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electrical:12v:drop-in_lifepo4 [2024/09/08 20:39]
frater_secessus [myth: lithium doesn't need absorption] |
electrical:12v:drop-in_lifepo4 [2025/08/28 15:17] (current)
frater_secessus [choosing a drop-in LFP battery] fleshed out |
| ===== drawbacks of lithium ===== | ===== drawbacks of lithium ===== |
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| * Li is relatively expensive up front | * Li has (historically at least) been relatively expensive up front. Prices started plummeting somewhere around 2024. |
| * Li cells need a [[#bms_functions|BMS]] to protect them from damage.((some DIYers run Li "barefoot" (without a BMS) )) 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 cells need a [[#bms_functions|BMS]] to protect them from damage.((some DIYers run Li "barefoot" (without a BMS) )) 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 | * Li can be **damaged** by long duration at full charge or high voltage, or high ambient temperatures |
| ===== choosing a drop-in LFP battery ===== | ===== choosing a drop-in LFP battery ===== |
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| There are many factors here which only you will be able to assess. | There are many factors here which only you will be able to assess. Basic due diligence will involve: |
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| | * reading and understanding the specs (see below) |
| | * searching for youtube teardowns and testing on that particular model |
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| | Shortcut: Will Prowse has [[https://www.mobile-solarpower.com/lithium-batteries.html|a list of recommended, tested batteries]] which can be trusted. |
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| | **Heads up**: do not use lowest-possible-price as the main criterion for a battery unless there was a test/teardown available for it. |
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| ==== voltage ==== | ==== voltage ==== |
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| 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) | 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) |
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| | 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 [[electrical:12v:alternator|alternator charging]] is present, [[electrical:12v:self-jumpstarting|self-jumpstart]], etc. Read the battery manual for specifics. |
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| 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 overvoltage. Charging is typically disabled when one or more cells rises to ~3.65v (~14.6vv for the pack) |
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| | === low temperature (~freezing) charge cutoff === |
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| | [not present in all BMS] |
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| | LFP cells are damaged by charging when the cells are at ~freezing temperatures.((discharging too, but the limits are much colder)) |
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| | 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. |
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| | [[opinion:frater_secessus:self-heated_lifepo4|externally warmed vs. self-heated LiFePO4]] |
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| === high temperature charge/discharge cutoff === | === high temperature charge/discharge cutoff === |
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| 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. | 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. |
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| === cell balancing === | === cell balancing === |
| 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] | 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] |
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| === low temperature (~freezing) charge cutoff === | |
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| [not present in all BMS] | |
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| LFP cells are damaged by charging when the cells are at ~freezing temperatures.((discharging too, but the limits are much colder)) | |
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| 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. | |
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| === heater control === | === heater control === |
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| [rare] | [less common] |
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| BMS in batteries with [[electrical:12v:drop-in_lifepo4#self-heating_batteries|a self-heating function]] trigger the heating when they sense that cell temps are at some defined setpoint. | BMS in batteries with [[electrical:12v:drop-in_lifepo4#self-heating_batteries|a self-heating function]] trigger the heating when they sense that cell temps are at some defined setpoint. |
| - verify that charging completes as expected. If cell voltages are visible verify their balance is improving. | - verify that charging completes as expected. If cell voltages are visible verify their balance is improving. |
| - 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)**. | - 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)**. |
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| ==== an approach to greater longevity ==== | ==== an approach to greater longevity ==== |
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| > Bottom line, stay within the manufacturer recommended specs, and you should be fine, go beyond that (more conservative) and you should be extra fine. -- [[https://diysolarforum.com/threads/sok-206ah-battery-concerns.32368/post-397575|Dzl]] | |
| | >> ... the reactions that cause [LiFePO4] aging are strongly correlated with voltage - David Howey, Professor of Engineering Science at the University of Oxford((https://youtu.be/1LygMkJpN6Q?si=MiLB6m-QUjPSzcs8&t=2372)) |
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| | >> Bottom line, stay within the manufacturer recommended specs, and you should be fine, go beyond that (more conservative) and you should be extra fine. -- [[https://diysolarforum.com/threads/sok-206ah-battery-concerns.32368/post-397575|Dzl]] |
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| [//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] | [//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] |
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| =====self-heating batteries===== | ===== waking lithium batteries ===== |
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| Lithium cannot be charged in freezing temps. We can either: | Lithium banks can go dormant at low voltages in order to protect themselves: |
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| - cut off charging in the chargers; and/or | - the BMS turns off the DISCHARGE channel to keep cell voltage from going any lower |
| - warm the batteries, either internally (self-heating) or externally (typically with warming mats). | - 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 |
| | - which means smart chargers don't see battery voltage |
| | - and think there's a problem so they won't start(("too smart for their own good")) |
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| | What we need is a //dumb// charging source to get the party restarted. Another battery. Power supply. Charger with a "wake LiFePO4" mode, vehicle starter battery, etc. |
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| Self-heating is convenient, and does not require lithium- or temperature-aware chargers. The downsides are: | If you want to use the starter battery there are several possibilities: |
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| * typically substantially more $$$ than warming pads | * Rigs with IGN-triggered relays can briefly turn the key to ACC then back off.((don't leave it on ACC long or the dead Li will suck power from the starter battery. Either turn ACC off again or actually start the vehicle.)) |
| * can miss out on charging opportunities. Not a big deal with smaller solar-only setups, but can really hamper alternator or big solar. | * 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 |
| | * If you have no other options you can remove either the starter battery or house battery and locate them so a pair of jumper cables can connect them. |
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| ==== how self-heating batteries work ==== | |
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| The last issue is a function of how they work. | |
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| - When the BMS detects dangerously-low temps it deactivates charging to the battery cells | |
| - any charging power is sent to the internal warming pads, typically ~50w | |
| - 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. | |
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| === how this could cause a missed charging opportunity === | |
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| Imagine a half-hour drive on a freezing morning with a 50A [[electrical:12v:b2b|DC-DC charger]].((Doesn't have to be DC-DC but it makes the math easier because charging current is more stable.)) You could pump 25Ah((minus the energy it took to hold temp overnight)) into an externally-warmed battery, or you could use the alternator to run the 50W internal heater and get 0Ah replaced. | |
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| With only small solar the Wh consumed overnight and Wh not produced in the morning might be a breakeven. | |
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| ==== external warming ==== | |
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| If your preferred battery has everything but heating you might want to [[https://diysolarforum.com/threads/lifepo4-heating-pad-for-cold-temperatures.5/page-26|DIY a heater]]. It is typically much cheaper than built-in heat and offers more control. | |
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| 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 [[electrical:12v:alternator#disabling_alternator_charging|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. | |
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| >> 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. | =====self-heating batteries===== |
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| | [see [[opinion:frater_secessus:self-heated_lifepo4|self-heating vs DIY warming]]] |
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| === acceptable charging voltage ranges === | === acceptable charging voltage ranges === |
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| We can assume **the alternator voltage is acceptable to the starter battery** because the manufacturer designed that system.((and we can observe that the vehicle starts on demand)). So we only have think about whether or not the alternator voltage is acceptable to the house bank. | We can assume **the alternator voltage is acceptable to the starter battery** because the manufacturer designed that system.((and we can observe that the vehicle starts on demand)). 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 [[electrical:12v:psoc|the way lead batteries do]]. |
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| For the following thought experiment we will use some a typical alternator output voltage of 14.2v and house bank charging voltage [[electrical:solar:charge_controller_setpoints|setpoints]] ("Absorption" or "Boost" voltage, Vabs); check your vehicle's alternator voltage and battery manufacturer charging specs to make your actual decision. | For the following thought experiment we will use some a typical alternator output voltage of 14.2v and house bank charging voltage [[electrical:solar:charge_controller_setpoints|setpoints]] ("Absorption" or "Boost" voltage, Vabs); check your vehicle's alternator voltage and battery manufacturer charging specs to make your actual decision. |
| | AGM | 14.2v - 14.5v | 14.4v | | | AGM | 14.2v - 14.5v | 14.4v | |
| | Flooded | 14.4v - 14.8v | 14.6v | | | Flooded | 14.4v - 14.8v | 14.6v | |
| | LiFePO4 | 13.6v - 14.4v | 14.0v((this is a matter of some debate. LFP mfg charging recommendations are often [[opinion:frater_secessus:lifepo4_charging_voltage|quite high]] - secessus)) | | | LiFePO4 | 13.6v - 14.4v | 14.2v((this is a matter of some debate. LFP mfg charging recommendations are often [[opinion:frater_secessus:lifepo4_charging_voltage|quite high]] - secessus)) | |
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| * going into a period where you will need max capacity | * going into a period where you will need max capacity |
| * to perform a capacity test | * to perform a capacity test |
| * to reset the BMS amp/SoC counter | * to [[electrical:12v:battery_monitor#drift_and_reset|reset the BMS amp/SoC counter]] |
| * to top-balance cells((to the degree this works)) | * to top-balance cells((to the degree this works)) |
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| Further reading: [[https://www.youtube.com/results?search_query=offgrid+garage+absorption|Off-grid Garage videos]] testing various absorption approaches | Further reading: [[https://www.youtube.com/results?search_query=offgrid+garage+absorption|Off-grid Garage videos]] testing various absorption approaches |
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| | ==== myth: you shouldn't Float lithium ==== |
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| | 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.((Li batts with active balancers or other parasitic loads may be drawn down by them)) |
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| | Reminder: lead requires Float because |
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| | - lead banks need to be held at 100% SoC whenever possible for their long-term health |
| | - the self-discharge rate is so high that they lose capacity just sitting there |
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| | Neither of these is true for Li, which dislikes sitting at 100% SoC and has vanishingly-low self-discharge rates.((but see [[https://www.technomadia.com/2020/06/what-killed-our-rv-lithium-batteries-8-5-years-of-lifepo4/|this cautionary tale]] about add-on balancers depleting/killing a $4,000 bank)) 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.((when a fresh charge cycle begins)). 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. |
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| | 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:((and using nominal 12v math)) |
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| | * <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.((When charging from shore power 13.4v will eventually charge and hold at 100% SoC, which may be undesirable)) |
| | * >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. |
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| | 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. |
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| | ==== myth: you can't equalize LFP ==== |
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| | LFP does not require [[electrical:12v:charging#equalization|Equalization]] (controlled overcharge) the way lead does.((in a sense [[opinion:frater_secessus:lifepo4_charging_voltage|charging LFP at higher-than-necessary voltages]] is akin to EQ)). |
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| | Having said that, folks who regularly charge to less than 100% SoC might use the EQ setpoint to schedule occasional forays to 100% in order to [[electrical:12v:battery_monitor#drift_and_reset|reset the Ah counters]]. A user who regularly charges to 13.5v might use EQ to drive the bank to 14.4v every few weeks. |
| ==== myth: you can't charge Li with a lead battery charger ==== | ==== myth: you can't charge Li with a lead battery charger ==== |
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| Depends on the charger and how your Li wants to be charged. Most **fully-configurable** chargers can be used to charge Li.((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.)) 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. | Depends on the charger and how your Li wants to be charged. Most **fully-configurable** chargers can be used to charge Li.((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.)) 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. |
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| | This info can also be used to make a custom (USER) profile for LiFePO4 banks. |
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| Here is the order of operations: | Here is the order of operations: |
| * **Absorption voltage** (Vabs) - whatever charging voltage your battery manufacturer recommends.((see the section on longevity in this article)) | * **Absorption voltage** (Vabs) - whatever charging voltage your battery manufacturer recommends.((see the section on longevity in this article)) |
| * **Absorption duration** - whatever the battery manufacturer recommends, typically 0 to 20 minutes.((charging voltages ≥14.0v typically require no absorption duration at all)) | * **Absorption duration** - whatever the battery manufacturer recommends, typically 0 to 20 minutes.((charging voltages ≥14.0v typically require no absorption duration at all)) |
| * **Float voltage** (Vfloat) - Something like 13.4v((3.35vpc)) is a good compromise. See the discussion on float below. | * **Float voltage** (Vfloat) - Something like 13.3v-13.4v((3.35vpc)) is a good compromise. See the discussion on float below. |
| * **Absorption reconnect** - this voltage is the setpoint below which Absorption(("boost" in Renogy/EpEver nomenclature)) 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. | * **Absorption reconnect** - this voltage is the setpoint below which Absorption(("boost" in Renogy/EpEver nomenclature)) 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.((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.)) | * **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.((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.)) |
| Note: if you are willing to pay minimal attention, even a single-voltage power supply or [[electrical:12v:alternator#combiners|relay]] would work. Stop charging if/when the voltage hits your desired setpoint. | Note: if you are willing to pay minimal attention, even a single-voltage power supply or [[electrical:12v:alternator#combiners|relay]] would work. Stop charging if/when the voltage hits your desired setpoint. |
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| ==== myth: you shouldn't Float lithium ==== | |
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| 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.((Li batts with active balancers or other parasitic loads may be drawn down by them)) | |
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| Reminder: lead requires Float because | |
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| - lead banks need to be held at 100% SoC whenever possible for their long-term health | |
| - 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.((but see [[https://www.technomadia.com/2020/06/what-killed-our-rv-lithium-batteries-8-5-years-of-lifepo4/|this cautionary tale]] about add-on balancers depleting/killing a $4,000 bank)) 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.((when a fresh charge cycle begins)). 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. | |
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| 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:((and using nominal 12v math)) | |
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| * <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.((When charging from shore power 13.4v will eventually charge and hold at 100% SoC, which may be undesirable)) | |
| * >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. | |
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| 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 ==== | ==== myth: you must use DC-DC for alternator charging Li ==== |
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| Depends on the battery, the alternator, the use case, and even the [[electrical:12v:alternator|combiner]]. For example, [[https://www.youtube.com/watch?v=VY2b71zoyvg|Battle Born says]] this about direct-charging lithium: | If you have been successfully charging an AGM bank through a relay then an LFP bank of similar rated capacity will likely charge similarly. The actual results depend on the battery, the alternator, the use case, and even the [[electrical:12v:alternator|combiner]]. |
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| | [[https://www.youtube.com/watch?v=VY2b71zoyvg|Battle Born says]] this about direct-charging lithium: |
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| >> Yes, you can. Under most circumstances you don't even need to modify your system. | >> Yes, you can. Under most circumstances you don't even need to modify your system. |
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| They do recommend [[electrical:12v:alternator#lithium-specific|a BIM]] or [[electrical:12v:b2b|DC-DC charger]] //for banks >300Ah//. | They do recommend [[electrical:12v:alternator#lithium-specific|a BIM]] or [[electrical:12v:b2b|DC-DC charger]] //for banks >300Ah//. |
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| | If not already present, a small switch to [[electrical:12v:alternator#disabling_alternator_charging|disable the combiner]]((same goes for [[electrical:12v:b2b|DC-DC chargers]])) at will is a good idea. |
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| - Li doesn't like to be held at 100% State of Charge for long periods | - Li doesn't like to be held at 100% State of Charge for long periods |
| - In practice a solar-charged bank doesn't bounce between Float and Absorption during the course of a day | - In practice a solar-charged bank doesn't bounce between Float and Absorption during the course of a day((unless a heavy enough load pulls Vbatt down to the re-Bulk setpoint)). |
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| The 2nd point takes a bit of explaining. A [[electrical:solar:charge_controller|solar charge controller]] completes Absorption then falls to into Float where it will remain as long as the sun((and system capacity)) 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. | The 2nd point takes a bit of explaining. A [[electrical:solar:charge_controller|solar charge controller]] completes Absorption then falls to into Float where it will remain as long as the sun((and system capacity)) 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. |
