i live in Ulaanbaatar the coldest city and i own a hyundai verna 2006 with 1.5 turbo diesel engine. i replaced it's 12 v 80 Ah battery and glow plugs about a 3 month ago. last night we had -40 c and in the morning i tried to start my car at -36 c. heated glow plug few times then cranked. after repeating cycle 5 times my battery gone out of juice. only if i had charge for 2 or 3 heat and crank attempts my engine must start. i felt it because every time when i crank engine was becoming more softer, crank speed was becoming faster and faster almost started. after that i thought if i had another 80 ah 12 v battery in my trunk it'll crank like a boss. is it possible to do this ? if yes . then how ? can this cars alternator fully charge these 2 total 160 ah batteries when engine is running ?
Yes, this is possible.
What you need to consider is how you connect them.
1) connected directly with heavy starter cable - works and alternator will charge them both. Possible disadvantage is that if you leave the lights on then both become flat ie no security...
2) have a split charge system and a switchable connection to bring in the second battery to help starting - this is how I would do it (and have done it like this in the past).
A split charge system - usually found with motor homes etc