Tony,
It is hard to respond without some additional info. What type of batteries do you have (group 27, 31, 4D - deep cycle, etc.), and how old are they? I would expect any battery you might find on a C36 to be able to fire the compressor on your fridge. It sounds like your batteries, or at least whatever combination you use for your house bank, may be shot.
The Ample Power site is a good place to learn about 12v batteries and their use. Check out the Primer on their tech documents tab.

Agree with the Ample Power references - [url]http://www.amplepower.com/tech_docs/index.html[/url]

Also check out Mainsails offferings, which cover many subjects in fair detail -
[url]http://www.pbase.com/mainecruising/boat_projects[/url]

One thing I'd recommend is to evaluate your energy usage, then decide what sort of system will meet those needs the best. There are a range of options for battery types, size and charging systems, which vary greatly in price.

Tony,
I agree with your plan to deep-six the old humming charger. Another thing to check into is the wiring to/from your batteries. Four batteries, in any configuration, is not the way your boat came from the factory. Which means the PO made some changes. Which means it may or may not have been done correctly. Most owners opt for a large house bank, and a smaller, dedicated starting battery. If you currently have two starting batteries, something was not thought out well, and/or it was an attempt to compenstate for another problem. The links above are good ones. Do some reading, and then ask questions. You may get slightly different responses, but in the end you must decide how YOU want YOUR boat to be. Hope this helps.

There a quite a few threads on wiring up battery banks in a more efficient way. See, for example, these threads, and the links inside them:
[url]http://www.c36ia.com/forums/showthread.php?t=1844&highlight=wiring[/url]
[url]http://www.c36ia.com/forums/showthread.php?t=1802&highlight=wiring[/url]
[url]http://www.c36ia.com/forums/showthread.php?t=1776&highlight=wiring[/url]
[url]http://www.c36ia.com/forums/showthread.php?t=1557&highlight=wiring[/url]
[url]http://c34.org/bbs/index.php/topic,6604.0.html[/url]
[url]http://forums.catalina.sailboatowners.com/showthread.php?t=137615[/url]
(If you haven't done so already, try using the search function in the forum (its on the dark blue bar towards the right) to find additional threads on these subjects. )

Our current (pardon the pun) set up is a pair of 6V golf cart batteries as a house bank and a group 27 as a starting battery. We're not heavy power users and seems to do OK for us during summer cruises. All three fit under the seat closest to the nav table.
The best enhancement of all was to rewire so that the house bank is fed by its own 2ga cable directly from the alternator, and the starter battery is charged via a Blue Seas battery combiner. That means no longer having to be mindful of the battery selecter switch. Also added a Victron monitor in place of the useless analog volt gauge (fits the existing hole in the panel) so we have some idea now of what's happening in the system.

I fixed a fridge compressor issue by disconnecting the power cable to compressor, cleaning the contacts and pluging it back in. Apparently I was loosing enough voltage across the pins preventing the compressor from starting.

John & Kathy Impag--
Pooka 1339TR