[nativepaul]

Don't replace your car battery with a deep cycle one, while deep cycle batteries are the best for irregular charging such as charger use will see asking it to crank your car will ruin it quickly, just use your existing car battery (engine off) and don't take over half its capacity out or get a deep discharge battery put it in the trunk and charge it at home.

Think of a deep discharge battery as an AA consumer battery and a LiPo as a car battery, a 12s2p pack of 2500mAh AA cells has the same energy as a 4s 5000mAh lipo pack but there is a difference, like LiPos the car battery can pump out an enormous Ampage but will be damaged if you flatten it completely, suffering degradation when discharged over 50%, and like the AA pack a deep cycle battery won't die if you completely discharge it and is perfectly happy discharging 80% of its capacity but like putting the AA pack in a truggy and trying to do backflips, starting a car with a deep discharge battery is asking too much of it, the voltage will drop lower than a car battery will and it will be degraded.

I have a deep discharge battery that I keep in the garage, charge it off my Hyperion charger from a PSU at home and put it in the back of my car before going out for two reasons:-
(1). The convenience of charging at a pit table rather than keep walking to my car.
(2). I do boats as well as cars, I have a small car with a 50Ah battery and some of my boats have very large batteries which use a significant amount of my car battery to charge, and if I leave 50% charge in my car battery to give it a decent life I didn't get many charges in before I have to go home but with a 110Ah deep cycle battery discharging down to 20% I can play much longer.
You can work out roughly how many charges you can get from a given battery, I will use my 4s 5000mAh pack for an example:-
4s x4.2v=16.8v x5Ah= 84 watt hours
50Ah x 12v = 600 watt hours, or 300Wh to its safe 50% capacity
Add 20% to the battery being charged to account for a charger being about 80% efficient and you have 84x100/80 a 105 watt hour draw from the battery, divide the charging battery Wh by the charged battery Wh and I find my car battery is good for 3 charges down to its safe 50% capacity although 5 charges would still see enough juice to start the car and get me home, doing that regularly would lead to a significantly shorter battery life.
switching to my 110Ah deep discharge battery gave me
110x 12v = 1320watt hours or 1056Wh to its safe 80% discharge for long life and that gives me enough for 10 charges of my truggy/buggy that takes 105Wh to charge.