Amazingly everyone agrees in an amicable way about utilising battery technology......not.
How 'off-grid' are you going?
If in the car to a high spot then you may as well just use the battery that's in the car and save lugging anything else around. If you get into serious high ground DX transmitting for hours on end then start thinking about another battery, maybe a leisure type and invest in a good charger for home. Better that than finishing up some DX for the day then finding out you can't start the car!
When travelling on foot, bicycle, motorcycle or whatever then you'll obviously need to lug around a battery.
If you just want to give it a taster then £15-£20 will buy you a 7aH or more Motorcycle lead acid battery or a typical alarm style AGM from the local DIY outlet. Bit like carrying a brick around with you weight wise.
If you want to give a similar amperage LiFePo4 lithium battery a go expect to pay four to five times more and it'll still feel like you're carrying a brick.
The lithium will last longer than the lead acid as regards charge cycles but when you work out the cost difference it's still quite similar overall. (Four to five times the price for four to five times the number of recharge cycles.)
How long will they last on a charge?
Bit of a piece of string question really - receiving doesn't take much battery life whereas transmitting obviously does and that depends on how much power you're transmitting. Your own personal mileage will vary probably considerably from anyone else. (Especially those that like to 'boost' figures to make for interesting reading.
You're bound to get many more recommendations and find a plethora of stuff on the internet all ably assisting you into more confusion.
It's all quite exciting to think about operating off-grid to start with but as you delve deeper you may find the novelty wears off. You can spend silly money on battery and recharging technology which will feel awesome with a good DX day and a total waste of cash if you don't get a single contact.
But, there's many that regardless of the contacts they make enjoy the adventure of running off grid. Just look at the SOTA, (Summits On The Air), guys who trek miles across moor and mountain and simply enjoy the whole experience. As for CB radio, driving up a hill to add tens or hundreds of metres to your antenna height is easier than adding scaffold poles to your QTH home base setup! Plus you can get away from all the QRM associated with typical human habitats.
It might just be worth it all to bag that elusive DX.
All the best and have fun,