Has anyone ever used something like this:

http://www.amazon.com/36-Volt-Battery-Status-Indicator/dp/B003TQTW3S

Curious if they would provide any benefit while on a trolling motor only lake.

They will benefit by letting you know how much you've drained your battery. They won't do anything to make your battery's life any longer.

Some trolling motors such as my Traxxis have something similar on the head. Push a button and you can see the state of charge/discharge of the battery.

I used it once or twice, but that was it.

I have a voltmeter on my Nitro, but it is hooked up to the starting/electronics battery.

But, even voltage readings by themselves are not reliable indicators of a batteries condition. A load test is what counts. It puts a large amperage draw on the battery for a specified time. A "weak" battery will lose voltage quickly. One in good condition, not so much.

As a rule, older batteries will not hold the voltage of new. I have two for my canoe's trolling motor. When new, and fully charged, they were well over 13 volts. Now, after much use, when they are "fully" charged, according to the charger, they read about 12.4 volts, and they definitely don't have the punch they had when new.

It could be useful though it won't tell you if a battery is going bad as Rhino stated. I use this cheap, little Minn Kota tester. You can get it with a volt reading . . .

http://www.jamestowndistributors.com/userportal/product.do?part=167668&BASE

or you can get it with marks corresponding to full, 2/3s, 1/3, and 0 charge.

http://www.google.com/products/catalog?hl=en&q=Minn+Kota+battery+tester&gs_upl=1453l6857l0l7701l28l16l2l9l11l0l331l2820l1.11.3.1l24l0&bav=on.2,or.r_gc.r_pw.,cf.osb&biw=1440&bih=744&um=1&ie=UTF-8&tbm=shop&cid=2296434891490531481&sa=X&ei=FCafTp65Js_fsQLj8LDxCQ&ved=0CEQQ8wIwAA

However, when I am on the water I can determine with accuracy when batteries are getting low, based on how well the t-motor is pulling the boat.

I would recommend getting a 12 volt BDI (battery disharge indicator and a switch so you can switch it to each battery. That way you can tell if one battery is dropping more than the others, a good indication there's something going wrong with that battery. With one that only reads the total voltage, you can't tell anything about each individual battery.

Like this but for three batteries if running a 36 volt system. http://www.basspro.com/Battery-Gauge/product/5197/136656

Here's one from Cabela's that will do all but I think it only reads when you flip the test switch for each battery selected. http://www.cabelas.com/product/Boating/AutoBoat-Batteries-Chargers/Auto-Boat-Battery-Accessories%7C/pc/104794380/c/104698080/sc/104205780/Battery-Gauges/700417.uts?destination=%2Fcatalog%2Fbrowse%2Fboating-auto-boat-batteries-chargers-auto-boat-battery-accessories%2F_%2FN-1102373&WTz_l=SEO%3Bcat104205780

I made my own with Curtis BDI used in industrial lift trucks but I also installed a relay operated by the main power swich to turn it off/on. That way I didn't have to remember to switch the sucker off when I loaded the boat.

As said it will tell you the remaining charge but you will know that as the TM slows down. Probably nice to have but for me I'd opt for more tackle.

Most low end fish finders have a nice volt meter? built in. I'd go that route if you only have 1 battery.