Saturday we were fishing an Xtreme Series tournament, by about 8:00 we had three fish, thought it started out to be a productive day. Then my fish finder on the bow started beeping and flashing "low battery".  I have a three bank on board.  Charged my batteries Friday night as usual until I got an "all green".  By 8:30 I had a completely dead cranking battery.  Luckily the others had a charge, switched them out, got started and for fear of running down all the batteries, we called it a day. Any advice on what to do first, other than buy a cheap tester to see if it's the batteries or the charger?  

Thanks

a cheap battery tester would be the ticket if you don't have a sealed unit.  You can have one bad cell and 5 good ones and many times it will show charged.  I'd pull the battery and take it to a shop where they can check it for you.

"Charged it Friday night as usual There's your problem. Always charge ASAP after use. A battery sitting uncharged gets sulfate on the plates and reduces its capacity. Test the battery, but if you regularly run it down on a weekend and don't charge till Friday it will kill the battery.

In case I misinterpreted and your starting battery is dead, if it is two years old I would simply replace it and hope that that is the problem because that is your cheapest fix.

Deep Cycle Battery Maintenance

1. Use an automatic charger with trickle charge maintenance.

2. Charge ASAP after use.

3. Don't run down to dead. Stay above 30% or higher if possible

4. Refill with distilled water only.

You need to charge the dead battery full.

Cheap Muliti meter if it doesn't have 12.65 or more, 11 anything the battery junk.

Any repair place can load test a battery most don't even charge you. Takes longer to get the tester but it must be charged.

Twice this year I've run into people that don't understand how important cleaning all battery connections until shiney and covering with vasaline. That includes all breaker switch connection and the battery terminals clean and shiney.

Todays electonics will not stand for crap connections.

Garnet

If they are batteries that have removable caps on them, go to NAPA and get you a good hydrometer, one with the thermometer (about $8), charge the batteries, with the onboard charger, after the charge gets started, use a digital voltmeter and take a voltage reading across each battery.  Should be a high 13VDC - low 14 VDC on all three, if not wait about an hour and check again.  If all three banks are not at least a high 13 you may have a charger problem.  After the charge has cut off and showing a complete charge, check each batteries cells with the hydrometer, should be a minimum of 1.260 and all cells should be within .005 of each other, acutally even closer than that but that is the max difference they should read.

If the batteries are sealed, maintenance free, charge them, check the charging voltage as above.  Let it charge until it shows they are fully charged and take them to a place that has a computerized tester that will load test and check the conductance and have them tested.   The ones that will load test and check conductance are not cheap so not many places will have one, so you will have to ask.  Most have the tester that checks conductance, but will not load it.