Just make sure they are isolated so there can be no voltage drain. Sounds like you have twins? I give mine a trickle charge every now and then after the boat's been sitting for a while and yeah it doesn't hurt to crank up the donk every month or so through the down time.

 Just dont stop using the boat!!

I use a ctek on my batteries every few months or after a long drain! A good multi stage charger is good for the battery !

Thanks. I have them isolated. Do you have a suggestion on a good brand of trickle charger? I'm not sure what a CTEK is though. Is it a different type of charger?

 Second CTEK charger, 

Yeah Sea-kem, I got a seafox with twin 115 mercs. Love it.

Thanks for the advice. Will get a Ctek.

 I have just made up my own solar panel charging setup for my boat.

you can get fully sealed and flexible solar panels, match that to a solar battery charge regulator and mount it all nicely on your boat.

then just let the sun take care if the rest

I keep it outside scano. That set up sounds great. Do you leave it on the roof permanently or pack it up when you use your boat?

Ctek or projecta multi stage charger. 8 amp will do.

These are multi stage chargers that will analyse the charge state of your battery and apply the correct charging stages to bring it back to full charge.

Always isolate you batteries when not in use, charge them one at a time.

I usually charge mine once a month.

My boat is sometimes not used for long periods and not had too many problems with my single battery, (carry a mini super booster as backup just in case though). I just isolate it via a switch and whack the charger on for a bit after using it or if not using it charge it when I'm up at mums where I store it (this may be up to 3 months apart). Solar no good in the shed for me.

Main killer of batteries is them having low level of charge for fair - long period this varies with type of battery.
Cranking Battery anything below 3/4 is bad if for anything but a short period and house battery (non cranking)below 1/2 charge.

Starbug, does that mean you can leave the charger on all the time and when it is full it stops for a while then starts as needed. That would suit me. The solar set up scano mentioned also sounds good. Do you know about how much I would pay for the charger?

 I got it mounted via Velcro to a flat panel at the rear of the boat next to one of the rod holders and near the bait board.

then just run the twincore wire from the solar panel to a basic 12v solar regulator that you mount inside the boat near the batteries. That will feed as much current as what is needed or even not needed to the batteries to keep them charged. They also run diodes in the voltage regulators and some solar panels themselves so that it won't suck power or get any back feed when there is no sunlight at night.

I leave aver it all connected for even when I got fishing so that I can leave my sounder and accessories on whilst stopped without the engine running to ensure I won't have dramas with the battery running low and not being able to start the engine.

only out it together a few weeks ago however very straight forward. Just google solar panel boat battery charging and plenty should come up. Gave mine a good test whilst out squidding last fortnight and it worked really well. Kept the battery well charged all day with the deckwash and sounder constantly on without the engine running for a lot of the time.

Thanks heaps guys. I'll have a look at both set ups and go from there. I like the idea of having a charge going in the battery while fishing and will definitely get a jump pack as a back up.

lots of good info above, but one thing to be wary of.

If you leave a trickle charger on your batteries all the time, the first time you will likely know they are puckeroo is when the charger is off for a while.

And that will be when you switch off the engine and drift for a while etc.

The trickle charger IS good for your battery but will disguise a failure until you actually need it to work.

 also interested in this, theres a few different cteks, what model ctek are you guys using? 

You guys have been very helpful, thanks again.