keeping motor cranking battery charged

How do you keep the cranking battery charged up?
Battery in question INTERSTATE 24M-RD Cranking AMPS 500 Reserve Capacity 75. Battery has some code hot stamped code "SP3OCTNC" on edge of top. Is this the date of mfg???
I just purchased a new boat in Oct 2008 and have about 24 hours on the MERC motor.
I find that when out trolling and watching my 997 Si and with the aerator pump running that the 997 shows WARNING LOW VOLTAGE after about 3 hours. I have the low voltage warning setting on 10.5V. Fluid level in all cells are not quite up to the full indicators, but are not down to the top of the plates.
I am not trolling off of the cranking battery, I have two deep cycle batteries for that purpose.

Something ain't right. First of all if you fish like most of us you do not use your big motor enough to keep the starting battery charged. You use it to get to your fishing hole and that may be it until time to go home. Outboards have small charging systems, at least the older motors do. For that reason I always recommend an on-board charger that includes leads for the starting battery. Of course if you always make long runs then you don't need it.
With that said top off your water level in the battery. Check your charging system to make sure its working. Check the voltage with the motor off then check it again with it running at approx 2000 RPMs. You can do this with your 997 by monitoring voltage. You should see an increase in voltage (13-15V) with it running. Make sure all battery connections are very clean and tight. When you are sure the battery is fully charged take it to your local auto parts store and have it load tested. Even brand new batteries can be bad. Your aerator does not need to run all the time to keep fish alive. One of these is a great battery saver. Livewell Aerator Pump Timer - Marine - Bass Pro Shops
If the battery is bad I would go to at least a group 27.

Battery age - Interstate Batteries FAQ :: How can I tell the age of my battery?

I agree that you need to charge the battery when the boat isn't being used, but watch how much water you put in the batteries. If you get to the bottoms of the split-tubes, you'll get splash-out of electrolyte in rough water. Your hold down strap won't last long that way. I always try to keep between an eighth and quarter of an inch of water over the plates. This can slightly shorten battery life due to the lower pH of the electrolyte, but my batteries usually last 5 or 6 years this way.

If you are using wing nuts on your starting battery switch them to stainless nuts. Tighten them with a wrench. Wing nuts have a bad tendency to loosen or not have the best connectivity. Also check your wiring for your charging system. Make sure you have no corrosion and make sure all connections are tight and clean. Also check your grounds on the engine...a loose or corroded ground will cause a low charge situation. Many outboards do not charge at a very high amperage...and like IBNFSHN said, short runs you'll eventually run her down. Most cars charge between 60-100 amps...most outboards...smaller ones and older ones will charge 10 amps or less....add that to short runs and you aren't putting back what you take out...then you add the aerator and electronics and a full day...you'll see that. Most newer ones will charge at a higher rate and even have car style alternators.
I like an onboard charger myself...I have a three bank and hook it up normally when I get back from a trip and before I go. Never have an issue. Just some things to think about.
Like IBNFSHN said...something isn't right...put the thing on muffs....put a multimeter on the battery and see what you are getting to it...should be above 12V...if not could be rectifier...(volatage regulator)....stator...alternator (if equiped)...you should not run down that fast.

THANKS FOR THE REPLIES.
I have a minn Kota battery charger that I have attached to the battery and plug it in when I get back home and before I put the boat in the storage yard. The charger indicates that it is at full charge.
The stamping on the top of the battery indicates that it was manufactured in July 2008.
I am going to take the battery back to the boat dealer and have them do a load test.
I have replaced the cranking battery with a deep cycle style 27. It is not a new battery, but one that I took off the previous boat I had and used it for trolling battery.

I had the same problem with my boat when i first got it. The starting battery didn't have enough capacity. I also turned off my low battery alarm on my 997---I'm not convinced it was all that accurate. I haven't had a problem since I got a larger battery.

Might Be Something With The Hb Unit Buddy Has One Its A Color Unit Low Volt Seems To Stay On All The Time The One On My Boat Did It A Couple Of Times Changed The Battery Didnt Do It Any More .