Easy to make your own if you are not too worried about a fancy charging indicator on the dash. The last one i made was just a big battery isolator switch on the positive wire between the main battery and the extra battery.
I am about to knock mine up for the extra battery i am putting in the back to run the camping stuff and inverters, you can activate the main relay with a flash wire going to ignition circuit or just have an isolator switch that you turn off when parked to stop the main battery going flat.
You only need one if you add an extra battery and want to isolate it from the main batteries some of the time. You might want to isolate it so you can leave the fridge running when camped and not risk running down the starter batteries.
You can use a heavy duty switch designed for the job which you normaly fit close to the batteries in the engine bay rather than on the dash. It's very simple, not a lot to go wrong and very reliable except for the fact that its reliability is dependent on the human operator remembering to use it
Moving up from that would be a heavy duty relay or solenoid in place of that switch which you can activate from a switch on the dash. Still relies on the human operator but is at least more convenient.
The next step is instead of having a switch on the dash controlling the solenoid you have some sort of voltage sensing device that turns on the relay when the voltage rises above a predetermined level i.e. when the engine is running. This saves the human operator having to remember to toggle the switch. You can buy solenoids / relays with built in voltage sensing for this job but I wouldn't get one of those. You can buy a caravan split charge box that has a voltage sensed output that can be wired to operate the solenoid (the device itself has limited current capacity). You can also solder a few components together and make your own.
My initial split charge install used a caravan split charge box to trigger a HD solenoid. I wired the caravan unit to the solenoid coil via an on-off-on toggle switch. One of the ON positions was connected to the caravan voltage sense output so when the engine was running the solenoid was powered. The off position let me disable the solenoid completely and the other ON position was to the main battery + so I could also manually force the batteries to be linked e.g. winching where the winch might pull the voltage down far enough to stop the caravan unit triggering. This worked great for a bit until the caravan box voltage sense stopped working and I didn't notice which meant the aux power battery was run down and had no power for the fridge but at least when I did eventually notice that something was wrong I could toggle my switch and force the batteries to be linked.
The next step is something like the IBS or National Luna etc controllers. They still have a HD solenoid to link the batteries and use voltage sensing to decide when to link the batteries but they add a few more bells and whistles to replace my toggle switch. The IBS has led bar graphs for the main and aux battery voltages plus another for the charge voltage, so you can see what's going on (if you press the 'display' button otherwise the bar graphs are blank). the IBS also has a low voltage beeper that will warn you if the batteries are getting low. Those are the main reasons I bought an IBS, so I would know next time something was wrong, but the IBS also has manual and auto link buttons so you can force a link for winching / dead starter batteries etc.
Those are all ways of linking your aux and main batteries and are what most people settle for but there is a major limitation - your aux battery will never be very well charged voltage drop and differing internal resistance in the batteries plus being charged from an alternator which is a single stage device means the charging is always well below what you want. It is possible to fool the alternator regulator to improve on that a bit but that's not a great solution either.
The next step is something like a Sterling DC-DC multi stage battery charger. You connect it in place of the HD solenoid and it draws power to charge the aux battery, boosting the voltage to whatever level is needed for that battery type and state of charge. You can google multi-stage chargers to get the low down but they put a lot more power back into the battery than your vehicle alternator will ever be capable of and so your aux battery will be in tip top condition for powering your fridge and keep the beer cool With these devices power doesn't flow the other way though so e.g. when winching you can't link the aux battery to main for a boost.
The next step is to combine something like an IBS and a DC-DC charger which brings the toggle switch back because AFAIK no one sells a single device that does both. I have my IBS solenoid trigger output connected to that on-off-on toggle switch. One ON position connects to the HD solenoid to link the batteries the way the IBS was intended so you can boost power for winching / dead starter batteries. The other ON position connects to the Sterling DC-DC charger for multi-stage aux battery charging and is my default position. The IBS display still tells me what both battery banks are doing but in charge mode it is powering the DC-DC charger instead of just linking the batteries. The IBS is a bit too clever for its own good though and starts beeping when it realises there are two different voltages in play when it should be linked and so have a common voltage
The next step then is to roll your own controller with a PIC chip and do away with the toggle switch and false alarms again!
Just getting back to basics though, I think that I have to consider my application a bit more candidly...
Operating few & far between (hopefully) for now therefore I figure I get the winch installed off the main batteries when I get to it & see how that goes for me.
Running a simple 12v/240v/gas three way fridge for overnight/weekending solo missions as my family aren't 'bovvered' about coming out with dad at present. If I were to install one of these 'ere old batteries I have & hook the fridge on to that directly with no charge system, would that keep my beer cold for the weekend?
Gav, thanks for asking the question before I did. Now that I have a 24v starter, I am in a different place. Lil Blue has the IBS but as it's only 12v start, it made sense. So IBS went between the two main underbonnet batteries and was simple with a split charge caravan system to the third battery. I fitted a voltmeter too so that I could monitor condition. I also had an override switch to kill it completely if I wanted to.
But with main batteries linked on the 80, I will want to put the beer cooler battery in as described above. Do I come off the starter battery or the second battery to run the charging system? Do I run my winch(s) off the main or second battery?
Should I actually use the power out box under the bonnet to run power to the fridge?