Baterries set up in parellel

You don't actually have a "55 amp" battery. You have a "55 amp-hour" battery.
Which means you can consume:
1 amp every hour for 55 hours
2 amps every hour for 27.5 hours
5 amps every hour for 11 hours

If you run 2 x 55 Amp-hour batteries in parralel - you actually have 1 x 110 amp-hour battery - which would double the above duration figures

What you need to know is how much current does your fridge draws. Then you can work out how long the battery will last/

Yes, You could get away with charging all at the same time (although it's not the perfect setup, it will work.)
However, I'd separate them during use- any time you parallel non identical batteries you'll get poor results. Even two supposedly identical batteries (type, age) being discharged in parallel can lose some efficiency as one can "drag" the other down.
If you parallel an old battery and a new one when off charge, the new one will simply discharge into the old one. ie: separate them when off charge.

From my experience, I think the problems associated with parrallelling non identical batteries are overstated. Yes, in an ideal situation an exact pair (brand, type, size, age) may be best, but non identical set ups seem to work fine in practice. In your case, providing both batteries are in good condition, parralling an 'old' 55 AH battery with a similar 'new' 55 AH is not a problem.

An advantage of keeping them in parrallel all the time is that they will discharge and charge at about the same rate. The problem of one discharging into the other should not be a big problem if both are in good condition.

A disadvantage is that one stuffed battery can quickly lead to two stuffed batteries.

I run two 120AH AGMs in parrallel. One is about a year older than the other. They are often (when my CT is connected) in parrallel with a standard vented 105AH deep cycle, although I normally don't leave them connected for long periods without the engine running. When engine is running all that is in parrallel with the starter battery. Had this set up for about 12 months and works fine so far.

See what other advice you get, but that is my experience.

Identical batteries are essentail when it is important that both always have the same state of charge e.g. two 6 volt batteries in series to deliver 12 volts.

The whole point of Dual Battery systems when camping is to allow unequal states of charge - it would be pointless making the batteries identical size.

This was extensively debated here
www.exploroz.com/Forum/Answer.asp?ForumQID=31280
and the conclusion was that "identical batteries are needed in a parallel dual battery setup" is a myth that should have been buried a long time ago.

Mike

thanks for all input, helps alot. Specially Phil and norm. that was what i was looking for. Have had consistant advice the same as you guys from the likes of Piranaha but wanted to see what actually works in the field. you make sense. was still unsure on 2 more points. Have been told that you can get upto 10 years battery life out of gel batteries, although considered rare, was wondering what was more realistic. its been suggested around 4 or 5 years ( i realize that the are a few varibles before anyone gets excited). also, is it better to leave new 55 amp hour battery permenantly in car or pull out and store and reinstall for extended stay trips, this is from the point of view of looking after the battery (life). and finally it has been suggested that gel batteries should be discharged complete every now and then as part of looking after them although this sounds wrong to me??

Thanks again for all the replys

Wayne, I don't know a lot about Gel batteries. I and many others on this forum use and swear by AGMs. For some basic info on AGMs paste this link to your browser:
www.fridge-and-solar.net

Also do a search on this forum. There is heaps of info on batteries, dual battery set ups, isolation, charging etc. Some of it will be a bit conflicting, but you should be able to sort the wheat from the chaff. There are a number of excellent posters on this subject with a lot more knowledge than me. Two of them are Collyn R and MileDiD, but there are others. Collyn is not posting at the moment and I get the impression Mike is becoming a bit less patient (sorry Mike), but there is heaps of great stuff available if you look from these and other great posters.

Get a glass of red and spend an hour or two using the search facility. You'll be surprised what you will learn.

One battery that needs looking into is the one developed by Dr David Rand of the CSIRO which uses a new supergell paste and is made in Australia and supposedly has the lowest resistance and takes the fastest recharge and can be used as both cranking and house together.
The price is now competitive with AGM but the problem is it is not yet made in 12V
but two 6Volts in parallel will give you 200AH of capacity and they carry a five year full replacement warranty.
Regards
Wheeleybin