A portable battery system is a very handy item to have for ham radio in general (think field day, special event stations, etc.) and is essential for an EMCOMM kit. I based my battery box around a deep cycle lead acid battery and I ended up choosing a group 27 battery with a reserve capacity of 160 (66.7 Ah). Reserve capacity refers to the number of minutes a fully charged battery is discharged at 25 amps before the voltage drops below 10.5 volts. This figure can be converted to amp-hours by multiplying the reserve capacity by 0.4167. According the Battery School within a BCI group size, the battery with higher ampere-hours (or RC) will tend to have longer lives and weigh more because of thicker plates and more lead. While lead acid batteries are heavy, they are also versatile and can be continuously trickle charged to keep them at full charge until they are needed. A deep cycle battery is appropriate for this application since their power level can be drained very low without damaging the battery, unlike a typical car battery.
For ease of transport and to keep the wiring organized and neat I store my battery in a plastic battery box and hardwired it to a Rigrunner 4005 power distribution block mounted on the box. I went with a small Rigrunner since this system will only be used with a few items at a time (1 or 2 radios, inverter for laptop power, low voltage lighting, etc.). I keep the battery charged using a NOCO Genius G3500 charger which is smart enough to not overcharge the battery and powerful enough to recharge the battery at a decent rate. After years of thinking about it I finally got around to standardizing all of my power connections with Anderson Powerpoles which make 12V power connections very modular and interchangeable.
This is a very useful and simple project that performs well for my purposes. I could easily parallel another battery with this one for increased capacity if the need arises in the future. You could buy a premade battery box with a Rigrunner, but doing it yourself is a lot cheaper.