I put together a battery pack using the 18650 rechargeable batteries. It may be overkill but I mounted these inside a waterproof Pelican case. I plan to use this battery pack for ice fishing as well as canoeing so I like the idea of the batteries being in a rugged waterproof case. I am getting a voltage reading of 20 volts with fully charged batteries. This is right at the upper limit for my Gamin sonar unit. There is enough room in the case that an spare set of five will lay in above the set of five installed in the battery holders.

sweet. Invent a combo charger/shade thingy that simultaneously charges and shades the unit for easier reading and give me half the money. :)

Yup good job Dave.

How long do these batteries last? Will a set get me through a 10 day trip? How expensive are they? Where can one find them?

Very nice setup.

Cool idea! For this year my plan involves a lawn
tractor battery.

Hope it works...

quote old_salt: "How long do these batteries last? Will a set get me through a 10 day trip? How expensive are they? Where can one find them?"

(A) Don't know yet. Haven't actually used it yet. I am optimistic based on my research.

Specs say .8 amp draw on the Garmin @10 to 18 volts w/ high voltage protection. I'm hoping my 20 volt reading will be tolerated OK till it starts dropping. If it faults out, I can pull one battery for if needed. (I will need to substitute a jumper for the pulled battery)

One set of batteries will put out 5 amps @ full draw down. If I get 4 amps before dropping below the 10 volt cutoff point, I should get 5 hours of run time. 5 hours actual run time should get me two full fishing days, based on only using the sonar for trolling and checking depths occasionally when searching for structure.

I also have a Vexilar FL18 I can use. Specs on that say a power draw of .175 amps and 10.5 to 15 volts, which is significantly less than the Garmin unit. I could run longer and would only need 4 batteries per set (Total peak voltage would be 16 volts). The disadvantage is the Vexilar is a flasher and the Garmin has an LED screen. (which is why the Garmin draws as much power as it does.)

What I really need is a smaller LED screen, but these are the units I already have on hand.

As to the batteries themselves; I bought these off of Ebay for $1.00 each including shipping. They are rechargeable, dirt cheap and I've used them in my flashlights with no issues for several years.
Compared to a SLA battery, these weigh less and hold more amp hours per pound and bulk. I bought twenty of these batteries which gives me four (5)battery sets with a total amp hour of 20 Ahr. A typical SLA battery for a Vexilar is 7-9 Ahr and cost around 20 dollars also.

Pretty good idea with good amount of voltage.

Maybe this will help in your comparison:
I've run my Humminbird PiranhaMAX 210 Portable off of the standard set of 8 AA Energizer Advanced for seven 3-day trips, probably 6 hr days. Always carried a spare set of 8 and never needed them. Another unnecessary 4.3 oz carried... :-) Replacing the set this year -- maybe even with Ultimate Lithium -- and using the first set of 8 around the house. These guys say that mine draws 50-100 mA (light off/on).

quote schweady: "Maybe this will help in your comparison:
I've run my Humminbird PiranhaMAX 210 Portable off of the standard set of 8 AA Energizer Advanced for seven 3-day trips, probably 6 hr days. Always carried a spare set of 8 and never needed them. Another unnecessary 4.3 oz carried... :-) Replacing the set this year -- maybe even with Ultimate Lithium -- and using the first set of 8 around the house. These guys say that mine draws 50-100 mA (light off/on).
"

That is about 1/3 the power draw of the Vexilar so I am not surprised by your results. I so need a newer unit with a smaller screen to reduce my power consumption.

quote Huntindave: "

quote schweady: "Maybe this will help in your comparison:
I've run my Humminbird PiranhaMAX 210 Portable off of the standard set of 8 AA Energizer Advanced for seven 3-day trips, probably 6 hr days. Always carried a spare set of 8 and never needed them. Another unnecessary 4.3 oz carried... :-) Replacing the set this year -- maybe even with Ultimate Lithium -- and using the first set of 8 around the house. These guys say that mine draws 50-100 mA (light off/on).
"

That is about 1/3 the power draw of the Vexilar so I am not surprised by your results. I so need a newer unit with a smaller screen to reduce my power consumption."

Yeah. Not to steal your thread, but I couldn't be happier with my unit. Total weight 2 lb 7 oz, with batteries. (I don't bring the suction cup/bracket shown in the stock photo, just shoot signal through a wet hull.) Too bad it was discontinued and replaced by a slightly heavier, quite bulkier solution.

.8 amps is a lot of draw from a fish finder. All the extras like GPS, mapping, and color really add up. I have a little 'bird with all that and side imaging and supposedly draws .65 amps. I was surprised how fast it drew a lawn tractor battery down to 11v where it shuts itself off, supposedly to protect the electronics. I dug out my old no-frills Eagle and it ran close to 10 times longer.

I think Schweady is on to something...