What battery should I get for Hummingbird 688ci di?

As a general rule of thumb, assume about 75-80% "useable" battery before voltage starts to drop below "useable" voltage for most units.

I don't think batteries are rated that way.  I think you get 100% useable battery when it is brand new out of the factory and that's how they are rated.  Through general usage in our application (or even just sitting on the shelf), you quickly go to 80% capacity or less, and then it is a slow steady decline from there until the battery dies months/years later.

-Allen

It's not, persay, a "rating".  As you use a battery, even the first time, the voltage output will decline from "full charge" of ~13.2V to ~10.2'ish V as you use that battery.  Some brand units (ie. FF/GPS) will shutoff when the voltage drops below a threshold.  I've found that most batteries will drop below that "threshold" at around 80% of rated cpacity.  Some higher ~ 90/95% but others are much lower.

Bill

We're sort of saying the same thing, but not entirely.  The problem is that we use these batteries more in a deep cycle fashion.  It only takes a few deep discharges to reduce their capacity to 80%.  it's less a function of the fishfinder, and more a function of how we use the batteries.

-Allen

Correct.  Different math behind say a "starter" load.  We're on the same page my friend!

When working in the industry, I always liked to premice a 80% with the customer so that their expectations weren't "best case" but rather "normal case" scenario.

~Bill

I use a 7ah sealed gel battery for my 788ci or 597 Humminbird, it will last a day easily.  I use a battery tender to charge the battery after each use and I do a top off charge once a month. 

With this math of yours Polepole.....1.1A draw*20hrs=22 using Elite 5xHDI  specs I'd need a 22Ah battery....Really?

Current 2014  Humminbird catelog shows all the 600 series as 615Ma. The 500 series are 200Ma.
I use a battery box power sys from OEX and it is a 10 amp. Had for 3 years and never a problem.
I am also looking at the 698ci HD SI combo for new kayak.

Somewhere I saw a Humminbird 700 series rated at .650a or so, but it qualified that as being without backlight. The Lowrance spec just says 1.1a draw. Wonder if thats with backlight. How much time you getting out of your 10Ah battery?

With this math of yours Polepole.....1.1A draw*20hrs=22 using Elite 5xHDI  specs I'd need a 22Ah battery....Really?

The math doesn't lie.   

But, you're probably seeing that there are a lot of nuances to it.  The first assumption is that you are actually drawing 1.1A.  Another factor built into this is that the usage model assumes optimizing for battery life.  If your typical day is 8 hours on the water, you could use an 8.8 Ah battery (actually a 9) and it will work.  But your battery lifespan will suffer.

-Allen

Thanks for the reply Allen. Well, if I use a totally uneducated guestimate of something in the neighborhood of .85a x 14hrs (two 7hr days) that suggests a 12Ah battery, which is doable. Not much bigger and not much more money but jeez, 10 lbs. Any relatively simple way to measure the amp draw of a depthfinder to really know what's going on?

Thanks for the reply Allen. Well, if I use a totally uneducated guestimate of something in the neighborhood of .85a x 14hrs (two 7hr days) that suggests a 12Ah battery, which is doable. Not much bigger and not much more money but jeez, 10 lbs. Any relatively simple way to measure the amp draw of a depthfinder to really know what's going on?

Use an ammeter to measure it!!!

I know what you mean about weight.  In this scenario, I'd just get two 7 Ah batteries and plan on having to replace them regularly.  Wait, that's what I do now!

-Allen

Two 7's is probably a simpler solution.    ....but I think I'll end up with a 12Ah. Thanks Allen for the help and article...I'm much better informed now.

Using the 10 amp I have never run out of power. I originally bought the 10 amp because I was going to also run the live well and FishFinder.etc It also came in a water proof box that solved the storage issue also. They didnt have battery bags when I set mine up. Water proof box to me is a must. Wet battery will drain down faster.

Your article (great article by the way) stated that....   "A 7AH battery can supply a 0.350 ampere (or 350 milliampere) current for 20 hours when the battery is in the new condition. In this case 0.350*20=7. However, this does not scale proportionally for higher currents (or lower currents) and it also reduces over time.

How does one apply a curve for different currents....or specifically .7amps or the Elite5x HDI's claimed 1amp draw? 

Your article (great article by the way) stated that....   "A 7AH battery can supply a 0.350 ampere (or 350 milliampere) current for 20 hours when the battery is in the new condition. In this case 0.350*20=7. However, this does not scale proportionally for higher currents (or lower currents) and it also reduces over time.

How does one apply a curve for different currents....or specifically .7amps or the Elite5x HDI's claimed 1amp draw?

Replace the .350 in the math with .700 (or 1.0).  That will tell you what size (Ah) battery you need to run for 20 hours.

-Allen

The Elite 5/7 HDI units draw right at ~1amp with everything at it's maximum; it goes without saying that it is less with no, backling, dimmer screen, etc... (down in the mA range). 

The Elite 5/7 HDI units draw right at ~1amp with everything at it's maximum; it goes without saying that it is less with no, backling, dimmer screen, etc... (down in the mA range).

I just tested the 5xHDI today and even though the manual claims a 1.1 draw it was running between a .35 and .45amp draw. This wasn't in the water but under most of the variables. I'm waaaay more optimistic of battery life especially since I just put a 14Ah battery in the back hatch.        I may know better tomorrow.

I went to Napa auto parts today and bought a 12.0 ah battery. Listed weight is 8.4 lbs.. I came home and measured my 7.0ah and it came in at 5.4 lbs. With my 20 % discount + tax it was 53 bucks. I recently bought and installed a Humminbird 688 ci HD combo. No DI. It drew the 7 ah down to non operational in 5 hours. It is a used battery that I have run and recharged for two years. New it was 25.00  Ill let you know how it holds up when I use the 12ah next if that helps.