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electrical:12v:deep_cycle_battery

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

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.

DoD vs. duty cycles

popupbackpacker.com_wp-content_uploads_2013_12_trojan-depth-of-charge-table.jpg 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:

  • discharging to 20% DoD3) daily will yield ~3000 duty cycles (~8.2 years of daily use, ~$0.21/KWh4))
  • discharging to 50% DoD5) daily will yield ~1200 duty cycles (~3.3 years of daily use, ~$0.21/KWh)
  • discharging to 80% DoD6) daily will yield ~500 duty cycles (~1.4 years of daily use, ~$0.26/KWh)

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.

flooded lead acid

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):

  • Vfloat 13.2v when not deeply cycled; 13.8v when cycled daily
  • Vabs 14.7v - 14.8v

6v golf cart batteries

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.

Trojan T-105

www.trojanbattery.com_images_image_library_trjn_t105_lr.jpg

The Trojan T-105 RE is the canonical deep cycle battery.

Trojan recommends a 14.8v absorption setpoint.

Other GC-2 batteries

Cheaper and easily exchanged batteries are often recommended as “learner batteries” that will be destroyed by newbie error.9)

These batteries:

“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)

carbon FLA

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)

VRLA

Words of Wisdom: “Generally, gel and AGM batteries have about 20% less capacity, cost about two times more, and have a shorter cycle life than comparable flooded lead acid batteries.” – Trojan20)

Valve Regulated Lead Acid batteries have the same chemistry as FLA but

  • the battery case is sealed (with overpressure safety valve)
  • the electrolyte is stabilized so it does not slosh. The electrolyte can be gelatinized or embedded in an absorbant material.21)

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.

AGM

This type of VRLA battery stabilizes the electrolyte in an Absorbent fiberGlass Mat (AGM).

  • PRO
    • able to charge/discharge huge amounts of current
    • can be mounted in odd positions, including upside down
    • generally maintenance-free which can be a benefit if mounted in inaccessible areas
    • no outgassing under normal conditions
  • CON
    • generally double the cost of FLA for about 20% less capacity22)
    • Premium AGM need much more charging current than flooded, and it may be more than solar-only setups can provide.23) The charging rate given by Lifeline, a respected AGM manufacturer, is at least C/524) or battery life will be affected. They also say it is ok to charge up to 5C25) (!).26) Odyssey specifies C/2.5 charging rates for their batteries.27)28)
    • even cheap AGM require more current than flooded: minimum C/5, max C/3.(20A-33A per 100Ah of battery capacity).
    • fully charged AGM are slightly lower voltage than FLA29) (12.6 vs 12.7).
    • outgassing is less likely but still need to be vented30)

Common setpoints (check your manufacturer's recommendation):

  • charge with greater current like C/3
  • Vfloat 13.2v when not deeply cycled; 13.8v when cycled daily
  • Vabs 14.6v - 14.7v31)

Also see SternWake's advice on charging AGM batteries.

Gel cell

This type of VRLA battery gelatinizes the electrolyte to stabilize it. While renowned for their duty cycles, they are rarely used in campers.

Carbon foam AGM

Firefly makes the Oasis (AGM) battery with carbon foam. Claimed benefits are:

  • increased current acceptance
  • Absorption stage required only 1x/month or so rather than every day or so. Since Absorption stage is long-duration, this can cut down charging time considerably.

chain store batteries

i5.walmartimages.com_asr_523d2f9f-6552-48c6-928c-9eebf09ab0e9_1.3ffce13c71014acbc0a6c469b62333dc.jpeg“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.

lithium chemistries

In theory LifePO4 lithium batteries should be near perfect for boondocking.

Pro:

  • can accept significant current throughout charging, unlike lead-acid which accepts relatively current during absorption.
  • can be cycled down to 80% depth of discharge compared to lead-acid's 50%.
  • higher number of rated cycles than lead-acid, perhaps 2000.33)
  • half the weight, smaller size
  • no mounting restrictions
  • no venting required
  • not rated over 20 hours like lead-acid; LiFePO4 Ah are rated at .5C.34)
  • 4S LiFePO4 cells are a good fit for nominal 12v systems. They are fully charged at ~13.8v and 80% discharged around 12.6v.

Con:

  • much more expensive
  • cannot be charged below freezing (use is ok)
  • can be damaged by heat

For best life (and to decrease the likelihood of cell imbalance) Charge to 13.8v and disconnect voltage at 12.6v.35), 36), 37), 38)

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)

Further reading:

Other lithium chemistries like “Li-ion” and “Li-po” are rarely used. They are prone to thermal runaway (fire) and 3S packs do not line up well with 12v use. They are fully charged at 12.6v and and 80% discharged at ~11.2v.41) These lower voltages may trigger low voltage disconnect on devices that expect lead-acid range voltages.

7S banks make a decent 24v bank. They are 29.4v fully charged and 80% discharged at 26.11v, compared to lead acid's 29.6v max voltage and 24.2v 50% DoD cutoff. This could preclude alternator charging, however.

2) , 5) 50% SoC
3) , 6) 80% SoC
7) 10A-20A per 100Ah of battery capacity
17) “Renewable Energy”
19) $175 for T-105RE vs $145 for T-105
24) 20A per 100Ah of battery capacity
25) 500A per 100Ah of battery capacity!
27) 40A per 100Ah of battery capacity
electrical/12v/deep_cycle_battery.txt · Last modified: 2018/12/12 13:46 by frater_secessus