What kind of battery do I need for My Kayak Fish Finder?
One of the most commonly found accessories on fishing kayaks is a fish finder. My first fish finder was according to the manufacturer, a depth sounder. There was always a disclaimer added that this device was really intended to find fish attracting structure although fish did sometimes show themselves. I never found this to be otherwise until I put a much more updated one on a kayak. I used to see what I thought might be fish but I was never able to confirm it. I did confirm on one of my sounders that the fish symbols it was showing were in fact weeds. Maybe it's the new technology or maybe it's the stealth of the kayak but I have seen and caught fish with the combination of the two and others have had the same experience. In a kayak a FF will help you see and catch fish. It is however a kayak and not the Bass Boat for which the device was intended so where is the power going to come from?
The easiest choice when we first started looking for FF power was the 7 AH (ampere-hours) SLA (sealed lead acid) hobby battery. They were available for $29.95 plus shipping from Cabela's among others, charger included. The price is now down to $26.95. There are other more elegant solutions to which we will get but price and availability are powerful arguments. I did what everyone does when they don't fully understand the problem in choosing this. I overdesigned. The fact is that most of these smaller FF's are not at all power hungry. While the design current is usually 200 ma (milliamperes,) it doesn't always need this much. The average draw is less and depending on which FF, possibly much less. In any event 7 AH is way more than you need. On the plus side of these batteries is that they are completely sealed and will operate upside down if necessary. They are really quite rugged and will survive discharges that would kill a wet cell. The big problem in my view is the weight and density. It's a 5# brick that you really have to tie down to avoid it causing damage over time if you let it bounce around. It can also unbalance a hull if you don't mount it on the bottom. The most frequent complaint I hear about almost every kayak is about weight. You almost have to take one of these things out before you load it on the roof of your vehicle and put it in before you launch. In the first case you're tired and in the second you're in a hurry.
There are also 4 AH SLA's available that are still more than you need but the weight problem is now reduced to just 3#'s. It's a better solution but not the optimal one anymore. One of my engineering Profs used to say that "optimal design has it that all the parts fail at the same time." Obviously it was generally hoped that that would be a long time but the point was that if the wheels are falling off the car what's the point of having a still perfectly working engine. A less durable engine might have been cheaper and yet no harm would have come of it. That principle is behind my thinking on all this stuff. Too little is worse than too much but too much is not a virtue. With that in mind I proceed to something smaller and more optimal.
There is nothing quite like a competitive market when a solution is needed and the digital camera market is on fire. With multi-megapixel capacity and full color screens you have a battery hog of the first order. This has driven the industry to develop ever-larger capacities with the same basic sizes. As I write this the biggest capacity in AA is 2500 mah in NiMH (Nickel-metal Hydride.) That's 2.5 AH and as it turns out it's still more than most of us need. You need 8 of them to achieve the necessary operating voltage but fully charged they will run a typical FF for between 11 and 30 hours in my experience. Total weight would come in at well under a pound. They have to be connected in series. This is just like a flashlight with positive touching negative in a column. The AA's fit nicely inside 1/2" PVC and you can wire up a couple of end caps with a little ingenuity. Spring load one end just like a flashlight Alternatively, you can buy a battery holder from Radio shack and seal it inside a waterproof bag. The advantage in mounting is obvious. Tape would hold something this light in either case and if it got loose it wouldn't hurt a thing. If you want to go really light then AAA's could get you a full day of operation. NiMH AAA's are now coming out with as much as 800 mah. That's roughly equivalent to an alkaline AA. These will fit snugly inside 1/2" CPVC pipe. I don't know why the C makes 1/2" become a different size but it does. You can carry 8 AAA's in your shirt pocket. You can leave this in the yak except for recharging and never give it a second thought. The hidden advantage of either of these mounts is that you can get the batteries up off the bottom of the hull and away from water that tends to collect there. I use a length of 1/2" PVC pipe and snap it into a broomstick holder that is pop-riveted to the underside of the deck. Jon uses the RS battery holder inside a waterproof bag that he places inside the bag in one of the hatches.
Incidentally, the operating voltage of an NiMH cell is 1.2. Eight obviously gets you 10.6v when connected in series. Since most FF's are designed to run on 12 volt boat batteries will this work? Not to worry. The literature says the operating range is 10 to 15 volts. Most will probably work slightly outside of this range but 10.6 works just fine. Incidentally, I emailed Humminbird on this and they said you need 12v to operate properly and my Matrix 10 will draw 1/4 amp. When I questioned them further they said they will operate on 8 volts. Society's litigiousness makes most companies err very much on the safe side when speaking in public. We all know we don't need the full 12v and I estimate the average draw on my Matrix 10 at 165mah.
As far as price and availability go, EBAY is the place. You can get chargers there as well. Keep in mind that the very latest and highest capacities sell at a pretty substantial premium over even the next size down. Don't buy more than you need. You have you ask yourself that question if you can get ten hours out of AAA's. Also make sure you get an automatic charger. These will shut off or cut down to a trickle at full charge. It's possible to ruin a battery by discharging it completely but most FF's will shut down on their own before this occurs. It is far easier to ruin one by overcharging. Properly cared for you can easily get 500 recharges out of these things. Too, most of the literature suggests that full capacity isn't reached until somewhere around 50 cycles.
There still remains the question of powering multiple devices. If your going to power a running light, possibly a compass and a GPS wouldn't you be better off with the 7 AH lead battery? Well first of all the running light of choice in my view, is powered by three AA's. They are held inside the tube that acts as a mast. It is completely self-contained. The light is one of those new bright LED's. They are extremely efficient light producers. A set of alkalines would probably last most of us a season. As an aside, NiMH batteries lose their charge over time. In this case alkalines might be a better choice since they have a much better shelf life but no need for an outside power source here. Just load them up in March and yank them out in December. Most GPS receivers operate on 3 volts, are self-contained and can operate as a compass if you want so the only thing that needs a separate power source is the FF. The current trend seems to be combining a GPS and a FF in a single unit. This is particularly attractive to a space-starved kayaker. There has to be some additional demand on the power source as a result but I would be very surprised if it was a significant number.
The bottom line question then is how long will my FF work on a single charge? Well FWIW, I ran tests my older Bottomline 1100. This is a small very basic unit. The display is only 128x64 with a maximum depth of 240' and peak to peak power is 1000 watts. The batteries are 2300mah NiMH's. They lasted about 33 hours. I then tried the same batteries on my Matrix 10 with GPS. This has a display of 240x160 and peak to peak power of 2000 watts. It got about 11 hours. All this in my garage, installed in my sawhorse-mounted 120. I have a set of 8 1600 NiMH's that lasted about 7 hours. Bigger displays, multiple transducers, more power and color displays all use more juice. You have to run your own tests. Take a hint though from the specs of the unit. Use the wattage figures for comparison. Don't take them literally. 2000 watts is probably enough power to run a 50 inch projection TV.
Another thing about NiMh batteries is that they tend to self-discharge. If the last recharge was a week ago you could have lost an hour's operation versus a fresh charge.
A final word about corrosion. Whenever dissimilar metals are in contact, corrosion is waiting for its chance. All it needs is an electrolyte. Salt water does just fine, thank you very much, but it's not necessary. Even the moisture in the air can get it started. If you throw in an electric current it's like throwing gasoline on a fire. The key is to keep everything dry. That's why they keep all those old planes in the Arizona desert. The most practical way of doing this is with a water dispersant such as WD-40. Several of us have had terrific results with a product called CorrosionX. Water simply doesn't collect on the protected surfaces. It is important to seal the batteries to keep them dry as well but CorrosionX is even more so. I love this stuff. Just lightly spray all exposed metal. You don't have to do this often but a few times a season should be enough.
A few thoughts on the care and feeding of NiMH batteries. They can under unusual circumstances exhibit memory but you'll not likely see it. You have to really discharge them down to nothing but a good charger might be able to bring them back. My father-in-law was no threat to Shakespeare when it came to quotable quotes but " don't ever buy cheap tools or cheap paint." was well worth remembering. I would add DON'T buy a cheap charger. Make sure it's an automatic. Keep it ventilated. If it's got a cover keep it open. They lose their charge over time so don't expect to fire them up next spring without a recharge.
I don't see a need to go further but if you want here's the basic electricity behind all this. And god knows how much else. Imagine if you will that we have a supply of battery sized cylinders filled with water. They have connectors at each end so that one can flow through to the next. One also has a spigot at one end that points straight out. If we set that one on its end with the spigot at the bottom and open the spigot water will shoot out a certain distance. If we then start putting the others on top one after the other the distance the water shoots will increase. In fact it will shoot a distance directly proportional to the number of cylinders. The height of the cylinders is called the head in hydraulics. It is directly analogous to voltage in electricity. If we connect batteries end to end, positive to negative the resultant voltage is the sum of the voltages of each of the batteries. Voltage is the electrical equivalent of pressure in water.
Going back to the water cylinders obviously the smaller they are the less water they can shoot even though they may be able to shoot it quite far. This is where amperes or amps come in. Amps are a measure of volume. You can have lots of volts but without amps not too much gets done. As kids we built spark machines with door buzzers and spark coils from old cars. We powered them with batteries. The resultant voltage was probably about 35000. It could generate a continuous spark about 11/2" long. It wasn't fun to have it go through you but the amperage or current was too small to do much harm. There just isn't enough POWER in a couple of dry cells unless you store it and release it all at once. Fortunately we didn't know that then. Bigger batteries as you must deduce can supply more amps.
To further confuse you in practice a volt is a relatively small quantity. Most household items operate on 110 volts as you know. A 200 Amp service is usually enough to supply even the very biggest houses. I don't think anything in my house sucks up more juice that the 12 amps that my wife's self-propelled vacuum cleaner. That's almost 2 horsepower!
How did I get that? Well that's next.
What we always talk about is amps and volts and amp-hours. What we should be talking about is Watts. A watt is equivalent to one volt and one amp or the equivalent. The number of watts consumed by a device is the product of the number of volts multiplied by the number of amps. The real question we should ask is how many watt-hours will our battery setup provide on the one hand and what average wattage does our device consume? The only way to find this out is to experiment with whatever devices you have. To further complicate things the rate of discharge can affect the number of watt-hours you can get. Fast discharges under heavy loads tend to give less. Too, if you're FF is set on automatic ranging it could require less power in shallow water than in deep.
Oh yea. A horsepower is equivalent to 745 watts. The vacuum cleaner is drawing 12 amps at 120 v so that's 1440 watts. So when you see the specs say 2000 watts peak to peak remember that that's only for a small fraction of a second and is useful only in relating to other FF's.
So there you have it. It really isn't that complicated is it? The really good part is that as long as you're using AA's or smaller you really can't do any harm. Don't use more than 8 batteries and you'll be fine. The worst thing that can happen is that it won't work. Too, be warned if you've never used a FF before you won't want to be without one again.