have room under the front deck to put this big battery to dedicate it to the trolling motor. That means I'm looking to get another battery to use as the cranking battery and to run the fish finder.
My question is, how big of a battery do I need for the 40 hp Yamaha, depth finder, and bilge pump? I'm thinking fairly small, to keep down the weight, but I'm not sure how much current the fish finder will draw.
Any advice is appreciated.

[nofearengineer]

Mississippi,

Most small fishfinders draw a 0.5 amps or less. Some much less. If you get a 100 AH cranking battery (you probably have bigger), you could run the fish finder alone for a couple of weeks and not totally discharge the battery. With your outboard's alternator topping off the battery (not sure what outboard you have, assume it has an alternator/regulator), it's a non-issue.

[champman]

Nofear sounds good on the amp-hours. I would go with a duel perpose marine deep cycle for starting, lights and depth finder and a good deep cycle marine for the trolling motor. Never use a standard car battery, it will not hold up very long.

[Bob Smith]

Originally Posted by champman

Nofear sounds good on the amp-hours. I would go with a duel perpose marine deep cycle for starting, lights and depth finder and a good deep cycle marine for the trolling motor. Never use a standard car battery, it will not hold up very long.

Ditto on the dual purpose deep cycle battery. A series 27 size would take care of your needs more than sufficiently and last a very long time on a rig like yours. If you ever installed a livewell or bait keeper system, you could also run it on this battery set up. I would never chance it by running the T-motor and depth finder. I am not sure if it is myth or truth, but I have never done it or seen a factory boat rigged that way.

[bassboogieman]

A marine cranking battery, size 24 should be enough. I use a size 27 for my cranking battery (turns over a 225 just fine) and runs my livewells, 2 sonars, GPS, all my onboard electronics. You won't require a battery larger than a 24. Even if you don't run the outboard and get it recharged during the day, you'll have plenty to run your electronics all day. Just make sure it's a MARINE cranking battery.

[nofearengineer]

Originally Posted by Bob Smith

I would never chance it by running the T-motor and depth finder. I am not sure if it is myth or truth, but I have never done it or seen a factory boat rigged that way.

Bob, it is definitely truth. By their very nature, brushed DC motors like those in electric trolling motors can introduce a lot of harmonics into the power system. Every time the commutator "commutes", or changes polarity, there are transients that can cause small, but definitely significant voltage irregularities.

This can be fixed, however, if one must only use a single battery. All it takes is a filter between the hot and the neutral wires, which is fancy terminology for a couple of capacitors. Usually, the pair will be like a 10uF cap and a 100uF cap.

I am not sure, but I would think that most modern fishfinders would have this filter already incorporated into their circuitry. They're not particularly expensive, but then again, leaving them out would give HB and Lowrance the opportunity to make some more money off of fisherman. I actually saw a HB filter kit today for $59.99. What a frigging ripoff for $2 worth of capacitors.