To clarify how **I prefer** to do this,
also accounting for the difference in "usable amps" between the two chemistries.
Split off from this previously hijacked thread
eBay search for pricing data
at writing, two options for a nominal 12V bank including delivery and at 900Ah, pretty small for running aircon more than a few hours:
$4100 from a USA seller, $4.86 / Ah
$3212 from the Chinese one, $3.57 / Ah
in theory both equally covered by eBay's NQAMB guarantee, but for the Chinese one you have to pay for the return shipping if they turn out to be sub-par, but in eBay's opinion still "as described"
That shipping cost would end up being thousands, so, in effect a very risky purchase.
In fact, I would not purchase from either seller, only from a known trust-worthy vendor as reported by a friend or multiple trusted forum members having dealt with them satisfactorily in the past, and proven stable over time.
Most sellers through platforms like eBay, Amazon, Ali Express, Taobao.com, Tmall.com and 1688.com*are selling less than brand new Grade A factory output, many even used as if new.
So of course you need to be able to do a proper accurate capacity benchmark upon arrival to judge whether they're worth keeping, before the warranty return period is finished.
Buying from the factory direct is really the best strategy, but so far that's always turned out to be more expensive.
The relationship between average DoD% and cycling lifespan is proportionally similar for LFP, shallower cycling yields 3-5x more cycling, can easily get to 10,000 - barring user error, bad design, failure of protection gear or accidents.
But, even if you're happy with only 4-5000, never let resting voltage get near 3Vpc, and the stop-charge setpoint should stay around 3.45Vpc, depending on the C-rate.
That works out to around 70-75% DoD on average, or 630-675 usable.
The GCs are well under $1200 for six pair, yielding 1290Ah, or around 650Ah usable
So comparable ballpark usable capacity.
Note the Deka GCs are every bit as good as say Trojans, or Superior, Crown and US Battery.
Very few would spend the extra money on Rolls / Surrette, mostly yacht owners looking for 15-year lifespans, cruising where replacements are just not available.
You also need a known-accurate and reliable BMS, perhaps
separate balancing gear,
DMM & ammeter and
ideally a variable-voltage and current-limiting power supply,
along with the high-current charger you'll need for routine daily usage when shore power or an AC genset is available.
Not counting the voltage-adjustable and current-limiting gear required for charging your LFP bank off alternator while driving, and/or panels and solar controller, which may not be needed atall.
If you have to ask how much you can't afford it all.
Only a tiny number of which if any are required using the Deka GCs, and much cheaper versions good enough as opposed to with LFP.
You **might** get away with less than $2,000 for these infrastructure components, but I doubt it, even $3k won't get you the usability & security when shooting for being "certain" of over a decade's lifespan.
And if you don't have the confidence (skilz, knowledge) to select install configure maintain all these components, you'll want to either buy a (**much** more expensive per Ah) pre-packaged "systems solution" from Lithionics, Victron or Mastervolt, or
hire a trusted / recommended professional to help your DIY efgorts, and those are pretty thin on the ground, especially in North America.
Not so bad in Europe, Australia etc, where local LFP distribution channels are **much** more mature, and good quality deep cycling lead banks are also **much** more expensive
thus tipping the scales much more readily in LFP's favour.