The 90Ahr AGM battery in the engine bay is being charged via the bumper socket on my driveway every night and that provides adequate electrical power for my daily driving needs.
In a way, that is probably the optimum level of work, cost and trouble I should go to if cost-saving is my goal. However, my goal is to maximise mpg, so I shall make my life complicated and go the extra mile to fit a 2nd battery, 2nd mains charger, a DC/DC converter and a whole lot of extra wiring. My main reason for being so unreasonable is to extend my alternator-free driving range to about 300 miles, which I'm unlikely to exceed very often in any one day. So that's my main reason - to get as many miles per gallon as possible on every occasion. But this 2-battery setup will also probably extend the life of the batteries considerably, as on most days I will only be discharging each battery by 20% or so.
My general advice to anyone more sensible than myself, would be to get the biggest AGM battery you can fit in your engine bay and charge it every night from the mains via a good quality charger also mounted in the engine bay. That's relatively simple and should save money as well as a considerable amount of fuel. The savings you make on daily commutes are far more significant than savings on occasional longer journies. For longer trips, just use the alternator.
That said, I'm going to fit a 2nd battery.
I had just bought a DC/DC converter for this purpose when a friend asked me why I didn't simply add a 2nd battery in parallel with the first one. I did a little reading around that and found that basically he has a point. Wiring two batteries up in a simple parallel arrangement would work well, but there are potential pitfalls compared to running the 2nd battery via a DC/DC converter. The main thing is that unless the 2nd battery is right next to the 1st battery then you need seriously fat cables to connect them together. Also, you need to connect both +ve and -ve terminals together with these 'fat as your finger' cables, and ground only the rear-mounted battery, to avoid current imbalances. (And those cables would also have to be fused at either end with say 300A fuses because any short on the cable length would otherwise be disasterous, with exploding batteries, fire, total loss of the vehicle or personal injury being distinct possibilities.) The two batteries should also be of the same type, same manufacturer, same Ahr rating and same manufacture date to avoid problems. Those cables and fuses would cost close to £200, and would not be easy to route through the vehicle. You'd have to be super-careful to avoid any chance of abrasion. (The 300A fuses will stop the batteries exploding but may not blow in time to prevent a fire in the event of a short to ground.) So not straightforward, but if I were starting from scratch I might well do it that way for the sake of simplicity and efficiency. Since I don't have room in the engine bay for two large batteries, I might consider mounting both batteries in the rear so they are both being charged and discharged at the same temperature. Or just use one enormous battery in the rear.
However, I have got half-way along the path of connecting a 2nd battery via a DC/DC converter, and that approach does have advantages (like not needing monster cables anywhere) so I shall go ahead and complete the set-up. I got a DC/DC converter a week ago...
E914 12A 12v-12v Battery Charger (12V Turn On)
...and I'm currently collecting together all the bits and pieces to wire it up. The unit will put out a constant 12A at 14.0v. It has a built-in donor battery voltage cut-off of 12.0v, to protect the donor battery from becoming discharged too deeply, and I think that is probably about right for my purposes, as 12.0v at 12A load probably corresponds to about 12.2v resting voltage, which corresponds to about 50% state of charge. It has a low current 'control wire' which I will wire up to a manual switch for now but which I plan eventually to wire up to the ignition switch (or perhaps to the in-tank fuel pump which only runs when the engine is running) so the unit will switch on and off automatically. I will then have a three-position switch somewhere on or near the dash so I can switch the DC/DC converter ON, or OFF or to AUTO. (It would be good to be able to overide the donor battery cut-off voltage threshold, so I can use the 2nd battery in an emergency to charge the starter battery even if the donor battery is also 50% discharged, but I'm not sure how to do that just now.)
I'm going to set the system up temporarily in the passenger footwell with a spare (non-AGM but otherwise sound) car battery. If that seems to work OK I shall buy another AGM battery of around 110Ahr, mount it securely in the spare wheel well, and route all the cables through properly from the engine bay. The 2nd mains charger will also go in the rear of the vehicle, as will the DC/DC converter.