Drop-in Lithium (LiFePO4)

• the most common lithium chemistry for 'dwellers is LiFePO4 (LFP hereafter), an extremely stable version of Li-Ion.
• there are two basic types of LFP batteries: drop-in (pre-made retail batteries) and DIY (built from components by the user)
• LFP must not be charged in freezing temperatures.
• drop-in LFP have electronics (BMS) for to help prevent damage to the cells.
• most “problems” reported by LFP owners are user error / misunderstanding, not actual problems with the battery. Read the manual!

about these summaries

“Drop-in” batteries are pre-built LiFePO4 (LFP) lithium (Li) batteries that are more or less the same size and voltage as lead-chemistry (Pb¹⁾ hereafter) 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.

• LFP has very low internal resistance, so they will display less voltage sag and can charge/discharge at up to 1C, or 100A for a 100Ah bank. Note that for longevity reasons this is often restricted to 0.5C, or 50A for a 100Ah battery.⁴⁾
• If treated properly LFP can have many times the cycle life of Pb.⁵⁾
• LFP are not vulnerable to partial states of charge like Pb. If anything they prefer sitting at PSoC.
• LFP charges more efficiently than Pb, so less charging power is wasted. Solar charging in particular will act larger than it is.
• flat voltage curve - stable voltage over much of its state of charge
• very little reduction of capacity at higher discharge rates compared to lead⁶⁾
• 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.

Specific to drop-ins: ready to use; no building required.

• Li is relatively expensive up front
• Li cells need a BMS to protect them from damage.⁷⁾ 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.
• LFP's nominal 12.8v is slightly higher than PB's nominal 12.0v. This can cause subtle misbehavior on components that are expecting a specific trigger voltage.

Specific to drop-ins:: can be “black boxes” with no practical way tell what is going on inside. Some do have bluetooth or other methods to interact with the insides. Of course, lead batteries are black boxes, too. Although technical folks would be curious to know the inner workings, it is not a given that Normal People require that information.

There are many factors here which only you will be able to assess.

Most 'dwellers will use 12v (12.8v) LFP banks. Higher voltages are available but the use cases that require those voltages are beyond the scope of this article.

If you are replacing a lead-chemistry bank whose capacity met needs, you can find the equivalent in LFP by multipying the lead capacity by 0.625. For example, if your 200Ah lead bank met needs it would require ~125Ah of LFP to replace it.

If you are starting from scratch (no existing battery bank) it's a bit of math like any other bank:

• daily power requirements x days of autonomy / usable percentage⁸⁾

Example: if you needed 1,500Wh/day and wanted two days of autonomy, you would require 3,750Wh of LFP.⁹⁾ (1,500Wh x 2 days / .8 usable)

Roughly from most common to least common:

LFP cells can be damaged by undervoltage. Discharge is typically disabled when one or more cells drops to ~2.625v (~10.5v for the pack)

If the BMS has shut down due to low voltage/SoC it usually requires external voltage to “wake” it. Connect another battery, start the vehicle if alternator charging is present, self-jumpstart, etc. Read the battery manual for specifics.

LFP cells can be damaged by overvoltage. Charging is typically disabled when one or more cells rises to ~3.65v (~14.6vv for the pack)

LFP cells can be damaged by use at high temperatures, so the BMS will disable use at extreme temperatures.

The BMS will have stated charge/discharge limits. They may be the same or different; when they are different the discharge current limit is usually higher than the charge current limit. Examples in a 200Ah battery might be:

• 200A charge, 200A discharge
• 100A charge, 150A discharge
• etc

Note that just because you can charge/discharge at higher rates doesn't mean you have to or that you should. Generally speaking LFP prefers current to stay around 0.2C (40A for our 200Ah example) for longevity and cell balance. Also if you will only ever need to discharge at 75A then a 200A BMS is not required.

[not present in all BMS]

LFP cells are damaged by charging when the cells are at ~freezing temperatures.¹⁰⁾

Lack of low temperature cutoff is not necessarily a deal-breaker. Maybe you live in a hot location. Maybe your chargers have low temperature cutoff. Maybe you externally warm your battery.

externally warmed vs. self-heated LiFePO4

The shape of the voltage curve¹¹⁾ can the cell to leave the flat middle first to “run away” from the others, voltage-wise. Example of mild imbalance when charging to 14.0v:

  Cell #1 - 3.500
  Cell #2 - 3.502  
  Cell #3 - 3.518 <- runner  
  Cell #1 - 3.480 <- lagger  

In this case the voltage adds up to 14.0v (good) but some cells are out of whack with the others (bad). The difference between the highest and lowest cell voltage is 38mV;; this difference is called the voltage delta. The pattern would exaggerate as charging continues until one cell (probably #3) trips the BMS' cell overvoltage charging protection. Triggering charge/discharge protection before the battery is completely charged/discharged reduces the apparent capacity of the battery.¹²⁾

Various balancing schemes exist to try to hold back runners. Note: Gentler charging and discharging also tends to improve cell balance.

Passive balancing is the most common BMS cell balancing strategy, present in most drop-in BMS.¹³⁾ The BMS identifies the runner[s] and dissipates some of their charging current¹⁴⁾ with a small resistor. This dissipation reduces the voltage to slow or stop the runner. Passive balancing are usually quite low, perhaps able to bleed off 60mA-100mA. It might take a very long time to balance – wildly imbalanced cells might require a temporary change in charging setpoints.

Passive balancing is total loss: the excess is converted to heat in the resistor.

Active balancing is less common but can usually balancing cells faster and to closer tolerances. The general approach is like Robin Hood – take from the rich (overvoltage cells) cells and give to the poor (undervoltage cells).

The most common active balancer is the double-tiered capacitor type like the Heltec.

Capacitive balancing wastes less power since most of the excess is transferred to the low cell[s]. There are efficiency losses involved (~50%¹⁵⁾) but it's far less than the 100% loss of passive balancing. The main drawbak of capacitive balancers is the current spec (like 5A) only occur at huge deltas; the rest of the time the balancing current is much lower.

In theory inductive active balancers would get around this delta/current relationship. [as of this writing in 2024 I know of no drop-ins that use inductive active balancers - secessus]

[not present in all BMS]

Bluetooth¹⁶⁾ so an app can monitor (and possibly interact with) the BMS. Tech hobbyists typically want to know everything that is going on under the hood. It is debatable whether or not Normal People would benefit from visibility into the inner workings.

[less common]

BMS in batteries with a self-heating function trigger the heating when they sense that cell temps are at some defined setpoint.

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.

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 Absorption¹⁷⁾ and stops charging¹⁸⁾, 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 banks¹⁹⁾ while charging MPPT controllers will typically outperform PWM.

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

• Absolute maximum²⁰⁾ - 1C (100A for 100Ah battery)
• Manufacturer recommended maximum - ≤0.5C (≤50A for 100Ah battery)
• recommended / 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.

If you choose to end Absorption based on current, you might start out with 0.05C and see where that gets you. Adjust as needed.

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.²¹⁾
• charging to 3.4vpc (13.6v for a 4S pack) and holding voltage for 4+ hours yielded 98.2% SoC.

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 voltages²²⁾ 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.²³⁾

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

1. reduce Absorption (“boost”) voltage to 13.8v²⁴⁾, (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).

… the reactions that cause [LiFePO4] aging are strongly correlated with voltage - David Howey, Professor of Engineering Science at the University of Oxford²⁵⁾

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.

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.

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

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 14.2v²⁶⁾ 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

Lithium banks can go dormant at low voltages in order to protect themselves:

1. the BMS turns off the DISCHARGE channel to keep cell voltage from going any lower
2. so the battery internally has ~11.0v or whatever but does not pass that voltage to the terminals on the outside of the battery case
3. which means smart chargers don't see battery voltage
4. and think there's a problem so they won't start²⁷⁾

What we need is a dumb voltage source to get the party restarted. The starter battery will do nicely.

• Rigs with IGN-triggered relays can briefly turn the key to ACC then back off.²⁸⁾
• Rigs with voltage-sensing relays will have to actually start the engine or press a manual override switch to activate the VSR and wake the lithium bank.
• Rigs with diode- or FET-based isolators would start the engine to spin the alternator and get power flowing through the isolator to the sleeping lithium

[see self-heating vs DIY warming]

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.²⁹⁾, and the effect may be worse at low temperatures.

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

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 relay³⁰⁾ 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.^{31) 32)}
2. acceptable charging voltages - the alternator voltage needs to be acceptable (not necessarily optimal) to both batteries. Read on.

We can assume the alternator voltage is acceptable to the starter battery because the manufacturer designed that system.³³⁾. So we only have think about whether or not the alternator voltage is acceptable to the house bank. And remember that lithium chemistries don't need to be fully charged the way lead batteries do.

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.

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 ³⁵⁾ 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 impossible³⁶⁾ 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.

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.³⁷⁾

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 cells³⁸⁾

• both lead and li chemistry charge acceptance will taper when charged by a relay/isolator, and
• both will charge ~at a DC-DC charger's rated output until Absorption voltage is reached

If we observed closely we would see some differences: the LFP taper is shaped a bit differently, and that lead Absorption tends to start earlier and last longer.

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 13.6v 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.

Lithium doesn't need Absorption in the way lead does.³⁹⁾ 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.⁴⁰⁾. 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

Depends on the charger and how your Li wants to be charged. Most fully-configurable chargers can be used to charge Li.⁴¹⁾ 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 specs.
2. if you want to maximize life from your bank consider charging less aggressively
3. Read and understand the charger's specs and functionality
4. decide whether the charger can charge to the way you (or the manufacturer) wants

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.⁴²⁾
• Absorption duration - whatever the battery manufacturer recommends, typically 0 to 20 minutes.⁴³⁾
• Float voltage (Vfloat) - Something like 13.4v⁴⁴⁾ is a good compromise. See the discussion on float below.
• Absorption reconnect - this voltage is the setpoint below which Absorption⁴⁵⁾ 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.⁴⁶⁾
• 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.⁴⁷⁾

Note: if you are willing to pay minimal attention, even a single-voltage power supply or relay would work. Stop charging if/when the voltage hits your desired setpoint.

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.⁴⁸⁾

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.⁴⁹⁾ 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.⁵⁰⁾. 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 we can start with some ballpark assumptions:⁵¹⁾

• <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.⁵²⁾
• >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.

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.

So if isolator charging might work and DC-DC charging does work⁵³⁾ 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.⁵⁴⁾
• you want to be able to self-jumpstart

see this section

Why would a manufacturer of pricey DC-DC chargers want to publish a video of a big LFP pack destroying a low-output car alternator⁵⁵⁾ 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-438740,⁵⁶⁾ 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.⁵⁷⁾

The results:

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

• “traditional” alternator
  • 1.500rpm⁵⁸⁾ - 65.1A. 43deg C.
  • 3.000rpm⁵⁹⁾ - 78.9A. 43deg C.
  • a 126 and 128deg C interior temps were shown but it was not clear what they they are from.
• Balmar
  • 2,100rpm⁶⁰⁾ - 50A, 42deg C external, 74deg C internal.
  • 3,600rpm⁶¹⁾

charging lithium batteries at low RPM results in the altenator overheating.⁶²⁾

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 means⁶³⁾ ($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 }

Sterling also made a video, but left comments on. They use a 90A Bosch alternator⁶⁴⁾ and 1 Sterling LiFePO4, presumably 100Ah. The battery is connected to an additional load (ahem) and has a heavy dedicated NEG return.⁶⁵⁾ 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}

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 alternator⁶⁶⁾

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]⁶⁷⁾

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.⁶⁸⁾ Don't do that.
• 97A from the 90A alt: 165C at the coils.⁶⁹⁾. Seriously stop doing that.

Then, as with Victron, reduce alternator RPM so it struggles.⁷⁰⁾

• 92A out of the 90A alt at reduced RPM. 184C at the coils.⁷¹⁾ You're going to wreck it

At this point they disconnect the load and battery charge acceptance begins to drop. Within 5 mins⁷²⁾ 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.

But the variability in charge rate, alternator output voltage, battery voltage-The calculations for the majority of the period would just not be correct.⁷³⁾

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 complain⁷⁴⁾

Seems like it's worthy of further study.

• mega-thread on DIYsolarforum, including using a resistor to decrease current.
• YT vid where Ron tested 400Ah of LFP with an isolator on his Sprinter.
• YT vid where both 280Ah and 100Ah LFP batts are charged with a Toyota Hilux and and RV.
• technical info and data collected by secessus on direct-charging setups

Microcyling means bouncing between two⁷⁵⁾ 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 sun⁷⁶⁾ 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.

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 loads⁷⁷⁾ but the mechanism is concentration polarization rather than Peukert.⁷⁸⁾ At normal discharge rates (<1C) LFP capacity is relatively stable.

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.

[The market moves so fast that this section was badly outdate. This version is more general. Please verify stats and specs before ordering. - secessus]

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

• premium - Battle Born, Lithionics
• midrange - SOK, Epoch. And there are companies like SunFunKits that will build LFP batteries with the customer's choices of options.
• inexpensive - Li Time (was Ampere Time), Weize, Power Queen, Roypow / Power Urus, etc

To make an informed decision:

1. read and understand the specs ← srsly
2. search Youtube for teardown videos
3. search Will's DIY Solar Forum to see what informed users are saying

• Marine How-two arcticle on drop-ins. ←- highly recommended
• Takeaways from a 13yo battery
• How to Find Happiness with LiFePO4 batteries (archive version)
• Practical Characteristics of Lithium Iron Phosphate Battery Cells
• Overkill Solar BMS manual (pdf).
• Test: Charging to 3.4vpc and 3.5vpc with and without absorption (YT)
• Battle Born: Can I charge my batteries using the alternator? (yt)