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electrical:12v:drop-in_lifepo4

[draft]

Words of Wisdom: “Pricing of lithium-ion batteries is slowly changing from obscenely expensive to only moderately unaffordable…” – Rob Beckers1)

Drop-in Lithium (LiFePO4)

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“Drop-in” batteries are pre-built LiFePO4 (LFP) lithium (Li) batteries that are more or less the same size and voltage as lead-chemistry 12v batteries.2) 100Ah is the most common capacity, but larger and smaller batteries do exist.

The other approach to lithium is DIY3), where one selects cells, BMS, and other components and builds it themselves. That topic is beyond the scope of this article. People interested in DIY might want to read the DIY LiFePO4 Battery Banks subforum over on DIY Solar. You can also see the Lithium Battery Summary article for high points of a debate on lithium suitability for long term use.

Beginning in roughly 2022, the market for drop-in Lithium batteries started growing extremely quickly and prices started coming down out of the stratosphere. Knock-off branded Chinese lithium batteries have also come up in quality, with a slew of different “brands” (that are all idendical) starting to feature things like bluetooth monitoring, cold cut-off, and internal heaters.

The DIY Solar Forums are a good place to check for updated info, as well as links to teardown reviews and disassembly videos where the internal component quality has been given a detailed review.

Drop-in lithium benefits

  • Drop-ins are ready to use. Buy it, install it.

Drop-in lithium limitations

  • Drop-in batteries are typically “black boxes” with no practical way tell what is going on inside. Some do have bluetooth or other methods to interact with the insides.
  • Drop-ins are not always repairable. Some, like SOK metal-cased batteries are reportedly repairable.
  • Drop-ins are very expensive compared to DIY lithium

overall lithium benefits

  • Li batteries (Li) have greater energy density than lead (Pb), so are smaller for a given capacity. They are also much lighter for the same capacity.
  • Li batteries have very low internal resistance, meaning they can charge/discharge at up to 1C, or 100A for a 100Ah bank. Note that for longevity reasons this is often restricted to C/2, or 50A for a 100Ah battery.
  • If treated properly Li can have many times the cycle life of Pb.4)
  • Li batteries are not vulnerable to partial states of charge like Pb. If anything they prefer sitting at PSoC.
  • flat voltage curve - stable voltage over much of its state of charge
  • very little reduction of capacity at higher discharge rates compared to lead5)
  • much less voltage sag under heavy loads

overall lithium limitations

  • Li is relatively expensive
  • Li cells need a BMS to protect them from damage.6) For example, Li can be damaged by overvoltage, undervoltage, charging below freezing (32F), etc. Some batteries have low-temp cutoff and/or internal heating to address the cold-charging limitation. Most Drop-in Lithium batteries will have a BMS integrated into them, but raw cells do not.
  • Li can be damaged by long duration at full charge or high voltage, or high ambient temperatures
  • the flat voltage curve makes gauging SoC by voltage extremely challenging, and battery “gauges” designed for lead chemistry batteries will not work. An amp-counting battery monitor will be more useful with Li.

charging

Note: there are many myths about charging LFP.

There are two main approaches to charging a drop-in:

  1. Normal - charging to manufacturer specs, which should get you through the warranty period; see charging specs below. Battle Born, for example, recommends “14.2V – 14.6V for bulk and absorption and float to be 13.6V or lower”, and absorption duration of 20 minutes.
  2. Advanced - charging/using more gently, which may extend life dramatically.

chargers

Some chargers are labeled “lithium-compatible”. This can mean:

  • built-in configurations for LFP, which may or may not match mfg recommendations. The Renogy Rover Elite 20A, for example, defaults to 14.4v Absorption7) and stops charging8), then allows the bank to drop to 13.2v before starting absorption again.
  • the charger has a configurable USER mode where you can config the setpoints you want

If you already own a fully-configurable charger then you probably don't need to buy a lithium-specific one. [my settings are here - secessus]

Note: due to the relatively low voltages of LFP banks9) while charging MPPT controllers will typically outperform PWM.

current

Lithium can famously accept huge amounts of current; this does not mean it is good for it to do so.

  • Maximum10) - 1C (100A for 100Ah battery)
  • Manufacturer recommendation - ≤0.5C (≤50A for 100Ah battery)
  • cycle rating (longest life) - 0.2C (20A for 100Ah battery)

Lithium does not have a minimum charging current in the lead-battery sense. Charge as slow as you like.

voltage

LFP are typically fully charged at 13.6v, so Absorping at 13.8v for gentle overvolting and floating at 13.2-13.4v are common setpoints for folks who want to get maximal life from the bank. Observe cell voltages and return to mfg-spec voltage if/when balancing is needed.

Andy from Off-Grid Garage found at moderate charging rates like ~C/5:

  • charging to 3.5vpc (14v for a 4S bank) at and stopping yielded near 97.6% SoC
  • charging to 3.5vpc (14v for a 4S bank) and holding voltage yielded little further capacity (99.7% SoC)
  • charging to 3.4vpc (13.6v for a 4S pack) and stopping yielded 89% SoC.11)
  • charging to 3.4vpc (13.6v for a 4S pack) and holding voltage for 4+ hours yielded 98.2% SoC.

memecreator.org_static_images_memes_5578892.jpg Manufacturers need simple instructions that will still allow the batteries to meet their advertised lifetime and reduce customer support issues. In this scenario higher charging voltages have the following benefits to the seller:

  • allow use of conventional lead-chemistry battery chargers or “lithium” chargers (reduce customer confusion)
  • ensure batteries can deliver 100% of advertised capacity (reduce customer service calls)
  • fully charge the batteries under the most challenging scenarios (solar charging, where charging duration is constrained by sundown)
  • predictable SoC at end of charging (reduce customer service calls)
  • faster charging
    • higher charging rate - bigger differences between charger and battery voltage mean more current (customer happy with fast charging). Note that more current is not necessarily the key to battery longevity.
    • at higher charging voltages12) little or no absorption time is required. “Charge and stop.” (simpler for customers)
  • raise cell voltage so passive top-balancing can occur (customers get to see the vaunted cell balancer feature).

But higher charging voltages are more likely to cause cell imbalance and behavior that worries new users, like premature disconnect of charging and apparent battery voltage spikes.13)

To walk the battery back down from this precipice we need to lower charging voltage, at least temporarily:

  1. reduce Absorption (“boost”) voltage to 13.8v14), (3.45Vpc) or lower
  2. verify that charging completes as expected. If cell voltages are visible verify their balance is improving.
  3. optional: start moving back up by 0.05v or 0.1v increments if desired, watching as in step 2 above. Example: 13.8v, then 13.85v, then 13.9v, etc. There is little reason to charge >14.0v (3.5Vpc).

an approach to greater longevity

Bottom line, stay within the manufacturer recommended specs, and you should be fine, go beyond that (more conservative) and you should be extra fine. – Dzl

[Fine vs Extra Fine is like normal driving vs hypermiling; getting big MPG numbers is possible but requires forethought and a willingness to alter one's own driving style. – secessus]

In this section we are thinking about “extra fine”. One way to baby the bank is to treat it like the manufacturer did when getting the much larger cycle numbers like 8,000. These tend to be:

  • charge/discharge rates limited to C/5 (20A for a 100Ah bank)
  • at human-comfortable temperatures. There is some evidence that, in addition to stopping charge at freezing, charging currents should be limited below 60F.
  • not held at high states of charge (see below)
  • discharged no lower than 20% State of Charge
  • generally operated “between the voltage knees”

The overall idea is to treat the bank like there is no BMS, no safety net. Charging rates/voltages are conservative and charging takes longer. Note this only works if one has enough time for gentle charging; if you only have a 2-hour charging window hard-and-fast is the only option and we accept the shorter life trade-off.

an example of long life

The longest-lived, fully documented, instrumented LFP bank in actual use appears to be Maine Sail's 400Ah bank from 2009. It's still holding rated capacity. His charging regime is

  • charge to 13.8v (3.45Vpc)
  • at 0.4C
  • with 30mins Absorption
  • he is charging this bank from alternator, so he stops after Absorption. In other contexts with solar charging he has used 13.4v (3.35Vpc) as a quasi-Float (voltage floor below resting voltage)

Maine Sail's approach demonstrates several of the life-extending approaches discussed in this sub-article.

Note from secessus:

We don't have to slavishly follow what MS and the other early-adopters are doing, but we would be wise to pay attention. I am suggesting the values above would be a better default than every Li profile I have seen so far. I'd encourage new users to start from there and adjust as needed.

high SoC

There is some evidence that high states of charge may worsen capacity degradation at temperatures >86F:

When capacity degradation occurs in LFP cells at elevated temperatures, both temperature (30–60 ​°C) and SOC determine the degradation rate. High storage temperature, which is the most important factor, combined with high SOC result in the greatest degradation

charging voltage

If there is sufficient charging time, a lower charging voltage may offer these advantages:

  • gentler charging rate
  • less (or zero) time at 100% State of Charge
  • cells more likely to stay in balance

At gentle charge rates like 0.2C, the following patterns emerge:

  • ≤13.4v will not fully charge the bank
  • 13.4v will get the bank to ~85% over a couple of days, then stabilize there
  • 13.6v will charge to 100% SoC with several hours of Absorption
  • 13.8v will charge to 100% SoC with token Absorption (10-30 minutes) and cells tend to stay in balance.
  • ≥14.0v will charge to 100% SoC with no absorption. Cell voltages tend to diverge.

Some drop-in BMS only start top-balancing at 14.2v15) but increasing voltage to that level tends to cause imbalance. Catch-22.

If charging at lower voltages the initial charge (and occasional charges thereafter) might be to mfg spec voltage. This might allow the BMS to reset “full” and top-balance to the degree that such balancing works.

Also see: Will Prowse's Lithium Battery Longevity: Double or Quadruple the Life of Your Lithium Battery

self-heating batteries

Lithium cannot be charged in freezing temps. We can either:

  1. cut off charging in the chargers; and/or
  2. warm the batteries, either internally (self-heating) or externally (typically with warming mats).

Self-heating is convenient, and does not require lithium- or temperature-aware chargers. The downsides are:

  • typically substantially more $$$ than warming pads
  • can miss out on charging opportunities. Not a big deal with smaller solar-only setups, but can really hamper alternator or big solar.

how self-heating batteries work

The last issue is a function of how they work.

  1. When the BMS detects dangerously-low temps it deactivates charging to the battery cells
  2. any charging power is sent to the internal warming pads, typically ~50w
  3. when the BMS detects the temps are ok it turns the charging back on. The warming may be switched off, or may continue to warm the battery further to a given temp setpoint.

how this could cause a missed charging opportunity

Imagine a half-hour drive on a freezing morning with a 50A DC-DC charger.16) You could pump 25Ah17) into an externally-warmed battery, or you could use the alternator to run the 50W internal heater and get 0Ah replaced.

With only small solar the Wh consumed overnight and Wh not produced in the morning might be a breakeven.

myths

myth: you can charge Li as fast as you want

BMS are often configured to limit charging to 1C (100A for a 100Ah battery) as an absolute maximum. Manufacturer's who actually warranty the battery often recommend .5C (50A) or even .2C (20A) for longevity. Charging at too high a rate for conditions can cause permanent damage to the battery.18), and the effect may be worse at low temperatures.

Charging Li at very high rates may also strain the alternator.

myth: you can't use a combiner to charge batteries of different chemistries

There are two different challenges here:

  1. different resting voltages - if the lead rests at 12.8v and LiFePO4 at 13.6v then when charging stops the lead batt could put a drain on the li batt. The charging relay19) is primarily there to stop house loads from draining the starter battery, but this separation also means the starter battery cannot drain the house battery.20) 21)
  2. acceptable charging voltages - the alternator voltage needs to be acceptable (not necessarily optimal) to both batteries. Read on.

acceptable charging voltage ranges

We can assume the alternator voltage is acceptable to the starter battery because the manufacturer designed that system.22). So we only have think about whether or not the alternator voltage is acceptable to the house bank.

For the following thought experiment we will use some a typical alternator output voltage of 14.2v and house bank charging voltage setpoints (“Absorption” or “Boost” voltage, Vabs); check your vehicle's alternator voltage and battery manufacturer charging specs to make your actual decision.

Chemistry Acceptable Vabs Optimal Vabs
Gel 14.0v - 14.3v 14.2v
AGM 14.2v - 14.5v 14.4v
Flooded 14.4v - 14.8v 14.6v
LiFePO4 13.6v - 14.4v 14.0v23)

Let's think about some combinations. In all these cases alternator charging is extremely useful for Bulk stage charging but may not be sufficient on its own:

  • easy to meet charging requirements by alternator alone
    • starter battery + LiFePO4. Charging LFP at 14.2v is a good balance between moderate voltage and charging time. Lithium is not affected by partial states of charge so you can charge as little or much as you want. Caveat: alternator charging should be disabled if you drive long enough to reach your desired state of charge. Specifically, LFP should not be held at high voltage after reaching full charge.
  • unlikely to meet charging requirements by alternator alone - could theoretically meet charging by alternator alone if given sufficient time. Unfortunately most people don't drive enough hours 24) to complete Absorption; incomplete charging damages lead batteries. less one is driving for many hours each day it is unlikely that there will be sufficient time to complete Absorption. For this reason solar or other long-duration charging source is often added to handle Float and late Absorption.
    • starter battery + AGM. 14.2v from the alternator is in the acceptable range, but just barely. Extremely long Absorption would be required and adding solar is highly recommended.
    • starter battery + Gel. The alternator is putting out the exact Vabs spec'd by the manufacturer. Caveat: gel can be damaged (electrolyte cavitated) by excess current. Ensure your setup charges withing the mfg current specs.
  • Useful but impossible25) to meet charging requirements by alternator alone - cannot reach Vabs in any case. Solar is nearly mandatory, although DC-DC charging will do it if one is driving many hours a day.
    • starter battery + flooded. Alternator charging is useful for Bulk stage (shoving Ah into the bank) but it cannot meet the minimum Vabs.

myth: you have to charge Li to 100%

Lead batteries require fully charging to 100% state of charge (SoC) but lithium batteries do not. Charging them fully at higher voltages can cause cells to become further out of balance. Leaving them at 100% SoC can cause degradation.26)

This paper found that charging to only 50% SoC resulted in extension of

lifetime expectancy of the vehicle battery by 44–130%. When accounting for the calendar ageing as well, this proved to be a large part of the total ageing.

Having said that, there are valid reasons for charging to 100%:

  • going into a period where you will need max capacity
  • to perform a capacity test
  • to reset the BMS amp/SoC counter
  • to top-balance cells27)

myth: if you don't charge to 14.4v the cells won't balance

Charging at higher voltages (and higher rates) causes cell imbalance, which the (usually comically-undersized) balancer attempts to correct.

This occurs because the cells hit the high-voltage “knee” at slightly different times, at which point they “race” ahead of the others. Stay below the knee and the cells will tend to stay in balance on their own.

  • in cyclic use (like offgrid) ~13.8v (3.45Vpc) is sufficient to reach 100% with some absorption, without antagonizing cell balance.
  • when on shore power (like on a power pedestal) voltages as low as 13.4v will slowly bring the bank up to 100% SoC.

Overcharging lithium to run the cell balancers is like driving up to every red light at 100mph in order to trigger the ABS. It's causing the problem in order to solve the problem.

myth: lithium doesn't need absorption

Lithium doesn't need Absorption in the way lead does. In some circumstances, however, an Absorption duration can help match charging setpoint expectations to state of charge reality.

Most charts and tables showing voltage v. SoC assume moderate rates of charge like 0.2C (20A per 100Ah of capacity). At that rate the charts are reasonably accurate.

Very high charge rates, as sometimes seen with alternator charging or large shore power chargers, can throw off these chart estimates. 0.8C charging might only yield 80% SoC when one expected 90%. Conversely, very low charge rates may result in unintentional overcharge; 98% instead of 90%.

One rule of thumb is that the bank is fully charged when you are at your target voltage and current acceptance has decreased to ≤0.10C. Other sources give 0.04C or other values that vary with the base charging rate.28). One could try tail currents in this ballpark and see which gives you the SoC you expect.

Further reading: Off-grid Garage videos testing various absorption approaches

myth: you can't charge Li with a lead battery charger

Depends on the charger and how your Li wants to be charged. Most fully-configurable chargers can be used to charge Li.29) Note that some so-called “lithium compatible” chargers may have presets that do not match the requirements of your particular battery, so read the specs.

Here is the order of operations:

  1. Read and understand your Li battery manufacturer's charging specs30)
  2. Read and understand the charger's specs and functionality
  3. decide whether the charger can charge to mfg specs

Armed with a full understanding, here is one approach to thinking about lead battery charger setpoints for lithium banks:

  • Absorption voltage (Vabs) - whatever charging voltage your battery manufacturer recommends.31)
  • Absorption duration - whatever the battery manufacturer recommends, typically 0 to 20 minutes.32)
  • Float voltage (Vfloat) - Something like 13.4v33) is a good compromise. See the discussion on float below.
  • Absorption reconnect - this voltage is the setpoint below which Absorption34) is restarted. Normally in a solar configuration Vfloat is held until sun goes down, solar conditions otherwise deteriorate, or a load is applied that is more than the solar can support. Start with a value like 13.2v and see how your system behaves. Adjust as needed.
  • Equalize voltage (Veq) - Li does not require equalization. If it cannot be disabled in the controller it is common to set Veq the same as Vabs so it becomes a non-issue.35)
  • Equalize duration - zero, or as low as the controller will allow. Will make no practical difference when Veq is set to Vabs.
  • Temperature compensation - Lead needs different charging voltages at different temperatures but Li does not. Change setting to 0mV/cell.36)

myth: you shouldn't Float lithium

Li certainly doesn't need Float voltage (Vfloat) in the sense lead-chemistry batteries do, but the Float setpoint is still useful for Li battery banks.37)

Reminder: lead requires Float because

  1. lead banks need to be held at 100% SoC whenever possible for their long-term health
  2. the self-discharge rate is so high that they lose capacity just sitting there

Neither of these is true for Li, which dislikes sitting at 100% SoC and has vanishingly-low self-discharge rates.38) So with lithium Float is used for a different purpose, as a voltage floor. It is a voltage below which the charger shouldn't let the bank fall while charging is present. Without Vfloat (or a very low one) the bank would charge then fall until reaching the “re-bulk” setpoint.39). After initial charging loads would run off the battery instead of the charging source. Having a sane Vfloat allows Li to “relax” after charging while retaining the desired amount of Ah/Wh capacity.

What Vfloat setpoint should actually be is a matter of some discussion and experimentation. Each setup (and use case) is different, but in general40)

  • <13.4v will allow the bank to settle below 100%
  • 13.4v will hold the bank near whatever SoC it was charged to. If in doubt, this is a good default for solar charging.41)
  • >13.4v will continue to charge the bank beyond the SoC it was charged to during Absorption. This may be useful if the Vabs value is set intentionally low.

If you cannot set a Float within the confines of the Li profile then leverage the USER or GEl profile, modifying as described in the previous section.

myth: you must use DC-DC for alternator charging Li

Depends on the battery, the alternator, the use case, and even the combiner. For example, Battle Born says this about direct-charging lithium:

Yes, you can. Under most circumstances you don't even need to modify your system.

They do recommend a BIM or DC-DC charger for banks >300Ah.

why an isolator?

So if isolator charging might work and DC-DC charging does work42) why would we even consider using an isolator?

  • you already have an isolator installed from a previous Pb bank. Might as well see if it meets needs “for free”.
  • even if you have no isolator installed an isolator is much less expensive, costing as little as 1/10th the price of DC-DC. If after testing you do decide to go DC-DC you can carry the isolator as a backup. Or daisy-chain the DC-DC behind it for units that use D+ rather than voltage triggering.
  • an isolator is likely to charge with more current than smaller (~20A) DC-DC units during shorter drives.43)
  • you want to be able to self-jumpstart

testing your isolator with Li

but that Victron video

Why would a manufacturer of pricey DC-DC chargers want to publish a video of a big LFP pack destroying a low-output car alternator44) at idle speeds? And why would they turn comments off? Oh, right.

The setup:

  • Victron 12v 300Ah Smart LiFePO4, no BMS. (website). ~50% DoD at the start of each test.
  • Victron BMV 712 to measure current into battery
  • “traditional” alternator (Expom ER-438740,45) used in small vans and passenger cars ≤2.0 liters in Eastern Europe. Quick googling shows it's somewhere between a 70A and 90A alternator.
  • Balmar alternator (six series, with external regulator and optional temperature sensor). This dual-fan line is available in 70A, 100A, and 120A models. The 96A cap shown in the monitoring software rules out the 70A model.
  • dedicated negative return (“ground”) wire between house battery and charging system. Typical relay-charging setups use the vehicle's chassis as the return leg, introducing a great deal of resistance.46)

The results:

Keep in mind that alternator RPM is typically 3x engine RPM.

  • “traditional” alternator
    • 1.500rpm47) - 65.1A. 43deg C.
    • 3.000rpm48) - 78.9A. 43deg C.
    • a 126 and 128deg C interior temps were shown but it was not clear what they they are from.
  • Balmar
    • 2,100rpm49) - 50A, 42deg C external, 74deg C internal.
    • 3,600rpm50)
charging lithium batteries at low RPM results in the altenator overheating.51)

Yes, a 300Ah LFP bank can smoke a ≤90A alternator at idle. Is anyone surprised? That's why we don't don't charge at idle, and don't direct-charge big banks off small alternators. Or run big DC-DC off small alternators for that matter. In other breaking news: carrying 1000lbs of cement bags in the back of your Civic will cause damage, too.

They go on to list the workarounds:

  1. install a high output alternator that can handle demand at idle ($hundreds)
  2. fit an externally-regulated alternator with temp sensor, as shown in the video (~$850)
  3. or (surprise!), install a Victron Orion Tr-Smart DC-DC charger (~$250)
  4. {not mentioned: adding resistance by using the chassis for negative return, or by other means52) ($0 to ~$34).

{ I would very much like to have seen the regular alt and all three four workarounds demonstrated at the same RPM settings. – secessus }

and that Sterling video

Sterling also made a video, but left comments on. They use a 90A Bosch alternator53) and 1 Sterling LiFePO4, presumably 100Ah. The battery is connected to an additional load (ahem) and has a heavy dedicated NEG return.54) The alt has no cooling airflow as one would have on a moving vehicle (or even an alt behind a radiator fan).

{Note from secessus: I rather like Sterling stuff, but will call out FUD anywhere I see it}

the video

why do we put on our lithium batteries that you must use a battery-to-battery charger….

Because Sterling's main product line is pricey battery-to-battery (DC-DC) chargers.

what I want to show is what temperature the alternator will go to when you start putting maximum current through the alternator55)

Any alternator will overheat at continuous max current. This is like saying “don't buy Nike shoes because if you jump off a building you'll break your legs.” Duhhh.

The question is: how much current will a LiFePO4 pull from the alternator in a normal install? The sellers of DC-DC chargers are uninterested in showing us this. Spoiler: about 0.32C, or 32A per 100Ah of capacity.

try and keep your alternator down below 80% [output]56)

I'd say even lower, 50% for road vehicles with internal regulation.

The testing, using external load to drive up the current:

  • 99.9A from the 90A alt: 150C at the coils.57) Don't do that.
  • 97A from the 90A alt: 165C at the coils.58). Seriously stop doing that.

Then, as with Victron, reduce alternator RPM so it struggles.59)

  • 92A out of the 90A alt at reduced RPM. 184C at the coils.60) You're going to wreck it

At this point they disconnect the load and battery charge acceptance begins to drop. Within 5 mins61) acceptance has dropped to 76A.

At 11:45 he re-applies the artificial load “to see how much hotter it will get”. ???

DC-DC chargers are good enough technology that there is no need to trick customers into buying them.

the comment section

But the variability in charge rate, alternator output voltage, battery voltage-The calculations for the majority of the period would just not be correct.62)

Correct; direct-charging does not have consistent current so cannot be precisely calculated.

we do have customers who simply use our non current limiting charge systems on lithium and rarely complain63)

Seems like it's worthy of further study.

further reading

myth: floating will cause microcycling

Microcyling means bouncing between two64) voltage setpoints. The concern here is that each one of these bounces would count as a cycle and subtract from the rated 3,000 cycles or whatever the manufacturer claims. Those who take this position suggest setting Float voltage to the same as Absorption voltage to avoid microcycling. The drawback to this approach is two-fold:

  1. Li doesn't like to be held at 100% State of Charge for long periods
  2. In practice a solar-charged bank doesn't bounce between Float and Absorption during the course of a day

The 2nd point takes a bit of explaining. A solar charge controller completes Absorption then falls to into Float where it will remain as long as the sun65) cooperates. Absorption can be re-triggered if voltage falls below the Absorption Reconnect setpoint, but that setpoint is even lower than Vfloat. If that happens the solar charging has already been overtasked and we will get a “microcycle” during that day in any case if the sun comes back.

myth: lithium doesn't lose capacity under heavy loads

Lead batteries famously yield different capacities at different discharge rates; this is the reason they are rated at a specific rate (C/20).

Lithium batteries do exhibit lower apparent capacity under extreme loads66) but the mechanism is concentration polarization rather than Peukert.67) At normal discharge rates (<1C) LFP capacity is relatively stable.

BMS functions

Lithium cells can hurt themselves if left unsupervised. A BMS (Battery Management System) is device that tries to keep them out of trouble. It's like a fence around the yard that won't prevent skinned knees but should keep the kids out of the street where they could get really hurt.

Drop-in Lithium batteries all have a BMS integrated inside of them, so there is typically less to worry about. However, not all of them will have the same protection features so do your research carefully.

Typical BMS protections may include:

Charging cut-off

Charging is disabled for a few different reasons: temperature extremes, high cell voltage, overcurrent.

Note that the BMS overcurrent protection kicks in only at the limit, typically 1C (100A for a 100Ah battery). See the section on the benefits of even gentler charging.

low temperature charging cutoff

People who camp in cold weather may want to select a battery that has low temperature cutoff, which disables charging near freezing.68). The belt-and-suspenders solution is both low temperature cutoff and a way to keep the batteries warm. Some pricier batteries have internal heating, or you may DIY a heater.

It may be possible to add low temperature cutoff to a battery whose BMS lacks that feature. It requires the charging source that can be disabled on demand. In this approach the we are reversing the logic so that the default state of the charging source is disabled and it is only enabled when battery temps are warm enough to safely charge.

For belt-and-suspenders you could add a $10 12v temp controller or NO thermal switch in series [with the disabling method]. Stick the probe (or switch) on the bank and only complete the [disabling ] circuit when the ignition is on and measured temp >0C or >2C or whatever you prefer.

discharge cut-off

Discharge is disabled for similar reasons as charging: extremes in temperature, cell voltage, or current.

The low temperature discharge cut-off is typically much lower (like -20C) than for charging.

wake-up

After low voltage cutoff drop-ins often go into a sleep mode. The “wake-up” procedure will be detailed in the battery manual but typically involves a “dumb” charger that does not check for battery voltage before beginning charging. Some chargers (Victron SmartSolar for example) can emulate this dumb mode in Lithium mode in order to wake the battery. See details in product descriptions below.

This issue occurs because the charging and discharging channels are separately controlled. When the battery is too low the discharge channel is disabled which means battery voltage is not exposed to the outside world. The charging channel remains active but the charger doesn't know that. “Jumping” the battery works because the jumper cables are not looking for voltage.

cell balancing

Cell balancing attempts to keep all the cells in the same voltage ballpark. The method used in most drop-ins is passive balancing, which applies a small amount of braking a bit on runaway cells.69)

The image on the right shows the LFP voltage curve: a lower knee (below ~3.2v) on the left, a wide, flat section in the middle, then the upper knee (~3.4v) on the right. Between the knees the voltage stays relatively flat and cells stay in relative balance. The first cell[s] that hits a knee will race in that direction ahead of the others.

This matters because voltage extremes, high or low, are not good for the cell. Cell imbalance can change effective capacity, since a wayward cell could trigger cell voltage BMS protection before the bank is fully charged or discharged.

Consider these two batteries, using made up round numbers to illustrate the point. Cells are arranged in series so the volts add up:

  • balanced70)
    • overall battery voltage = 13.6v
    • cell 1 = 3.400v
    • cell 2 = 3.400v
    • cell 3 = 3.400v
    • cell 4 = 3.400v
  • unbalanced71)
    • overall battery voltage = 13.6v
    • cell 1 = 3.300v
    • cell 2 = 3.400v
    • cell 3 = 3.400v
    • cell 4 = 3.500v ←- a “passive” balancer will try to slow this one down72)

In both cases the overall battery voltage is 13.6v, but in the unbalanced battery cell #1 is lagging (reducing capacity) and cell #4 is too high. The balancer will attempt to rein in #4 but the effect is tiny. Using a typical balancing current of 50mA, if you are charging at 20A that means the cells are receiving 5A except cell #4 which gets only 4.95A, about a 1% difference. Tiny balancing currents and the propensity of cells to race away explains why balancing is so gradual.

minimizing cell imbalance

It is more practical to minimize imbalance than to try to rebalance them after the fact. Here are some approaches:

  • discharging gently, maybe a max of C/2
  • charging gently (minimum current needed to get the battery charged in the amount of time available). 50mA only makes a 1% difference when charging at 20A but 4% difference at 5A.
  • charging voltages closer to 13.6v than to 14.4v.73).
  • charging to a lower SoC, like 85%-95% instead of 100%.

bluetooth

Some batteries have BMS with bluetooth or other forms of connectivity. With these you would be able to see more information, like individual cell balance, statistics, setting and the status of any protections.

Brands and specs

The most popular drop-ins at various price points appear to be:

  • premium - Battle Born
  • moderate - SOK
  • inexpensive - Chins and Ampere Time (no cutoff) or Weize (cold cutoff)

Charge/discharge rates are expressed in C. Heated models typically shunt charging current to the internal warmer until the battery comes up to safe temp.74)

[Please verify stats and specs before ordering. This is a best-effort listing but things change fast in this segment - secessus]

Battle Born

With cold cutoff. BB also has a line of heated LFP.

Will Prowse teardown video. Absorption 14.4v for 20 minutes, Float 13.6v. Recommended charge rate C/2.75)

Victron

These batteries have cold cutoff. The batteries themselves do not have bluetooth, but are intended to be connected to other Victron gear that can talk to the batteries. Victron's value proposition is increased inter-device communication when installing a Victron ecosystem.

teardown pics and discussion

Charge 14.2v @ C/2, Float 13.6v.76)

SOK

SOK models are popular with tinkerers because the case and internals are easy to disassemble for service, inspection, and troubleshooting.77)

With cold cutoff. Will Prowse teardown video

Charge 14.6v, max 50A, 20A recommended.78),79)

wake-up procedure

Rebel Batteries

Rebel Batteries - with cold cutoff and bluetooth (BT demo). Will Prowse teardown video

RB says “just make sure [the charger] has a Lithium charging profile”.80) The shore power charger they recommend charges at 14.3-14.4v @ 20A. The BMS has a max charging rate of 100A.

Weize

Cold cutoff. Will Prowse teardown video

Ampere Time

Now known as LiTime.

No cold cutoff. Will Prowse teardown video

AT recommends charging at 14.4v81) at 20A.

PowerUrus

Budget PowerUrus line by RoyPow

Will Prowse teardown, low temp cutoff confirmed. Bluetooth.

Renogy

Renogy's original “Smart Lithium” line has no cold cutoff. They are releasing a new line of temp-protected, bluetooth enabled batteries.

Charge 14.4v @ max C/2.82)

Initial activation may require a quick press-and-release of the button rather than longpress.83)

Relion

Unheated versions appear to lack cold cutoff. No bluetooth.

Charge 14v - 14.6v, 1C up to 100A max. Float 13.3v - 13.8v.84)

Kilovault

Kilovault HLX series, internally heated. Recommended by Marine How-To.

Lithionics

Chins

Base models with no cutoff or bluetooth and “Smart” models with 5A internal heating and bluetooth (BT demo).

Will Prowse teardown video of base model; he notes they are similar inside to Ampere Time.

Absorption 14.2-14.6v, C/5 charging recommended, C/1 max.85) Heating element on supported models appears to turn on near freezing then turn off at 50F.

An after-sale email from Chins to an Amazon buyer86) said:

Charging Limit Voltage: 14.2V
Over Voltage Disconnect Voltage: 14.6V
Over Voltage Reconnect Voltage: 13.8V
Float Charging Voltage: 13.8V
Low Voltage Disconnect Voltage: 11.6V
Low Voltage Reconnect Voltage: 12.4V
Under Voltage Warning Voltage: 12.8V

There have been reports that the BMS does not disconnect the battery at low voltages. External LVD is always advisable.

According to Amazon answers87) reset due to LVD is done by applying enough voltage to raise to 11v: “when the battery voltage reaches about 11V, the battery's BMS will be activated.”

batteries with troubling reports

further reading

2)
6S lead nominal voltage is 12.0v; 4S LiFePO4 voltage is 12.8v
3)
do it yourself
4)
treated improperly they can suffer a sudden death
5)
there is a non-Peukert mechanism that starts to attenuate apparent capacity at very high C rates
6)
some DIYers run Li “barefoot” (without a BMS)
7)
user-adjustable
8)
probably a good thing – secessus
9)
under 13.6v for most of the charging cycle
10)
max for our drop-ins; EV packs can have much greater throughput
11)
good for 90% SoC targeting? – secessus
12)
≥14.0v
13)
“apparent” because the controller and battery monitor will report the spike, but since the BMS is disconnected the battery cells do not experience the spike
14)
13.6v may be required over many days for particularly bad cases
15)
3.55Vpc
16)
Doesn't have to be DC-DC but it makes the math easier because charging current is more stable.
17)
minus the energy it took to hold temp overnight
20)
unless self-jumpstarting
21)
note, however, a gotcha with VSRs
22)
and we can observe that the vehicle starts on demand
23)
this is a matter of some debate. LFP mfg charging recommendations are often quite high - secessus
24)
typically 5-6 hours from 50% SoC
25)
practically
26)
lithium metal “plating”
27)
to the degree this works
29)
Some simpler controllers that only have selectable presets like AGM or gel may have a preset that overlaps with the correct charging specs for your battery. Read the specs carefully.
30)
if you are adventurous and want maximal life read and understand what the Li cells actually need
31)
see the section on longevity in this article
32)
charging voltages ≥14.0v typically require no absorption duration at all
33)
3.35vpc
34)
“boost” in Renogy/EpEver nomenclature
35)
some folks who charge to lower voltages like 13.6v may use Veq to raise bank voltage into the 14s for various purposes. See the section on longevity.
36)
lead defaults are something like -4mV/cell
37)
Li batts with active balancers or other parasitic loads may be drawn down by them
38)
but see this cautionary tale about add-on balancers depleting/killing a $4,000 bank
39)
when a fresh charge cycle begins
40)
and using nominal 12v math
41)
When charging from shore power 13.4v will eventually charge and hold at 100% SoC, which may be undesirable
42)
as long as we don't oversize it for the bank and the alternator
43)
On longer drives the DC-DC will be able to provide higher voltage than the alternator so its current will remain stable while isolator current drops with the voltage delta
44)
2:50
45)
label visible at 2:51
46)
the resistance of steel is ~10x that of copper
47)
~500 engine RPM
48)
~1000 engine RPM
49)
~700 engine RPM
50)
~1200 engine RPM - 94.3A, unstated alt case temp, 130deg C internal. Their conclusion:((5:27
51)
3:13
52)
see below
53)
0:30
54)
3:25
55)
0:49
57)
4:12
58)
5:09
59)
5:45
60)
6:45
61)
11:07
64)
or more
65)
and system capacity
66)
and perhaps extreme temperatures
68)
Lithium is permanently damaged by charging below freezing
69)
Active balancing, more complex and expensive, takes current from the highest and gives it to the lowest.
70)
it will never be this perfect
71)
greatly exaggerated
72)
an “active” balancer would steal from the rich (#4) and give to the poor (#1
73)
the batt will eventually reach 100% State of Charge (SoC) when charged at 13.6v
74)
some run the warmer and rely on the BMS to release charging cutoff when safe to charge.
76)
printed on case of 100Ah
77)
may void warranty
81)
+/- 0.4v
85)
description and chart on https://amzn.to/3ueMs7F
86)
forwarded by personal correspondence to secessus
electrical/12v/drop-in_lifepo4.txt · Last modified: 2024/02/18 14:28 by frater_secessus