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opinion:frater_secessus:agm2lifepo4 [2023/12/28 11:42]
frater_secessus [Can I just drop in the LFP and go?] |
opinion:frater_secessus:agm2lifepo4 [2024/03/23 13:13] (current)
frater_secessus [shore power charging] |
| * charging only above freezing 32F/0C | * charging only above freezing 32F/0C |
| - what your existing charging system provides (or can be configured to provide). | - what your existing charging system provides (or can be configured to provide). |
| | * alternator voltage and rating if a [[electrical:12v:alternator|combiner]] or [[electrical:12v:b2b|DC-DC]] is present |
| | * [[electrical:solar:charge_controller|solar charging]] (profiles, configurability, output rating, protections if any) |
| | * [[electrical:converter|shore power charging]] (voltage, output rating, stages if any) |
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| If the existing system cannot be configured to meet LFP's needs then you are looking at replacing some components. For more specific guidance please share details about your setup, use case, budget, and priorities. | If the existing system cannot be configured to meet LFP's needs within [[electrical:12v:alternator_details#current|the alternator's comfort zone]] then you are looking at replacing some components. For more specific guidance please share details about your setup, use case, budget, and priorities. |
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| Note: **undercharging lithium is not an issue** unless you are running out of power. This is a radical change from lead-battery charging. | Note: **undercharging lithium is not an issue** unless you are running out of power. This is a radical change from lead-battery charging. |
| ==== Will I need a special LFP solar charge controller? ==== | ==== Will I need a special LFP solar charge controller? ==== |
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| Probably not, assuming your existing [[electrical:solar:charge_controller|controller]] has a profile that meet's the LFP's stated needs, or has [[electrical:solar:charge_controller_setpoints|configurable setpoints]]. | Probably not, assuming your existing [[electrical:solar:charge_controller|controller]] has a profile that meet's the LFP's stated needs, or has [[electrical:solar:charge_controller_setpoints|configurable setpoints]] that you can adjust to meet needs. |
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| ==== What do I use for Boost, Float, etc? ==== | ==== What do I use for Boost, Float, etc? ==== |
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| Take your battery manufacturer's specs and [[electrical:12v:drop-in_lifepo4#charging|apply them thusly]] | Use a lithium profile or take your battery manufacturer's specs and [[electrical:12v:drop-in_lifepo4#charging|apply them thusly]] |
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| Note: advanced or geeky folks may want to take [[electrical:12v:drop-in_lifepo4#an_approach_to_greater_longevity|a different approach]]. | Note: advanced or geeky folks may want to take [[electrical:12v:drop-in_lifepo4#an_approach_to_greater_longevity|a different approach]]. |
| Some controllers do use temperature sensing to cut off lithium charging near the freezing point. Some only do this in the preconfigured Lithium profiles (Renogy, on supported models). | Some controllers do use temperature sensing to cut off lithium charging near the freezing point. Some only do this in the preconfigured Lithium profiles (Renogy, on supported models). |
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| Note that having a temp probe left over from lead battery charging does not mean low-temp cutoff is supported in the controller. With lead banks temp sensing is used to adjust charging voltage for conditions, not turn off charging in freezing weather.((lead batts actually thrive on charging in freezing temps since higher SoC means it takes deep cold to freeze them solid)) | Note that having a temp probe left over from lead battery charging does not mean low-temp cutoff is supported in the controller. With lead banks temp sensing is used to adjust charging voltage to battery temperature((higher voltage when cold, lower voltage when hot)) and are not intended to turn off charging in freezing weather.((lead batts actually thrive on charging in freezing temps since higher SoC means it takes deep cold to freeze them solid)) Read your controller docs to see if the temp probe can be used to sense freezing temps with Li banks. |
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| | In addition, some batteries have a low temperature charging disconnect built into the BMS, or internal [[opinion:frater_secessus:self-heated_lifepo4|heating to skirt the issue]]. The owner can also [[https://diysolarforum.com/threads/lifepo4-heating-pad-for-cold-temperatures.5/page-26|add external warming]]. |
| ===== alternator charging ===== | ===== alternator charging ===== |
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| Alternator charging of aux battery banks is a balancing act, regardless of battery type. We want enough current that we get adequate charging in the time allotted, without drawing so much current that we [[electrical:12v:alternator#effect_on_alternator|stress the alternator]] or battery bank itself. | Charging LFP by alternator is understood by few but confidently((and often hysterically)) discussed by many. [[https://www.youtube.com/watch?v=VY2b71zoyvg|Battle Born summarizes the subject]] like this: |
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| | >> ...can I use the alternator to charge the batteries? The answer is yes, you can. Under most circumstances you don't even need to modify your system |
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| | Alternator charging of aux battery banks is a balancing act //regardless of battery chemistry//. We want enough current that we get adequate charging in the time allotted without drawing so much current that we [[electrical:12v:alternator#effect_on_alternator|stress the alternator]] or battery bank itself. |
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| Excess charging current //can// be an issue with charging from the alternator. Discharge and charge current is expressed in [[electrical:12v:battery_capacity|C]]. A 100Ah LFP is 100A at 1C, 50A at 0.5C, etc. Common defaults: | Excess charging current is rarely a problem with solar, but //can// be an issue with charging from the alternator. Discharge and charge current is expressed in [[electrical:12v:battery_capacity|C]]. A 100Ah LFP is 100A at 1C, 50A at 0.5C, etc. Common current specs: |
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| * 0.2C - the rate at which battery manufacturers rate cycle life -- this might be telling us something.. | * 0.2C - the rate at which battery manufacturers rate cycle life -- this might be telling us something.. |
| * 1.0C - typical limit enforced by BMS | * 1.0C - typical limit enforced by BMS |
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| Alternator charging rates should probably be ≤0.5C, with 0.4C probably being the sweet spot. So ≤80A for a 200Ah bank. | From the battery's point of view, alternator charging rates should be ≤0.5C, with 0.4C likely being the sweet spot. So ≤80A for a 200Ah bank. From the alternator's point of view, current should be no greater than [[electrical:12v:alternator_details#current|the alternator can comfortably provide]] under expected charging conditions. |
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| [[https://www.youtube.com/watch?v=VY2b71zoyvg|Battle Born says this]] about charging LFG by alternator: | |
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| >> ...can I use the alternator to charge the batteries? The answer is yes, you can. Under most circumstances you don't even need to modify your system | |
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| | As the Battle Born quote above suggests, for average LFP banks and average cargo van alternators these criteria will often be met with no reconfiguration. If the criteria cannot be met then we will need [[electrical:12v:b2b|DC-DC charging]] or some other approach to manipulate voltage and/or current.((diode-based isolator, dedicated second alternator, etc)) |
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| ==== disabling alternator charging ==== | ==== disabling alternator charging ==== |
| ==== Can I still use my existing DC-DC charger for alternator charging? ==== | ==== Can I still use my existing DC-DC charger for alternator charging? ==== |
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| Yes, unless it has some weird hardcoded [[electrical:solar:charge_controller_setpoints|setpoints]] that are incompatible with the LFP bank. | Yes, unless it has some weird hardcoded [[electrical:solar:charge_controller_setpoints|setpoints]] that are incompatible with the LFP bank. Or is rated higher than the charging current your LFP should receive. |
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| If it was already handling a similar-size AGM bank then it will probably be fine. | If it was already handling a similar-size AGM bank then it will probably be fine. |
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| Caveat: charging LFP with DC-DC //can// be somewhat harder on the alternator than charging AGM with DC-DC. AGM current acceptance tapers in Absorption which means the DC-DC will not be demanding full powah from the alternator. Depending on the setup Lithium can make the "full pull" right up to 100% state of charge. In practice this might only make a real difference with long drives and heavily discharged batteries. | Caveat: charging LFP with DC-DC //can// be somewhat harder on the alternator than charging AGM with DC-DC. AGM current acceptance tapers in Absorption which means the DC-DC will not be demanding full powah from the alternator. Depending on the setup Lithium can make the "full pull" until the bank is near full charge. In practice this might only make a real difference with long drives and heavily discharged batteries. |
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| An esoteric caveat: it's conceivable there are edge cases where a vandweller has an oversized DC-DC charger (60A?) that has not destroyed a small alternator (90A?) yet because the small lead bank (100Ah?) has only been pulling a fraction of the alternator's capabilities. Shoehorning in a 300Ah LFP bank would max the 60A charger and place serious demands on the undersized alternator. | An esoteric caveat: it's conceivable there are edge cases where a vandweller has an oversized DC-DC charger (60A?) that has not destroyed a small alternator (90A?) yet because the small lead bank (100Ah?) has only been pulling a fraction of the alternator's capabilities. Shoehorning in a 300Ah LFP bank would max the 60A charger and place serious demands on the undersized alternator. |
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| IF **other forms of shore power charging are available** or **shore power charging will last at least overnight** the [[electrical:converter|shore power charger]] can be a simple, inexpensive, single-stage charger set to ≤13.4v. This will allow near full charge and load support without overcharging. | IF **other forms of shore power charging are available** or **shore power charging will last at least overnight** the [[electrical:converter|shore power charger]] can be a simple, inexpensive, single-stage charger set to a relatively low voltage (13.4v - 13.6v?) This will allow near full charge and load support without overcharging. |
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| If **shore power charging is the only source of charging** or if **time on the charger is limited** then a more aggressive, expensive, higher-capacity multistage charger may be warranted. | If **shore power charging is the only source of charging** or if **time on the charger is limited** then a more aggressive, expensive, higher-capacity multistage charger may be warranted. |
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| | If the van will **be on shore power for an extended period** (days, weeks, months) then a small single-stage converter charger set to quite low voltage (13.2v?) would hold the battery at middling states of charge. It only needs to be a bit bigger than the expected loads so gradual charging can occur. |
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| [Note from secessus: there is [[https://diysolarforum.com/threads/why-you-cannot-charge-lifepo4-below-0-degrees-celsius.2912/post-199774|some evidence]] that LiFePO4 fares better when cell temps are held to warming temps than just above freezing. For this reason I warm my bank to 50F.] | [Note from secessus: there is [[https://diysolarforum.com/threads/why-you-cannot-charge-lifepo4-below-0-degrees-celsius.2912/post-199774|some evidence]] that LiFePO4 fares better when cell temps are held to warmer temps than just above freezing. For this reason I warm my bank to 50F.] |
