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opinion:frater_secessus:lfprationality [2024/03/21 17:45] frater_secessus [LiFePO4, alternator charging, and Nancy Reagan] |
opinion:frater_secessus:lfprationality [2024/03/21 20:30] (current) frater_secessus [limiting current from the alternator] |
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- | The discussions get a little weird. | + | The discussions get a little weird. |
+ | ===== "what you need is a Yeti mug" ===== | ||
- | ===== just say no to drugs ===== | + | **Yeti aficionado: |
+ | **Me: | ||
+ | |||
+ | **Newbie: | ||
+ | **Me:** It is possible to brew too much coffee to put in //any// mug. | ||
+ | |||
+ | **Yeti aficionado: | ||
+ | **Me: | ||
+ | |||
+ | |||
+ | **Yeti aficionado: | ||
+ | **Me:** Or use a big regular mug? | ||
+ | |||
+ | |||
+ | **Yeti aficionado: | ||
+ | **Me:** Rtic and even Walmart' | ||
+ | **Yeti aficionado: | ||
+ | **Me: | ||
+ | |||
+ | **Yeti aficionado: | ||
+ | **Me: | ||
+ | **Yeti aficionado: | ||
+ | **Me: | ||
+ | |||
+ | |||
+ | **Yeti aficionado: | ||
+ | **Me: | ||
+ | |||
+ | **Yeti aficionado: | ||
+ | **Me: | ||
+ | |||
+ | |||
+ | |||
+ | |||
+ | **Yeti aficionado: | ||
+ | **Me:** < | ||
+ | |||
+ | |||
+ | Ok, enough silliness and satire. | ||
+ | ===== what DC-DC is designed | ||
+ | |||
+ | Here are the problems DC-DC are designed to solve. | ||
+ | |||
+ | ==== limiting current from the alternator ==== | ||
+ | |||
+ | An alternator has an Amp rating and [[electrical: | ||
+ | |||
+ | * the alternator can output about half of its rating continuously //while driving at normal speeds//. | ||
+ | * both the vehicle' | ||
+ | * very conservative use of the alternator might be 1/4th of its rated capacity((while driving)), moderate use might be 1/3rd of rated, and aggressive use might be 1/2 of rated. | ||
+ | * if the alternator is overtasked (especially when airflow is impaired) it can overheat and fail. | ||
+ | * DC-DC chargers have stable current outputs; | ||
+ | |||
+ | === 200Ah example === | ||
+ | |||
+ | Let's assume we have a 200Ah bank and a 150A alternator. | ||
+ | |||
+ | First things first - **how much current would the bank pull** if you connected a wire from the starter battery POS to the house bank POS? | ||
+ | |||
+ | * **theory** tells us that I=V/ | ||
+ | * an **often-repeated (and wrong)** answer is 200A, because the BMS cuts off at 1C. | ||
+ | * **testing** tells us(([[electrical: | ||
+ | * current at middling states of charge averages ~0.2C, or 40A. | ||
+ | * current at high states of charge((upper knee)) averages ~0.1C, or 20A. | ||
+ | * current at low states of charge((lower knee)) averages ~0.32C, or 64A. <- we need to think about this | ||
+ | |||
+ | 64A is clearly above our 50A target for continuous current but below our 75A max. There are two factors in play that make this Generally Not A Big deal: | ||
+ | |||
+ | - direct charging current starts tapering immediately after charging starts (the V in I=V/R is ever-decreasing); | ||
+ | - LFP voltage pops out of the lower knee quite quickly after charging starts (the V in I=V/R is suddenly reduced). | ||
+ | |||
+ | |||
+ | Related idea: - **how much current would the bank pull** if you connected 50A DC-DC between the starter battery a wire from the starter battery POS to the house bank POS? | ||
+ | |||
+ | The obvious answer is 50A, but there' | ||
+ | |||
+ | === 100Ah example === | ||
+ | |||
+ | Let's do it again with the same setup but a 100Ah bank. | ||
+ | |||
+ | * the DC-DC will charge at 50A <- 0.5C is a little high, we should think about that | ||
+ | * direct charging current at middling states of charge averages ~0.2C, or 20A. | ||
+ | * direct charging current at high states of charge((upper knee)) averages ~0.1C, or 10A. | ||
+ | * direct charging current at low states of charge((lower knee)) averages ~0.32C, or 32A. | ||
+ | |||
+ | Now which one is harder on the alternator? | ||
+ | |||
+ | |||
+ | === 300Ah example === | ||
+ | |||
+ | Once again, 300Ah bank | ||
+ | |||
+ | * the DC-DC will charge at 50A | ||
+ | * direct charging current at middling states of charge averages ~0.2C, or 60A. <-- not good, as this will be continuous | ||
+ | * direct charging current at high states of charge((upper knee)) averages ~0.1C, or 30A. | ||
+ | * direct charging current at low states of charge((lower knee)) averages ~0.32C, or 96A. <-- clearly unacceptable | ||
+ | |||
+ | There is a good chance DC-DC will be required here to stay at our 50A continuous max. But there is little harm in turning the key to take quick measurements since our wiring is fused at 75A. If current is as high as predicted the fuse pops and we start thinking about a DC-DC. | ||
+ | |||
+ | ==== output specific charging voltage[s] | ||
- | **Nancy Reagan: | ||
- | **Me: | ||
- | **Nancy Reagan:** But illegal drugs kill people! \\ | ||
- | **Me: | ||
- | **Nancy Reagan:** OMG, also don't get near pools! \\ | ||
- | **Me: | ||
- | This claim that a thing is fundamentally different is called [[https:// | ||