Amperage question

[Pooch]

I've purchased a Eagle 640c to use on Fly-in trips. I don't know how to figure how large of a battery I'll need to get a full days fishing out of it as a portable unit.

The units draws 900ma with lights on and operates from 15 down to 10 volts. Below ten volts the unit shuts off. Fully charged SLA battery is 13.5 volts. So does that tell me that I have roughly 3.5 volts of usable battery available?

If my old battery is rated at 12 volts/3.3 amp hours, does that mean 3.3 amp hours fully discharged or 3.3 amp hours available of 12 volt power?

I currently have the unit hooked up on that same battery and running in simulator mode just to see how long it will run before shutting itself off. (transducer isn't being used so that could make a difference, too)

There surely must be a formula to figure this out.

Thanks,

Pooch

[Hot Runr Guy]

There probably is a formula, but I'm not smart enough to know it. I do think the transducer will make a difference, why not drop the 'ducer in a bucket of water, and let her run until she dies?
HRG

[Pooch]

That's next. I wanted a "base line" first. Then out of curiousity add in the transducer and run it again.

If I'm figuring it right, the 3.3 amp hour battery in decent shape will run it for about four hours (maybe).

Thanks for the interest HGR.

Pooch

[Guest]

If your unit draws 900 milliamps - thats .9 amps. You said your old battery is rated for 3.3 amp hours. Take 3.3 amp hours and divide that by .9 amps. This means your graph will run for 3.66 hours on your old battery. The math works like this - (Amp Hours) / (Amps) = Hours This is not 100% accurate but it should get you pretty close. Find out the amp hour capacity for your new battery and your are good to go. Generally, your unit will draw a lot less current (amps) with the backlight off.

[retire55]

Pooch:

My fishing is always to remote areas where I don’t always have access to power to recharge batteries. Consequently, I’ve always relied on deep cycle battery power to provide power to my sonar unit. I’ve been running a Lowrance X-85 for the last 8 years. It draws approximately 400 milliamps per hour with the back light off.

When I was shopping for a battery I made the following assumptions. For a 10 hour fishing day (with the sonar unit on), I’d use about 4000 milliamps of battery power in a day. Therefore, on the typical 6 day fishing trip, I’d use 24,000 milliamps of battery power. Based on these assumptions, I purchased a 33 amp hour 12 volt deep cycle AGM battery to provide power to my sonar unit for a 6 day fishing trip. The 33 amp hour battery theoretically can provide 33,000 milliamps of power. In 8 years, I’ve never once run out of battery power for my sonar unit.

The only possible draw back to this scenario is that a 33 amp hour battery typically weighs about 25 pounds. An alternative is to use smaller batteries but more of them. I could run the X-85 on 8 D-cell batteries connected in series for about 3 days. I’d require another 8 D-cell batteries to power my X-85 for the last 3 days. Eight 1.5 volt D-cell batteries, connected in series, provide the required 12 volts and D-Cell batteries can provide 18,000 to 20,000 milliamps.