I currently have a Humminbird 898 and have all my electronics and pumps hooked up to my starter battery. I have not had any issues with the battery cranking the motor yet but I have noticed that it doesn't seem to hold a charge as long. After a couple of hours on the water I notice the voltage going down to about 11.0v and after 5 hours the other day it went down to 10.2v. I don't ever remember it going below 12v when I was on the water and was wondering if the battery is going bad? Its between 1-2 years old. I'm getting these voltages from the display on the Humminbird unit.

I was planning on upgrading to a Helix 12 and I'm worried that its going to drain the battery much faster. I don't have room under my center console to add any more batteries to separate the starter and electronics and running wires through those BS pipes under the deck (I have a 23 center console) is just about the most frustrating thing on the planet so I don't want to relocate any batteries.

This is my battery http://www.walmart.com/ip/20531538

Seems like your battery isn't recharging when running your motor. Might need to get that evaluated.

Agreed. You should measure something greater than 13V across your battery terminals with the motor running.
This would also be a good time to add a second battery to get all of that other stuff off of the starting battery. If your battery is of the "dual purpose" or marine starting battery, running it down to a deep discharge with the trolling motor is going to take it out pretty quick. The plates in those batteries just won't hold up to repeated discharges.

Bob, I don't think he has the TM connected to the starting battery. He seems to be describing the standard electrical plan for a boat. I'm guessing he has a bad battery, but certainly should check for alternator voltage when the engine is running above idle. Since he has a flooded cell battery, a hydrometer would be the best way to check it. I bet he has a dead cell in that battery.

When the engine is running it is a little over 13v and when it shuts off its 12v for a couple mins and then it starts dropping. It so far has always had enough juice to start the engine but I never remember it lowing voltage so quickly. I thought one test of a battery was if it had enough amps to crank the engine it is still good. I feel like it's just losing energy storage capacity.

You have a bad battery.

Yep, bad battery.

Put a full charge on it and take it in to be tested. Sounds like it has a bad cell. You're more than likely going to be stranded on the water with it eventually

Harbor Freight sells a resistive load tester for around $12. It's a good tool to have. Charge overnight, let it rest for 24 hours then load test it. it has to rest so you won't be reading a surface charge.

Surface charge is a capacitive charge on the battery cells giving an elevated no-load voltage reading. Seconds after a load tester is loaded onto a battery, the surface charge is long gone. I will admit that it's still not a bad idea to let the battery rest a bit, but just to see if it has a shorting cell that will kill itself in the brief rest period. If it does that, the battery is really a dead cause. But also, if its really that bad, you're going to see it when the load tester is engaged, not matter how long you do or don't wait.