Solotripper
I have never used those batteries but in this thread NotLight has made a battery holder with them.  I do not know if he has had them out for a real test yet but you might ask him.
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They are listed as 2000mAh and that would run most (non color) detectors for over 20 hours IF you keep the back light off.

The tube that holds the batteries is ½ inch PVC pipe.  You can use water pipe from the plumbing section of the hardware store or what I prefer is PVC conduit.  Menards sell PVC conduit in 4 ft sections and the PVC fitting for water pipe fit it. Cut a piece 17 inches long.  It will be cut a little shorter later when the exact length can be determined.
You will need two ½ inch PVC caps. Drill 1/8 inch holes near the center of the caps.
Two non-corrosive (brass or stainless steel) screws with 6-32 thread and five 6-32 nuts out of the same material. The best length would be ¾ inch to 1 inch and they need threads the full length. They can be any head shape but I prefer round head. Put a nut on one screw and tighten it against the head of the screw. Put the screw through the hole in one of the PVC cap with the head of the screw inside the cap. Add 2 ring lugs with the flat side of the lug toward the PVC cap and add one nut.   (Ring lug picture. (You need to Login or Register to view media files and links) ) Position the 2 ring lugs so they point 180° from each other and tighten the nut.  It will be difficult to tighten this nut in the future and it should never need to be loosened so make sure it is tight. Sand any burs off of the ends of the 17 inch piece of PVC pipe and glue this cap to one end. Mark this end cap as the negative end of the holder (NEG or –(black wire)). Set the glued pieces aside to dry.  Place a screw through the hole in the other PVC cap add 2 ring lugs and a nut. Position the 2 ring lugs so they point 180° from each other and tighten the nut. This nut should never need to be loosened.  Mark this cap as the positive terminal (POS or +(Red wire)). After the glue has had time to set on the pipe push the POS cap on the other end of the pipe firmly but so you can still remove it.  With a led pencil mark where the edge of POS cap is on the PVC pipe. Put 9 AA batteries flat end first into the pipe.  This is where you may need 3 or 4 hands.  Place the POS cap over the 9th battery and try to hold it straight.  It most likely will not reach the PVC tube.  Measure the distance from the edge of the POS cap to the led pencil mark on the PVC pipe. Take this measurement and subtract 2 inches from it and shorten the PVC pipe by this amount. With all the batteries out of the tube sand the POS end of the tube until the POS cap will slip on and off very easily. Put the POS cap on the PVC pipe with only the force you can apply with one finger and use the led pencil to mark this location on the PVC pipe.  Put 8 AA batteries flat end first into the pipe and add the POS cap.  The edge of the POS cap should be near (1/16 to3/16 inch) the pencil mark but not at the same location as without batteries. If it is in the same location as without batteries shorten the pipe 1/8 inch and re sand and re mark and re check. With 2 pieces of 1/8 inch shock cord about 13 inches each tie a over hand knot in each end of both pieces.  To tie the shock cord to the ring lugs use small nylon braid or heavy fishing line. Take about 7 inch of line and put it through the part of the lug where a wire would go and tie the ends together.  Use a larks head to attach the line behind one of the overhand knots in the shock cord. Connect the other end of the shock cord to one of the ring lugs on the other end of the holder and add the other shock cord between the other two ring lugs. Put the cap on the end of the pipe with less than 8 batteries in it and mark where the edge of the POS cap is on the PVC tube. Put 8 batteries in the tube and add the cap.  Note where the edge of the POS cape is it should be just short of going on as far as it did without batteries in the holder.  One more test pull the cap off about 1/8 inch and let go of it.  It should snap back to rest on the batteries if not sand to make a looser fit or tighten the shock cord.  Add a 6-32 nut to each screw to use to fasten your detector wires to the battery holder.  The holder is done but it needs to be mounted so the POS cap is free to slide on the pipe.  If it is going to be mounted with bungee cord so it can easily be removed all that is needed is a spacer to keep the POS cap from binding.  The lumber section of the hardware store usually has some wood about ¼ X 1.5 X 3 ft. cut 2 pieces about 1.5 inshes long and glue them to the side of the PVC pipe with GOOP.  What I prefer is mounting the battery holder to the side of the canoe under the gunnel.  If you don’t want to stand the canoe on end to get the batteries out of the holed the holder needs to be removable.  To make the holder removable I use TOOL HOLDERS like (You need to Login or Register to view media files and links) or TUBING HANGERS like (You need to Login or Register to view media files and links) .   The TOOL HOLDERS are easier to mount and hold tighter but are harder to get.  Both need the mounting screw put all the way through the wood and sticking out the back side then removed and shortened and reinstalled.   
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To use the TUBING HANGER pull the nail out and throw it away.  Drill a hole 90° to the nail hole and the pipe direction thru the plastic for a wood screw.  Use a larger drill bit to make a relief for the head of the screw.  I used a ¾ inch flat head wood screw which stuck out the back of the wood less than ¼ inch.  I used side cutters to cut it off and a file to reduce the bur before re installing.  To mount to the canoe I placed the clips on the tube.  Tipped the canoe up on its side so the place where I wanted to mount the holder was down.  Smeared a thin layer of GOOP on the bottom side of the wood and pressed it into place and let dry. You are ready to add batteries and connect the detector.

Well I pulled the trigger and bought a PirahnaMax 165, a basic no frill unit.

Thanks for link MP. I ordered a 10 AA battery holder after reading that 10 give you much longer battery life than 8.

I also bought 12 Panasonic Eneloop Lithuim batteries and a nice charger.

I was in Radio Shack getting some stuff for my power pack build and noticed they have battery holders for 6 AA's.

That made me wonder IF it would be worth it to use 2 x 6 AA holders wired in series as a power source?

I have the extra 2 batteries anyway, but am I thinking wrong about IF 10 is better than 8, 12 would be better than 10?

Unit draws 100 mA ( no backlight).
Humminbird says between 10-20 volts, so with 18, I'd be under that, but is there any benefit from doing it?

solotripper wrote on Jul 29^th, 2014 at 1:42pm:

Solotripper
I did not know Eneloop had Lithium or rechargeable Lithium batteries.  Where did you buy them?
I could not see any improvement in my detector performance above 10 volts.  I did see the unit drew more current the higher the voltage went.  So from what I was able to measure if you use more than the correct number of batteries they will last less time. If you do less batteries than the optimum number they will power your detector correctly for less time although they will still have considerable power left in them it will be at too low of voltage to properly operate the detector.
For non-rechargeable batteries I recommend 8 maybe 9 but not 10 Lithium batteries in series.  8 will give you the most hours of operation for the buck and will out last 10.
If you are using Alkaline I recommend 10 in series. I have had better results with Duracell batteries than any other Alkaline.  Duracell also sell a Duracell Quantum which has 33% more power than regular Duracell.  Costco sells both Duracell 2022 (regular Alkaline) and there Duracell Quantum. 
Most rechargeable batteries ( NiMH or NiCad) are lower voltage than the non-rechargeable so they require 10 batteries in series.  I have never tested the kind you bought.  Can you measure the output voltage of your 10 pack? Your fish detector may have a voltage readout if you do not have a meter.
EDIT
Just thought of something did you get Li-Ion or LiFePO4 batteries?  If you did do not connect 8 or 10 of those to your detector.  They are 3.7 or 3.2 volts and you only need 3 or 4 of them.  They also need a protection circuit to prevent over charge and discharge.

Your batteries are Ni-MH (Nickel-metal hydride). I looked up some discharge curves on the internet and they run out of power at about 1.1 volts. Another way of expressing that last sentence is as the battery is discharged at a constant current the voltage will slowly go down until it gets down to about 1.1 volts and then drop rapidly.  If you have 10 of those in series when they are fully charged they will put out about 14 volts but will drop to about 12.5 volts in about 2 to 4 hours of detector use. Then the voltage will stay fairly constant until the detector has been on for 14 to 17 hours. At 18 to 20 hours the voltage will be down to about 11 volts.  About 20 minutes later the battery voltage will be so low the unit will not work.  The math says 10 of those batteries (1900mAh to 2000mAh) should run your detector (100mA) for 19 to 20 hours.  I would need 3 sets of those for a 10 day trip.