a high quality wet cell marine deepcycle battery will do fine at that charging voltage (AGM & gel batteries won't handle it very well) -- look for a voltage regulator at radio shack, mount it under the dash wired between the battery and the fishfinders.

Really? Radio shack? Are they sold as voltage regulators, or do I have to ask for something else? Good to know...thanks!

$1.59 12V regulator:
http://www.radioshack.com/product/index....rentPage=search

max input = 35 volts, max output = 1 amp...wouldn't be good enough to regulate power from the alternator to the battery, but one ought to power both fishfinders. I've heard gps units are more power hungry.

it will drop a couple volts, so when the engine's not running & battery drops to 12.6 volts, output will be around 10.6 - most Eagle & Lowrance fishfinders tolerate 10 to 17 volts, don't know about others. they make a 15V version ("7815") if you can find one, might work better.

you'll have to solder the wires - hot wire to pin 1, ground to pin 2 and output to pin 3. what I'd do is, make a "plug & play" assembly by soldering short lengths of wires to the chip, and epoxy the whole thing to a small piece of plastic that could be fastened under the dash. otherwise the soldered joints might not hold up to shock & vibration. add some connectors to the wires & you're good to go.

I don't think you'd need a heat sink in this application.

Wow...cool. Have you actually used this product, or something like it? I could always have two, one for each fishfinder, if necessary.

Also, I see that its only a 1 amp reg...that is the output, but is there a max on the amperage going in to the reg? Come to think of it, I have no idea how many amps my motor puts out, and that reg looks so small.

That leads me to another question...what about my bilge and live well pumps? Are they okay at the higher voltages?

I've used those on other projects but not on the boat. to read more about it, google "7812" and click on the National Semiconductor or Fairchild Semiconductor links. Should have mentioned, I have no formal training in electronics, just like to dabble. It's not 100% certain to succeed, but that's a reliable device and I wouldn't hesitate to try it in your situation. only thing is the voltage drop - typically the output voltage will be at least 2 volts less than input, which I'd expect to still be ok...only downside to that is, voltage drops below 10 and the sonar cuts off, puts you back at square one.

generally speaking, electrical devices draw whatever amps they require, up to the capacity of the power source. your alternator is probably rated at nine amps. But the regulated circuit I described above would be independent of the battery charging circuit. the current flowing there would just be whatever is used by the sonar, which use just a few milliamps (1/1000th amp). you should have a 1-amp fuse on that circuit anyway.

pumps, lights, etc. have no electronic circuitry - pretty sure they can handle the extra voltage

Your best option, in my opinion, is to add the OMC regulator. Tommy will get you everything you need to install it. To much voltage is dangerous in my opinion. The gauges, except for the v meter, are not designed to operate at that much voltage and can destroy the battery by boiling all of the water out. You may not have to do anything but connect the regulator in on the buss bar of the motor. Good Luck