how many amphr battery for sonar plus light and 6hrs oper?

I want to run my sonar AND underwater fishing light for a maximum of say 6 hours...would a 12v 7ah do?

Any ideas w/be appreciated...

Really depends on the electrical requirements for the light

The 7 Ah battery will power the fishfinder alone for probably 20 hours or so in my experience... maybe more.  

I use a 5 Ah battery for my bait tank and it runs the pump for about 6 hours.... the 5 Ah runs my Fishfinder alone for about 12-16 hours.

Erik

Underwater Light? When and where do you go out at nite? I'm in SF as well, let's hook up for some nite fishing.

From sbennett...courtesy of kayaksportfishing.com...

The key thing to look at in batteries is the Amp-Hour (AH) rating. This tells you roughly how long you can supply how much current to whatever devices you're powering. Usually, the owner's manual will tell you how much current each of your devices will draw.

I haven't gotten around to wiring anything up on my yak yet, but in reading around, many people use a 12V, 7AH battery. Ideally the rating implies you can power 12V devices drawing a total of 7A of current for up to an hour. Or, 0.7A for 10 hours, 0.07A for 100 hours, you get the idea.

A typical fish finder (Eagle Cuda 128) draws about 110mA (0.11A) if you don't use the backlight. The cheapest of these Rule bilge pumps I can find info on (Rule 360) draws about 2.1A. So if you run the fish finder all the time and the pump 20% of the time, a rough idea of your current requirement is:

0.11 + 0.2 * 2.1 = 0.53A

This says you'd need a 5.3AH battery to operate under these conditions for 10 hours (0.53 * 10 = 5.3).

Naturally it's not quite that simple. Batteries drain faster at higher temperatures, and if you load them more (ie. supply more current to more devices), they'll also drain somewhat faster since the load will cause the battery to self-heat. Plus as the battery ages and goes through more charge/discharge cycles, it will also degrade over time. But if you plan for that, you should be OK. I would think the 7AH battery will get you a typical day's fishing using a fish finder and a low-current Rule pump, but I would probably use a higher AH battery for anything more than this.

Scott

Like Scott says ... it's not quite that simple.  There an post I made some time ago about Ampere ratings and use time.  It's not linear and there are so many other factors that go into it.  Let's skip all the mumbo jumbo this time as we're all older and wiser now.  Also ... I can't see to use search very well so the exact post eludes me (I'm sure Joel will find it).

I've never heard anyone say those 7 Ah batteries weren't enough.  I've heard lots of people say they are too heavy.  I agree with both.  I've never run a 7 Ah battery dry on a single day, powering a Matrix 17 only.  In fact I've used it for 2 days many times and never run it dry.  That's probably 10-14 hours of use for me.  I usually charge it by then, but I've used it for a 3rd day and didn't run dry either (except I try to conserve on the third day and don't have it on all the time).  Bottom line, I've never run it dry.  Once my 7 Ah batteries die, I'm going going to try the scwafish AA battery packs.

Oh ... I also have a 6V 5Ah battery running my bait tank.  I've  never run that dry either and I tend not to charge it that often, but I don't use it as much either.  6V because Sean built the tank and thought 12V pumped too much water ... I agree with him.

-Allen