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electrical:12v:drop-in_lifepo4 [2023/12/17 15:23] frater_secessus [and that Sterling video] |
electrical:12v:drop-in_lifepo4 [2024/06/19 19:36] frater_secessus [chargers] |
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* Li is relatively expensive | * Li is relatively expensive | ||
* Li cells need a [[# | * Li cells need a [[# | ||
- | * Li can be **damaged** by long duration at full charge or full discharge. | + | * 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, | * the flat voltage curve makes gauging SoC by voltage extremely challenging, | ||
+ | ===== sizing the bank ===== | ||
+ | Other than cost, there is little downside to having a larger LiFePO4 bank.(([[electrical: | ||
+ | |||
+ | * greater [[opinion: | ||
+ | * supporting larger discharge current - [[electrical: | ||
+ | * safely accepting larger charging currents when charging time is limited. | ||
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Lithium can famously accept huge amounts of current; | Lithium can famously accept huge amounts of current; | ||
- | * Maximum((max for our drop-ins; EV packs can have much greater throughput)) - 1C (100A for 100Ah battery) | + | * Absolute maximum((max for our drop-ins; EV packs can have much greater throughput)) - 1C (100A for 100Ah battery) |
- | * Manufacturer | + | * Manufacturer |
- | * cycle rating (longest life) - 0.2C (20A 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. | Lithium does not have a minimum charging current in the lead-battery sense. | ||
+ | If you choose to end Absorption based on current, you might start out with [[electrical: | ||
=== voltage === | === voltage === | ||
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Charging Li at very high rates may also strain the [[electrical: | Charging Li at very high rates may also strain the [[electrical: | ||
+ | |||
+ | ==== myth: you can't use a combiner to charge batteries of different chemistries ==== | ||
+ | |||
+ | There are two different challenges here: | ||
+ | |||
+ | - **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 [[electrical: | ||
+ | - **acceptable charging voltages** - the alternator voltage needs to be acceptable (not necessarily // | ||
+ | |||
+ | === acceptable charging voltage ranges === | ||
+ | |||
+ | 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)). | ||
+ | |||
+ | For the following thought experiment we will use some a typical alternator output voltage of 14.2v and house bank charging voltage [[electrical: | ||
+ | |||
+ | ^ Chemistry | ||
+ | | Gel | 14.0v - 14.3v | 14.2v | | ||
+ | | AGM | 14.2v - 14.5v | 14.4v | | ||
+ | | Flooded | ||
+ | | LiFePO4 | ||
+ | |||
+ | |||
+ | Let's think about some combinations. | ||
+ | |||
+ | * **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 [[electrical: | ||
+ | * **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 ((typically 5-6 hours from 50% SoC)) to complete Absorption; | ||
+ | * starter battery + AGM. 14.2v from the alternator is in the acceptable range, but just barely. | ||
+ | * starter battery + Gel. The alternator is putting out the exact Vabs spec'd by the manufacturer. | ||
+ | * **Useful but impossible((practically)) 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. | ||
+ | |||
+ | |||
==== myth: you have to charge Li to 100% ==== | ==== myth: you have to charge Li to 100% ==== | ||
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* to top-balance cells((to the degree this works)) | * to top-balance cells((to the degree this works)) | ||
+ | ==== myth: lithium batteries draw the full current until they are almost full ==== | ||
+ | |||
+ | |||
+ | * both lead and li chemistry charge acceptance [[electrical: | ||
+ | * both will charge ~at [[electrical: | ||
+ | |||
+ | If we observed closely we //would// see some differences: | ||
==== myth: if you don't charge to 14.4v the cells won't balance ==== | ==== myth: if you don't charge to 14.4v the cells won't balance ==== | ||
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* **Equalize duration** - zero, or as low as the controller will allow. | * **Equalize duration** - zero, or as low as the controller will allow. | ||
* **Temperature compensation** - Lead needs different charging voltages at different temperatures but Li does not. Change setting to **0**mV/ | * **Temperature compensation** - Lead needs different charging voltages at different temperatures but Li does not. Change setting to **0**mV/ | ||
+ | |||
+ | Note: if you are willing to babysit, even a single-voltage power supply would work. Stop charging when the voltage hits your desired setpoint. | ||
==== myth: you shouldn' | ==== myth: you shouldn' | ||
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=== testing your isolator with Li === | === testing your isolator with Li === | ||
- | see [[electrical: | + | see [[electrical: |
==== but that Victron video ==== | ==== but that Victron video ==== | ||
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Charging is disabled for a few different reasons: | Charging is disabled for a few different reasons: | ||
- | |||
- | People who camp in cold weather may want to select a battery that has "low temperature cut-off", | ||
Note that the BMS overcurrent protection kicks in only at the limit, typically 1[[electrical: | Note that the BMS overcurrent protection kicks in only at the limit, typically 1[[electrical: | ||
+ | |||
+ | === 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.((Lithium is permanently damaged by charging below freezing)). | ||
+ | |||
+ | It may be possible to add low temperature cutoff to a battery whose BMS lacks that feature. | ||
+ | |||
+ | >> For belt-and-suspenders you could add a $10 12v temp controller or NO thermal switch in series [with the disabling method]. | ||
+ | |||
+ | Note that some chargers have built-in low-temp charging cutoff already. | ||
+ | |||
+ | |||
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Some batteries have BMS with bluetooth or other forms of connectivity. | Some batteries have BMS with bluetooth or other forms of connectivity. | ||
+ | Having visibility into the state of the BMS/cells might be useful for tech geeks, but often muddies the waters for non-technical customers. | ||