Best battery for Garmin GTR 200, no alternator

Best battery for GTR 200, no alternator

Rebuilding a PA-11 with no alternator and want to install a Garmin GTR 200 and power it with an Aerovoltz battery.

GTR 200 draws 3 A during transmission, 600 mA during receiving and requires minimum of 9.5 V

Aerovoltz battery specs as follows:
4-cell: 1 lbs 2.5" (L) x 2.5" (W) x 4.25" (H) 8 Ah
8-cell: 1.7 lbs 4.5" (L) x 2.5" (W) x 4.25" (H) 15 Ah
12-cell: 2.5 lbs 4.5" (L) x 3.5" (W) x 4.25" (H) 20 Ah
16-cell 3.5 lbs 4.5" (L) x 4.5" (W) x 4.25" (H) 28 Ah

In comparison, a popular handheld transceiver draws roughly half the amps (1.5A T, 300 mA R and 70 mA standby) and the best Li battery pack provides 1.8 Ah. This battery Ah to current draw ratio is roughly less than 25% of the ratio we would have with the 4-cell Aerovoltz battery

We anticipate typical flights of 1 h or less with transceiver use for position reports while in pattern of uncontrolled airport, but would like to have enough battery power to use the radio for an 8-hour cross country. I do not find any data showing what the draw is when using the intercom.

Questions:

1. Which battery would prove best in this application? Looking for adequate but not overkill. The smaller the better if it will be enough.
   
2. Do we need to consider a special charger? (currently have a 500 mA battery maintainer
   
3. Do we need to install a BMS to prevent my partner from ruining the battery when he leaves the radio on and forgets to plug in the charger?