This link should help answer your question.

http://www.texaskayakfisherman.com/forum/viewtopic.php?t=137923

Amp hour rating is key. Using AA batteries in series to get your voltage up to 12v is not going to increase the Ah rating of the batteries. What ever current (amps) your fishfinder draws, divide that into the total amp hour rating of your battery setup to get an average timespan that your electronics will stay powered. This is a ballpark estimate, because the climate the battery is in, and the remaining amps will affect voltage output. The colder the climate the battery is in, the less current it will be able to supply, and the lower the amps drop, the lower the voltage will go. It's a diminishing return system.

For instance:

If your fishfinder draws 4 amps RMS (root mean square), and you are using a 12v sealed lead acid dry cell that has a 9 amp hour rating, expect your electronics to last no longer than 2.25 hours (9 AH / 4amps = 2.25 hours).

Also keep in mind that these over the counter deer feeder batteries are not rated for deep cycling, and you will get decreased cell life as you continue to drain and charge it. This is due to the electrolysis happening inside the cell becoming less efficient as the ion deposition from the anode / cathode begins to deteriorate over time.

*edit* as an addendum, you will find that mass of a battery and amp hours are directly proportional. It takes more mass of the anode / cathode to prolong the electrolysis process inside the battery. You need to find an amp hour rating that will give you a timespan of life for your electronics that makes you comfortable and just go with it.

Amp hour rating is key. Using AA batteries in series to get your voltage up to 12v is not going to increase the Ah rating of the batteries. What ever current (amps) your fishfinder draws, divide that into the total amp hour rating of your battery setup to get an average timespan that your electronics will stay powered. This is a ballpark estimate, because the climate the battery is in, and the remaining amps will affect voltage output. The colder the climate the battery is in, the less current it will be able to supply, and the lower the amps drop, the lower the voltage will go. It's a diminishing return system.

For instance:

If your fishfinder draws 4 amps RMS (root mean square), and you are using a 12v sealed lead acid dry cell that has a 9 amp hour rating, expect your electronics to last no longer than 2.25 hours (9 AH / 4amps = 2.25 hours).

Also keep in mind that these over the counter deer feeder batteries are not rated for deep cycling, and you will get decreased cell life as you continue to drain and charge it. This is due to the electrolysis happening inside the cell becoming less efficient as the ion deposition from the anode / cathode begins to deteriorate over time.

*edit* as an addendum, you will find that mass of a battery and amp hours are directly proportional. It takes more mass of the anode / cathode to prolong the electrolysis process inside the battery. You need to find an amp hour rating that will give you a timespan of life for your electronics that makes you comfortable and just go with it.

Lowerance says the unit draws .25 amps/hour how does that relate to RMS?

just go to Academy and get one of their batteries used in feeders. they work awesome

*Dazed and Confused, possibly with a little drool in the corner of my mouth* Huh?

Andrew, that was a great explanation! I've talked about batteries on occasion for years, but no one seems to care. The main cause of the deterioration of ion deposition is the formation of sulfate crystals on the lead plates. This process (called sulfation) can be reliably reversed using pulse technology chargers. I have been using this tech for years, and rarely buy lead-acid batteries for anything.