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electrical:12v:lifepo4_batteries_thread

Words of Wisdom:

“LFP batteries do not need to get back to 100% SOC, ever” – Maine Sail1)
“Others will likely say you should read a few of the previous pages in this thread and a lot of your questions will be answered. I'd suggest you pick and random number between 1 and 420 and start on that page and just read 10 pages. I promise you will learn about something relating to your inquiry.” – SailRedemption 2)

Synopsis of C&S LiFePO4 batteries thread

The Cruisers and Sailing forum has a 300+ page LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks thread.

There is gold in the thread but it can be hard to find. This page attempts to distill the thread down to best practices for RV/campervan use of LiFePO4 batteries. Links are provided so readers can go back to the thread for context-checking and critical examination.

Suitability for RV use

LiFePO4

  • can take full current during charging; there are no stages in the lead-acid sense
  • can use more nominal capacity than lead-acid (80-90% v. 50% depth of discharge)
  • are not damaged by low SoC
  • can be mounted vertically or on a thin edge (but not flat)3)
  • smaller and lighter than like-capacity LA
  • minimal Peukert effect; capacity remains stable under heavy load4)
  • minimal voltage sag under load5)
  • rated for many cycles (2000+, etc)
  • limitations found in Electric Vehicle applications generally not applicable to RV use

Con

  • high initial cost
  • sensitivity to excessive ambient heat (>95F?)6)
  • cannot be charged below 32F

Maine Sail has discussed the most common errors made by RVers vis-a-vis lithium.

Charging

three stage chargers

LFP batteries do not require multi-stage charging like lead-acid batteries; they are charged until full and then charging is stopped. LFP does not have an absorption phase like LA7) and does not need floating in the LA sense8).

That said, smart chargers can be used to charge LFP. Maine Sail describes the thought process of utilizing LA smart chargers with LiFePO4:

With a Li bank we are in bulk 99% of the time, on most boats with inadequate charging capacity. Once we get to “limiting voltage” how long do we stay there? Is this driven by time or current? After the limiting/absorption/CV stage voltage where do we go and how do we again re-trigger “charging”…“ – Maine Sail9)

Lead-acid stages are adapted to LiFePO4 charging like this:

  1. Bulk - the majority of charging, terminates when full (Vabs)
  2. Absorption - setpoint (Vabs) where LFP is “full” according to intended use. Note that full here is the target desired by the user, not necessarily 100% SoC. It is common to run a short Absorption stage as the charge controller will allow.
  3. Float - The setpoint (Vfloat) below which bulk charging starts again. Since LFP has a low self-discharge rate all charging stops until load pulls voltage below this setpoint. Common Vfloats include 50% SoC (12.8v), 80% SoC (13.25v) or 90% SoC (13.4v). 100% SoC + balancing charge (13.8v) float may be acceptable for batteries that are cycled daily 10) but others have warned about pushing for the final 5% of capacity.11) Note that as long as the charging system can hold Vfloat under load it will stay in Float and not restart Bulk charging.

Electric vehicle applications vs RV applications

There are important differences between how RVers/boaters and Electric Vehicles (EVs) charge and discharge LFP battery banks. EV batteries are used aggressively: charged at high current to full and drained at high current to empty. This can cause individual cells to get out of sync, voltage-wise, with others and warrant a BMS.

In Fractional C scenarios BMS failures may cause more damage than they prevent.12)

Fractional C

Boat and RV house batteries are generally charged and discharged with less amperage than the bank's capacity, hence Fractional C use.

  • charging at a rate less than the bank's Ah capacity (C/<1)
  • discharging at C/2 or less.

This can make LFP banks less complex and less expensive in an RV application. Assessing SOC by voltage is also more accurate with Fractional C charging/discharging than with high currents.

LiFePO4 charging for RVs: running barefoot

T1 Terry describes a gentle, relatively low-tech approach that can work well with vandweller use patterns. It does not use a BMS, judging that in a vandwellers' use the BMS introduces more hazards than it alleviates. This approach:

  • starts with a balanced battery
  • uses moderate terminal voltage (13.8v) to gently rebalance cells13),14), and even become more balanced over time.15) If they do get out of balance for some reason, run more conservative SoC voltages until they can be manually rebalanced.16)
  • charges and discharges at less than 1C, common in solar charging. i.e. less than 100A with a 100Ah bank.
  • keeps the voltage “between the [voltage] knees”, 13.8v - 11.8v.17) This is also called the “middle 80%”.
  • does not require strapping18) although it is a recommended best practice19)

Since the defining feature of this approach is the lack of BMS, it will be referred to as “barefoot” on this page.

LiFePO4 characteristics

LiFePO4 are fully charged at 13.6v (3.4v/cell).20) Voltage rises very quickly after LiFePO4 are fully charged (faster than the internal temps rise) so voltage can be used to assess full charge.21)

There is anecdotal evidence that sustained Vfloat at 13.8v (3.45v/cell) for months may not affect capacity.22)

LiFePO4 cells have a markedly lower full charge voltage than LiPo which is 3.7v.23) The LFP gives up energy density for stability and safety.24)

Rated cycles will be easier to attain by going no lower than 80% DoD, 12.4v (3.10v/cell).25) Shallower cycling may increase both cycles and lifetime capacity.26),27)

Under lab conditions LFP banks have been cycled 2000x to 100% DoD resulting in ~5% loss in capacity. MaineSail points out that regular battery temps above 80F would affect these results significantly.28)

So how do experts charge?

  • Marine
    • T1 Terry uses high cell voltage alarms, charges at 14.0v and floats at 13.8v.29) He explains his float setpoint: “By maintaining the 13.8v, any loads are powered by the charging device, not the battery, unless the load is greater than the charging current. if you start with full batteries, it's longer before recharging will be required. There is no point in keeping a charging device running just to maintain a float condition, but if the engine is already running, it may as well keep the battery fully charged.”30)
    • OceanSeaSpray charges at 14.0v and floats at 13.35v.31)
    • diugo charges at 13.9v and floats at 13.3v. HVD is 14.4v and LVD is 12.1v.32)
    • Many marine users charge to 13.8v and float at 13.4v: Maine Sail33), klaus5312334), ebaugh,35)36)
    • dlentz charges to 13.8v and floats at 13.2v37)
  • RV
    • On solar blars does a short 14.v “absorption” by a 13.4v “float”. He notes the float is high-ish but that the sun goes down so it will not be at that voltage a long time.38) When charging from shore power through a converter he does a very short “absorption” at 14.2v followed a 13.25v “float”.39)
    • 29chico charges at 13.8v-13.9v40) and recommends going no lower than 12.6v, approx. 10% SoC.41)
    • John61ct recommends charging at 13.8v and floating at 13.2v, when float is required by the controller.42)

notable voltage reference points

BANK CELL NOTES
16.80 4.20cell damage43)
14.40 3.60High Voltage Cutoff44)
13.80 3.45barefoot Vabs, with gentle overcharge for balancing. Max safe non-BMS voltage.45)
13.60 3.40fully charged
13.40 3.35barefoot Vfloat
13.25 3.3120% Depth of Discharge
12.80 3.2050% Depth of Discharge FIXME. Long-term storage.46)
12.40 3.1080% Depth of Discharge. Best for storage?47)
11.80 2.95barefoot Low Voltage Disconnect. Min safe non-BMS voltage. 48)
11.20 2.80100% Depth of Discharge49). Low Voltage Cutoff.50)

stress on charging equipment

”…lithium batteries can be very hard on alternators and DC to DC chargers that aren't designed for continuous 100% output as these battery will accept all they can get till they are full, no charge tapering caused by terminal voltage rise like lead acid batteries… overheating of charging equipment can be a problem.“ – T1 Terry51)

Increased alternator load may also cause premature destruction of belts52) or throwing a belt off the pulley.53)

banding

Banding (physical binding of cells to prevent expansion) may not be necessary in RVs:

“Normal use for a lead acid would be 0.1C charge, <1.0 C discharge, and the Winston cells would not need banding for the same parameters. If you plan to flex the muscles (pun intended) and charge at 1.0C and discharge at 3.0C, then the experts recommend banding.” – deckofficer54)

sizing

“Due to conservative a-hr ratings, nil Peukert, and being able to cycle to 80% DOD, you will discover that what ever a-hr rating worked fine for you in a lead bank, 1/2 that rating will give the same usable a-hr in LiFePO4. So if you were going to upgrade your lead bank to 400 a-hr, 200 a-hr LiFePO4 would be the same upgrade over your current 320 a-hr bank” – deckofficer55)

alternator charging

Unless the alternator's output voltage is reduced it can be high enough to damage your LFP cells (ie, > 13.8v).

7) LFP will pull full current until very nearly full
8) due to low self-discharge rates
electrical/12v/lifepo4_batteries_thread.txt · Last modified: 2017/12/09 15:02 by frater_secessus