Magic, perhaps your power requirements on your detector are greater than my basic unit.  The two Ryobi batteries tested may be getting a bit tired as they are over three years old, but they have been sufficient for three 7-10 day trips so far. 
So, so far for me,
Rechargeable Ryobi Lithium Drill Kit Batteries
2 batteries per trip (7.94 oz. each)
Complete Ryobi Kit (drill, two batteries, charger) $70,
3 Trips (so far) = $23.33 per trip
16 oz per trip
As the batteries may be getting tired I may add another for the next trip, which will bring the weight up all the way up to 1.5 lbs.
Anyway, whatever works, works.

MP/QUENTIN,

I'm glad your discussing the power/weight options between the AA Lithium packs and those 12volt drill packs.

I'm in the market for a new cordless drill. I see Ryobi has some 18volt rechargeable batteries/charger that you can retrofit previous models with. They have a long life model/and a compact version as well.

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I also see that some of the fish finders will handle 20 volts.
I see there rated at 4Ah which if I understand right is equal to about 4000mAH, is that close?

They're considerably more powerful than the 12 volt and have a built in battery level indicator which might be a nice feature.

Weight is always a factor, but I read where you have to be careful how you re-charge those AA so you get maximum benefit.

I like the idea of getting dual use out of something.

Quote:


I couldn't find weights for the 18v. I'm sure there posted somewhere but I doubt there lighter than a 10aa battery pack, but if your taking 2 of those and 1 18v pack would do the job, it might be a fair trade-off?

I looked at a few different low end fish finders.
At least two of them had the high rating at 20v.
I did a little Googling and some people say those high end voltages are deliberatley set lower than the unit could take if you had a spike in current.
Your the electric guru, so I would take your feelings into account IF I ever decide to go that route?

Although the 18 Volt  battery may be at 21 Volts unloaded as soon as a load is applied within seconds it will probably drop 1 to 1.5 volts.  I doubt that would damage a detector does it do you any good to have that higher voltage.  I ran my detector on an adjustable powersupply to and measured the current  and I even found my old data.
Eagle fish Mark 320 current draw. Voltage is
power supply meter with a fluke multimeter
in series to measure the current. Transducer is
glued to bottom of canoe not in the water.
Volts----Imin----Imax------Iave
8--------84ma----332ma--104ma
9--------88ma----372ma--111ma      
10-------88ma----456ma--116ma      
11-------92ma----476ma--121ma
12-------92ma----476ma--131ma
13-------96ma----612ma--132ma
14-------96ma----448ma--136ma
15-------100ma---492ma-141ma
16-------100ma---532ma      -148ma
17-------104ma---560ma      -153ma

As the voltage goes the current also goes although not by the same ratio.  If the detector performance also went up it might be worth it.  I do not think there is any improvement in detector performance above 11 volts.  My detector works well at 10 volts.
The 4 Ah 18 volt battery is probably made out of five 4 Ah 3.6 volt batteries in series.  If you run my detector on this battery it would start off drawing more than 153 mA and when the battery was drained it would be drawing about 140 mA.
If you cut that 4 Ah 18 volt battery apart took 2 of the cells and cut them off and threw them away the battery would be lighter and run the detector for longer.  The voltage would be between 10 and 12.5 Volts which would have a current draw of about 120 mA. To see the voltage profile of a 3 cell LiPo battery see the second graph on the page.  It is labeled LiPo5800.

I was reading through this plethora of information, I don't think anyone answered SolTriper regarding the use of 9volt  batteries; I like the idea for size and weight-but can these be used for depth finders? I also liked the Ryobi battery pack idea too, its just coming up with a receiver kit of some sort to plug it into, which lead to the idea of a few LED light mounted under the gun rails for in board lighting for night fishing.

Windsailor