I do not know what size the Ryobi one plus is but in the video they say if you step up to the lithium plus you get 4AH.  That is stated as a 18 volt battery.  I do not know what voltage a fully charged 18V battery puts out but using the same ratio as some of the stated value verses actual full charge voltage I would estimate fully charged lithium would be around 20 volts.  If this estimate is accurate I doubt it will destroy the detector but it is much higher than required to get good detector operations and my detector does draw more current the higher the voltage goes.  Many of the small detectors draw under 100mA and are probably still under 100mA at 18 volts. At 100mA a 4AH battery will last 40 hours.
If you want light weight 8 Energizer Ultimate Lithium batteries weigh 4.2 oz.  Today I built a water resistant PVC pipe battery holder that weighs   5.8 oz including the mounting hardware.         A set of 8 Ultimate lithium batteries will provide just over 3 AH at between 10 volts and 12 volts.  That will run the same fish detector 30 hours.  Two sets of batteries plus the holder would weigh 14.2 oz and power the detector for over 60 hours. Sams Club sell 8 batteries for about $15.
If you are willing to carry a little more weight look at the following battery
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This is a 5AH battery.  I have one and have tested it. I get over 4.9AH out of it. The price is $22.07 and if you spend a little time looking at it a popup will offer it to you at $20.89.  It needs a protection circuit added to it and information on that is 2 pages back in this thread. The protection box will cost about $10.  That will run your detector for 50 hours. Add another battery and you got 100 hours for less than $60.  I describe a way to charge it through the protection board and a resistor with an automobile battery charger.  I have charged my batteries this way several times. This battery weighs 13.8 oz and adding the box with the protection board in it brings the weight up to 15.6 oz. 

i know enough on how to make small electronics do what i want but all the technical stuff is way over my head. i found this article (scroll down to the fish finder portion) and then it has some really neat fish finder/battery info

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That was a good find jaximus  up

Anyone have any idea what units would draw that 28-35 mA ?

Using 8 alkaline batteries is a very common mistake.  Manufactures will picture alkaline batteries in there battery holders that hold 8 batteries. That said there are some detectors that list the operating voltage down to 7 volts.  Those detectors would use most of the power out of a pack of 8 alkaline batteries.

Dr. X,
I like the Rigid brand as well, but my weed whacker crapped out and they had a Lithium/Ion/corded one from Ryobi on sale, so I went with it.
Came with 1 battery and a charger that will charge all their batteries.
I'll probably work on using one of their battery offering for my Sept base camp trip?

Hopefully I won't have any battery failures with the Ryobi?
I must say I was duly impressed with it initially.
Good packing, quality construction, and you can actually understand the directions.