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opinion:frater_secessus:agm2lifepo4 [2024/10/07 11:15]
frater_secessus [what about low temp cutoff?] |
opinion:frater_secessus:agm2lifepo4 [2025/02/19 12:20] (current)
frater_secessus [Can I still use my existing DC-DC charger for alternator charging?] changed language |
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| ====== Upgrading an AGM house bank to LiFePO4 ====== | ====== Upgrading a lead-chemistry house bank to LiFePO4 ====== |
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| The pre-made LiFePO4 batteries (LFP hereafter) you seen on Amazon and elsewhere are designed and marketed as //drop-in replacements//: pull your lead battery and "drop in" the LFP. This approach works in many cases because the size, shape, voltage range, and charging requirements are in the same ballpark as traditional lead batteries. It's not a perfect scenario, but will meet the needs of the average user. People with special requirements or an interest in technical details can do the reading/testing required for an optimal setup. | The pre-made LiFePO4 batteries (LFP hereafter) you seen on Amazon and elsewhere are designed and marketed as //drop-in replacements//: pull your lead battery and "drop in" the LFP. This approach works in many cases because the size, shape, voltage range, and charging requirements are in the same ballpark as traditional lead batteries. It's not a perfect scenario, but will meet the needs of the average user. People with special requirements or an interest in technical details can do the reading/testing required for an optimal setup. |
| * 1.0C - typical limit enforced by BMS | * 1.0C - typical limit enforced by BMS |
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| 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. | From the battery's point of view, alternator charging rates should be ≤0.5C, with 0.2C((for [[opinion:frater_secessus:lifepo4_charging_voltage|high charging voltages]])) to 0.4C((moderate charging voltages like 13.8v / 3.45Vpc)) arguably a 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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| 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)) | 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)) |
| 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" until the bank is near full 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 charging current spends less time at full charging current because it spends less time in Bulk (full current) and more in Absorption (tapering current). Depending on the setup Lithium can make the "full pull" until the bank is near full charge. |
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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. |
| 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. | 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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| | ===== what about low temp cutoff? ===== |
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| | Some chargers 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 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]]. |
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| | ==== Wait, LFP can be damaged by charging in cold weather? ==== |
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| | Yes, discharging too but that happens at much lower temperatures than the 32F we use as a hard limit for charging LFP cells. |
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| | If you will be using the batteries in ≤freezing temperatures((it's actually the cell temps that matter, not the ambient temps)) you will need one or more of these protections: |
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| | * a BMS that shuts off charging in freezing temps |
| | * chargers that shut off charging in frezing temps((remember the alternator charging!)) |
| | * a way to warm the battery or otherwise prevent it from getting too cold |
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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 warmer temps than just above freezing. For this reason I warm my bank to 50F.] |
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