Words of Wisdom: “if [you're] the type to park the rig, let the batteries die and boost them for the next time [you're] using it, get the cheapest batteries out there. [You're] killing them anyway.” – twinboat1)
Deep cycle batteries are batteries which are designed to be deeply discharged (DoD) between full charges. For longest life, lead-acid chemistries (flooded or sealed/AGM) are discharged down to about 50% State of Charge (SoC) and then recharged fully every day.
In campervan scenarios the batteries hit their lowest Soc just before dawn and are fully charged sometime before sunset (the earlier the better).
Batteries can be charged with a smart or three-stage charger for optimal performance and longevity.
Deep cycle batteries are most commonly discharged to 50% DoD2), but can be discharged to other levels with radical results in the life of the battery.
On this graph from Trojan we see the following pattern:
Since drawing down 20% vs. 50% is the same cost over time it might be best to aim for 20% DoD and cycle to 50% DoD when needed. This would decrease the number of battery replacements while getting maximum use.
Drawing down to 80% DoD consistently is 24% more expensive and requires much more replacement effort. [Those things are heavy! – Secessus] Occasional/emergency drops to 80% DoD followed by full charging the next day will not inflict meaningful damage to the battery.
The most common deep cycle battery is flooded lead acid (FLA). They are least expensive in the long run but do require checking/refilling the cells with distilled water occasionally.
Common setpoints (check your manufacturer's recommendation):
Words of Wisdom: “The worst 6v GC-2 battery is still a better deep cycle battery than 99.98% of flooded 12v batteries sold in deep cycle applications.” – SternWake8)
6v golf cart batteries were designed for deep cycle use. They are extremely heavy and generally wired in series to make a 12v bank.
The Trojan T-105 RE is the canonical deep cycle battery.
Trojan recommends a 14.8v absorption setpoint.
Cheaper and easily exchanged batteries are often recommended as “learner batteries” that will be destroyed by newbie error.9)
“are very popular on RV forums if nothing else for the ability to be returned all over the country. Trojans for as wonderful as they are, can be ruined just as easily as any GC battery.” jimindenver10)
Deka GC-2 can be sourced from Batteries Plus.11)
Costco GC-2 (Enterprise, Interstate, etc) are about half the price of Trojans.12)
Energizer GC-2 are sold at Sam's Club. They are made by various manufacturers:
Energizer gc-2s could be made by johnson controls or USbattery or Exide or Eastpenn/deka or perhaps others depending on where they decided to source them at that time, and where in the country they are located at purchase. I would not want GC-2s made by johnson controls or exide. SternWake13)
Duracell GC-2 are a Deka brand made by East Penn.14) They can be sourced at Batteries Plus and Sam's Club.
The USbattery GC-2 can be sourced from Batteries Plus.15)
NAPA carries a GC-2 made by Exide.16)
Some manufacturers attempt to get around the Partial State of Charge limitations of lead chemistries by using carbon in some form. The idea is that the addition of carbon limits hard sulfation that can form in PSOC scenarios.
In the milder form, carbon is added to the negative plates. Trojan claims up to 15% more cycles from the RE17) line than non-carbon FLA in PSOC use.18) At this writing the cost premium for this ⇐15% increase in cycles is 21%.19)
Valve Regulated Lead Acid batteries have the same chemistry as FLA but
These two features combine to make a battery that can be mounted in positions other than upright and does not require the regular addition of water.
This type of VRLA battery stabilizes the electrolyte in an Absorbent fiberGlass Mat (AGM).
Common setpoints (check your manufacturer's recommendation):
This type of VRLA battery gelatinizes the electrolyte to stabilize it. While renowned for their duty cycles, they are rarely used in campers.
Firefly makes the Oasis (AGM) battery with carbon foam. Claimed benefits are:
“Deep cycle” / marine batteries found at Walmart and similar chains are not actually deep cycle batteries. Real 12v deep cycle batteries like the Trojan T-1275 are large, expensive, and very rarely seen in campers.32)
Having said that, if one is aware of the limitations of the battery it can play a role in the campervan, particularly as a “learner battery” that the beginner often murders. Lifespan on these batteries can be maximized by cycling lightly, perhaps 20% depth of discharge.
Pro: Local and 24hr availability in stores can valuable. Warranties are nationwide; save your receipt as it is usually required.
Con: Significantly less capacity than a real deep cycle bank; a group 27 wally world battery has about 1/2 the Ah capacity. Drastically reduced longevity when cycled to 50% DoD as with most campervan usage patterns.
In theory LifePO4 lithium batteries should be near perfect for boondockers who follow good weather.
The batteries can be charged with either a charge controller with a LifePO4 profile, or a smart charger with programmable setpoints. If the latter, something like a “Bulk” charge at 13.8, minimal “absorption”, and “float” in the 13.25-13.4v range.39) Technomadia charge to 14.2v, use a minimal “absorption”, and “float” at as the setpoint , float at 13.6v.40)
Non-LiFeP04 Li-ion [called “li-ion” hereafter] are usually built in 3S configuration and so run lower voltages than LFP or lead. Usable cell voltage is 4.2v - 3v, yielding bank voltage of 12.6v - 9v.41) This lower voltage may trip Low Voltage Disconnect on inverters or refrigerators.
There are hobbyists who build 4S packs and undercharge the cells to only 3.7v. This gives an operating range of 14.8v - 12v, a much better fit for nominal 12v systems. The downside is this configuration only uses 58% of the cells' capacity and will require more cells to achieve the same amount of Ah.
7S builds make a decent 24v bank. They are 29.4v fully charged and 21v discharged, compared to lead acid's 29.6v max voltage and 24.2v 50% DoD cutoff. Discharging 7s li-ion to 24v((3.42v/cell) could help avoid LVD, making 35% of the cells' capacity unusable. 24v banks in general could preclude alternator charging, however.