As suggested, measure the voltage with a voltmeter. (cost is about $10 – $20 at a home center).
I would try initially with the trolling motor running to create a load on your system to check first the battery, then the voltage at your trolling motor to determine whether there is significant line loss between the two points.
The drop in voltage will occur where there is a crimp connection, splice in the line, or small break in the insulation where water point can penetrate and corrode the copper wiring. The amount of voltage loss depends on the current load and the amount of resistance in the line. Any corrosion in connections should result in increased resistance and therefore a voltage drop across connections at that point.
The battery without load should be slightly above 12V. If it drops quickly to below 10V, then there likely would be a shorted cell or very weak battery.
The same would apply to the sonar, except is has a much smaller current load, so as a result, will have less voltage drop across corroded junctions. However, what you are looking for is that the voltages stays above 10V. For the sonar, you should not see very little drop, perhaps worst case, one might expect to see about 0.2V below the battery voltage. If you can follow the line and find any crimp connectors, use a stiff wire or large needle to probe into these points to test their supply level.
To check for current draw of the sonar, one can use the current (amperage) measurement of the voltmeter which will require you to move the connections to the meter to the appropriate locations and switch the dial over to the settings for current.
Then, insert the voltmeter’s leads in series with the sonar’s power. Current draw should be the same whether directly attached to the battery as you had done previously, as would be at the end of the wire.
It should be apparent, that these meters are good for measuring about 10 amps of current, so I don’t think I would do this with the trolling motor.
And, a note of caution… If you short the battery while doing the current measurements, you can fry your wires, so be very careful to avoid shorting any of the supply side wires to ground! Far better to have a 2nd set of hands and/or even to use black plastic tape to insulate your connections to the meter while doing these instead of trying to juggle holding two connections and not making a mistake and accidentally grounding a supply line.
I have an inexpensive Eagle Fisheasy 240 that I used to take some reference measurements that might be of some help to you. Using a small 12V lead acid battery to power this, the initial battery reading is 12.6V and drops to a steady 12.4V with the sonar powered. Putting the meter into the current measurement setting and inserting the leads in series with the supply to the sonar, the current draw is measured at 300 mA with the backlight on and 250 mA with the backlight off.
I have a old analog meter which I bought from Radio Shack years ago. While measuring the current draw, I noted that the meter shows three distinct jumps each 1 second interval which I expect is due to the operation of the sonar.
Without hooking up an oscilloscope with a series resistance, I have no way of knowing how much peak current the sonar draws during operation, but it obviously spikes upward, so one would expect the maximum supply loss to occur while the sonar is operating.
Hope this is helpful!
