I can fit two group 27 batteries under my body behind drivers seat.
At one point I carried 2 there and one in my engine compartment. Not soon enough, realized that I never needed that much battery capacity, then downsized to one group 27 and one group 31.
When the group 31 got weak and one cell was heating abnormally on the bottom when charging, I removed it and had only on Northstar Group 27 AGM for starting and engine.
Next month, it will have been my only battery for house loads and engine starting, for 3 full years and is approaching 700 deep cycles and in November will be 5 years old.
But I have a shunted battery monitor that lets me see how much capacity I take and return to the battery.
It takes about 11 feet of cabling to reach from my alternator (+) output stud to the underbody battery.
The quality of the cable terminations is often overlooked or perhaps not considered, but they play a huge part in how effective it recharges, and how safe it is.
Properly terminating battery cables cannot be done with vice grips and the hammer crimpers also have a suspect electrical connection.
While one need not go to ABYC standards for battery cables, the following is a good read on the correct way to make battery cables: https://marinehowto.com/making-your-own-battery-cables/
This outfit makes custom cables for a better price and from much better components than you will find in auto parts stores:http://www.genuinedealz.com/custom-cables
Most people will charge the secondary battery through some sort of isolation device which takes power from engine battery, but the stock alternator circuit to engine battery was never intended to have an additional depleted battery tacked onto the end of that circuit, and it acts like an electrical bottle neck severly slowing charging.
There should also be 2 new fuses installed in the tacked on circuit, each one withing a few inches of battery terminals.
if one takes power for second battery from alternator (+) stud, only one extra fuse is required, and the circuit is usually much shorter. Leading to more effective charging while underway.
But NO vehicles voltage regulator is designed to reharge a depleted additional battery quickly, or even fully.
Get a plug in charger and use it whenever you get home from an outing, even if the drive home was 6 hours.
For Lead acid battery longevity, it is best to always recharge to a true 100% recharge asap, and your alternator will NOT do that.
charging from 80% charged to 100% charged takes no less than 3.5 hours, and that is when the battery is brough upto the mid 14v range. Most systems will drop to the mid to upper 13's volt range after a few minutes and charging slows by 2/3rds at this lesser electrical pressure.
One never really knows how much battery capacity is remaining, until it can no longer power what they need. If their needs are low they might never know until the abused battery shorts a cell.
It sucks when one is takes by surprise with a failed battery from lack of proper and prompt recharging, and alternator only recharging is a guarantee that the depleted battery is never properly or fully recharged.