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electrical:12v:directcharginglfp [2023/12/15 11:29] frater_secessus [BMS disconnect will damage the alternator] |
electrical:12v:directcharginglfp [2024/04/04 11:51] frater_secessus [reasons NOT to attempt direct charging] temp |
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====== Direct charging LiFePO4 banks ====== | ====== Direct charging LiFePO4 banks ====== | ||
- | There is an [[electrical: | + | There is an [[electrical: |
This article will focus on current demand by LiFePO4 house banks. | This article will focus on current demand by LiFePO4 house banks. | ||
+ | |||
+ | ===== contraindications ===== | ||
+ | |||
+ | Direct charging is not a good fit for all scenarios. | ||
+ | |||
===== the formula in 60 seconds ===== | ===== the formula in 60 seconds ===== | ||
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There is an outlier that pulls 0.67C at ~10% state of charge.((spreadsheet says 25%, but a later post clarified it was in the bottom knee)) | There is an outlier that pulls 0.67C at ~10% state of charge.((spreadsheet says 25%, but a later post clarified it was in the bottom knee)) | ||
- | === why does acceptance | + | === why does C appear to decrease as bank size increases? === |
+ | |||
+ | Current acceptance does increase with larger bank capacities but this increase can be surprisingly small: | ||
>> IF you have other things in the network with a much higher resistance than the batteries (such as using the frame as a ground return path), changing the resistance of the battery bank [ie, increasing capacity] can have only a small effect. --MechEngrSGH((https:// | >> IF you have other things in the network with a much higher resistance than the batteries (such as using the frame as a ground return path), changing the resistance of the battery bank [ie, increasing capacity] can have only a small effect. --MechEngrSGH((https:// | ||
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- | Let's assume battery resting voltage is 13.0 and alternator voltage is 14.0 and circuit resistance is 20mR. | + | Let's assume battery resting voltage is 13.0 and alternator voltage is 14.0 and circuit resistance is 20mR (.020R). |
- | * I=V/R = (14v-13v)/ | + | * I=V/R = (14v-13v)/ |
| | ||
Let's add 5m Ohm of resistance: | Let's add 5m Ohm of resistance: | ||
- | * I=V/R = (14v-13v)/ | + | * I=V/R = (14v-13v)/ |
===== tweaking current with voltage ===== | ===== tweaking current with voltage ===== | ||
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If the SoC was very low (near cutoff) the combiner would slightly faster, ~7.5 minutes. | If the SoC was very low (near cutoff) the combiner would slightly faster, ~7.5 minutes. | ||
- | ===== assessing your own setup for direct | + | ===== reasons NOT to attempt |
- | There are good reasons //not// to attempt direct charging: | ||
+ | * you have [[electrical: | ||
+ | * you have a **small or already-overloaded alternator** | ||
* you **already own a DC-DC** | * you **already own a DC-DC** | ||
- | * you **need consistent, | + | * your BMS has no low temp charging disconnect and you have no desire to work around that((to be fair, this also rules out DC-DC that lack low-temp disconnect)) |
- | * you **don' | + | |
+ | * you want **stable charging rates** regardless of state of charge((acceptance will taper in Absorption if present)) | ||
+ | * if **alternator is your sole charging source**. | ||
+ | * you **don' | ||
* **choosing wiring** can require some research | * **choosing wiring** can require some research | ||
* [[electrical: | * [[electrical: | ||
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* In the van-relevant examples above LFP banks typically pull 0.33C, or 33A per 100Ah of capacity, so it might be acceptable to size them to 0.5C (noncritical). | * In the van-relevant examples above LFP banks typically pull 0.33C, or 33A per 100Ah of capacity, so it might be acceptable to size them to 0.5C (noncritical). | ||
* a set of heavy jumper cables and a clamp-on monitor might help assess max current draw before buying any wiring. | * a set of heavy jumper cables and a clamp-on monitor might help assess max current draw before buying any wiring. | ||
+ | |||
+ | |||
+ | |||
+ | ===== assessing your own setup for direct alternator charging ===== | ||
+ | |||
+ | [Note: | ||
+ | |||
+ | |||
If you are still interested, here is one approach to assessing your setup for direct charging lithium. | If you are still interested, here is one approach to assessing your setup for direct charging lithium. | ||
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- test it with a drive. | - test it with a drive. | ||
- repeat the last two steps with the Li bank at lower and lower states of charge, down to the lowest state of charge you expect to recharge from alternator. | - repeat the last two steps with the Li bank at lower and lower states of charge, down to the lowest state of charge you expect to recharge from alternator. | ||
- | - disconnect the isolator with **a switch** when you come to the desired SoC, when charge rate exceeds your preferences, | + | - celebrate |
**Caveats**: | **Caveats**: | ||
* Only alternator charge while driving ([[rv: | * Only alternator charge while driving ([[rv: | ||
- | * Pay attention while charging from alternator to keep from overcharging the Li or holding for long periods at high [[electrical: | + | * Pay attention while charging from alternator to keep from overcharging the Li or holding for long periods at high [[electrical: |
- | * Fuse the wiring from the chassis so the bank cannot pull more than you specify | + | * if current is excessive but you want to direct-charge, |
- | * if current is higher than you want consider adding resistance | + | |
===== the parts list ===== | ===== the parts list ===== | ||
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===== objections ===== | ===== objections ===== | ||
- | No one is insisting you must to direct-charge your LFP. This article intends to show direct-charging is practical and effective | + | No one is insisting you must to direct-charge your LFP or claiming it is a good fit for all use cases. This article intends to show direct-charging is practical and effective |
Let's address some other common objections. | Let's address some other common objections. | ||
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- why are you overcharging your bank so badly that the BMS is shutting down to keep you from damaging it? | - why are you overcharging your bank so badly that the BMS is shutting down to keep you from damaging it? | ||
- the starter battery is in the circuit and can absorb the transients. | - the starter battery is in the circuit and can absorb the transients. | ||
+ | |||
+ | ==== you will overcharge your battery ==== | ||
+ | |||
+ | If you are driving long enough to reach the desired state of charge (80%, 100%, whatever), you can [[electrical: | ||
+ | |||
===== addendum: | ===== addendum: | ||
This info has been moved to the Other Reports tab on [[https:// | This info has been moved to the Other Reports tab on [[https:// |