Ok, so I don't really know anything about electrical stuff, or whatever. I have a little box called a Kill-A-Watt to see the power draw for 120V A/C appliances. I recently got a trolling motor for my paddleboat however (yes I'm lazy) and have been using my 16ah sealed battery with it. What's the best way to know how much power you're drawing at any given time? There's gotta be some simple device that does it, or a way to measure it.

 

If you can multiply by 12 a DC amp meter will do the job.

Power = ElectroMotive Force (EMF) times Current.

or

Watts = Volts times Amps

You already know you have (roughly) 12 volts so all you need is the amps.

 

This is the cheapest DC Watt Meter I could find ($99.95):

http://www.newark.com/NewarkWebComme...KU=22H6392&N=0

 

Yeah, unfortunately I got the motor used and it came with no instructions, and there's nothing on the motor or control unit that says what the draw rating is. Looking at the above post however where it seems that such units for DC can cost around $100, it's not looking like there's a cheap ($20-30) device that does the same thing as an A/C Kill-A-Watt for 12v DC.

 

Have you tried using google to find information about the motor? If there are any labels at all you may be able to track down the specifications.

 

Depending on the draw of the motor, you can connect a multimeter between the battery and the motor and it will show you how much current it's using. Most multimeters have maximums of 10 or 20 amps, but if it's not a huge draw that may be suitable.

There are also clamp-on type meters that use an inductive type sensor to measure the current in the cable. Typically these are used in automotive shops for measuring the current draw of a starter motor, but I have no idea what they cost - probably a lot since DC induction is trickier than AC induction.

Unfortunately DC is harder to get a handle on the current without "opening" the circuit, it's possible, but not simple, and therefore not cheap, unfortunately.

 

I would assume that you are concerned with how much energy is left in the battery while "out to sea".

(You'll have the most draw going to the motor while accellerating, against the wind, faster speed etc just like your car)

If it were me looking for a quick low cost way to monitor available power I'd hook a simple volt meter across the (+) and (-) connections of the battery or motor.
I found a nice one for under $13.00 probably avaliable at your local Radio Shack store.

It's the panel mount meter that you can see at http://www.radioshack.com.
Paste this into the search (find) box: 22-410

Your buddy could solder the wires to the meter for you. For safety sake, be sure they are actually soldered (Not just twisted or wound around the terminals as some might do)

A couple of things to remember about this:
*Be CERTAIN that the (+) on the meter goes to the (+) side of the battery and the (-) side of the meter to the (-) side of the battery. If they are attached backward the needle will be forced backwards and can damage the meter.

*If you're worried about water getting into the meter you can always use silicone sealant (Bathroom cauking after the wires have been soldered) to seal any openings.

*Also, don't forget to disconnect the meter from the battery when not using it.

*And last, the best indication of remaining power is while the motor is in use. (A load is applied)
Oh yea...happy sailing!