The device I used is this one.

http://bluesea.com/products/7600

I have installed quite a few of them on yachts an not heard of any problems. The cutin point is adjustable and there is also an adjustable overvoltage protection. The unit has terminals for two batteries. In the default configuration it will combine when the voltage on battery A rises above the set cutin point. It can also be configured so that it combines if the voltage on battery A or B goes above the set point.

However, for as small a battery as you are talking about, maybe something like this would be more appropriate:

http://www.xantrex.com/web/id/103/p/1/pt/8/product.asp

On my boat, all charging sources including PV, wind, charger/inverter and the engine alternator are connected to the house battery bank (four Trojan T-105). A Digital Echo charge keeps the engine cranking battery topped up. I also have an 8D gel battery which is going to serve as a dedicated communications battery (ham and marine radios) and I will charge this one with the Blue Sea charging relay in the first link.

Of course, if you have the time to play around with stuff yourself and do not have to rely on these things for your safety out on the big blue ocean, you could use a Ghurd controller to control a big contactor capable of carrying the maximum expected current (and then some).