I'm going to jump straight on the Gel and discourage you from it. I thought the same, bought a Gel for our caravan and it's crap. I will never waste my money like that again.
Gel and AGM batteries are similar in that they are sealed (to an extent) requiring no input from the user under normal circumstances. There is a fundamental difference between the two that once understood, should be the reason why people never touch a gel battery again - they're just too damn sensitive.
Both batteries suspend their electrolyte between the plates. AGM batteries use a glass matt - like fibreglass without any resin - and this is saturated with battery acid. Gel batteries have a silicate added to the battery acid to turn the acid into a jelly.
The fundamental idea is a good one - the jelly holds in position, doesn't allow sulphation crystals to fall and short cells - but there's a problem with the execution.
If you charge a Gel battery at slightly too high a rate, or slightly too high a voltage, small bubbles will start to form in the Gel. This also happens in an AGM or a normal wet cell battery, but because there's no silicate added, in AGM and wet-cells the bubbles rise, pop and that's the end of it. In a Gel battery, the bubble stays. Get enough bubbles and there's not enough path for the chemical reaction to occur to produce electricity and that battery is dead.
On to the Jaycar stuff. That DC-DC converter is just way over the top and if you use it, it will probably never break - but it could also draw too much power from your alternator. At the most, you want to draw no more than 40A or so. Redarc or similar isolators (available from ARB) are cheaper, too.
The AGM battery that you've pointed us to is just over half a metre long! It's bloody huge! It won't fit in a normal battery tray, it's designed to be rack mounted (computer room). It's a bloody good investment if you've got a good management system - but for that, I wouldn't be looking at an isolator or Jaycar's DC-DC charger, I'd be inverting the power to 240V and throwing a
C-Tek 25A charger at it.
It's important to look at the return on investment. That AGM battery will provide about 80% of its capacity - easily 120Ah - and you're paying $700 for that, with a life expectancy of 12 years - let's say 10 to be conservative. It's $70 a year.
A spiral-wound AGM that you could buy from Rays Outdoors (yep, they sell 'em now) for $400 will last you 8 years, provide you with at least 90Ah of usable capacity. That's $50 a year, and these batteries will fit in a normal battery carrier.
A wet cell deep cycle will only give you about 50% of its fully charged capacity but will cost significantly less - and will not last as long. You're lucky to get 4 years out of them, but you can snag a 50Ah battery for $150. That's 25Ah of usable power - you'd need 4 of these to match the other two in terms of longevity and you need to maintain them - so your cost over time is $150 a year.
It is pretty obvious the spiral wound AGMs are the way to go. If you're a member at Rays (free to join and they don't spam you - I'm a member myself) you get a 5% discount normally, and sometimes they'll give you 25% off, taking the AGM price down to just $300. Rays also sell the C-Tek chargers, not sure if they sell 'em that big though - go take a look.
