This shows you the differences between two versions of the page.
Both sides previous revision Previous revision Next revision | Previous revision Next revision Both sides next revision | ||
electrical:12v:directcharginglfp [2022/09/17 11:56] frater_secessus [tweaking current with resistance] |
electrical:12v:directcharginglfp [2024/03/20 12:02] frater_secessus [assessing your own setup for direct alternator charging] |
||
---|---|---|---|
Line 4: | Line 4: | ||
- | ====== | + | ====== |
- | There is an [[electrical: | + | There is an [[electrical: |
- | Short digression: | ||
- | - LFP does not //need// to be fully charged | + | This article will focus on current demand by LiFePO4 house banks. |
- | | + | |
- | - if solar is present it will also "top off" the lithium if that is something the owner wants to do | + | ===== contraindications ===== |
- | - **overcharging** (holding Vabs after reaching 100% SoC) is actually a bigger concern, particularly if one drives | + | |
+ | Direct charging is not a good fit for all scenarios. | ||
- | So this article will focus on current demand by LiFePO4 house banks. | ||
===== the formula in 60 seconds ===== | ===== the formula in 60 seconds ===== | ||
Line 25: | Line 24: | ||
For us, this means there are **two factors** that dictate what a battery bank (lithium or otherwise) will demand of the alternator: | For us, this means there are **two factors** that dictate what a battery bank (lithium or otherwise) will demand of the alternator: | ||
- | * The **voltage difference between alternator and battery bank**((If alternator voltage is 14.1v and the resting voltage of the battery is 13.0v, then V=1.1v.)) -- so **the lower the battery' | + | * The **voltage difference between alternator and battery bank**((If alternator voltage is 14.1v and the resting voltage of the battery is 13.0v, then V=1.1v.)) -- so **the lower the battery' |
- | * the **total resistance** of the entire circuit((including the batteries themselves)) - wire gauge, length of wiring run, components installed in the circuit, battery chemistry and capacity, etc will make a difference | + | * the **total resistance** of the entire circuit((including the batteries themselves)) - wire gauge, length of wiring run, method of " |
You don't have to fully understand it or get a I=V/R tattoo, but do keep the basic idea in mind as we plunge ahead. | You don't have to fully understand it or get a I=V/R tattoo, but do keep the basic idea in mind as we plunge ahead. | ||
Line 32: | Line 31: | ||
===== analysis of van-relevant installs ===== | ===== analysis of van-relevant installs ===== | ||
+ | |||
+ | **TL; | ||
+ | |||
This section includes installs that one might find in a typical van: | This section includes installs that one might find in a typical van: | ||
Line 38: | Line 40: | ||
* single-alternator systems (no dual-alternator setups) | * single-alternator systems (no dual-alternator setups) | ||
* standard regulators (no external regulators, internal regulator hacks, etc) | * standard regulators (no external regulators, internal regulator hacks, etc) | ||
- | * with rated capacities ≥120A((where stated)) | + | * with rated capacities ≥120A((where stated)) |
+ | * plain relays, not voltage-boosting [[electrical: | ||
+ | * NEG return typically through chassis rather than separate NEG wire to battery. | ||
Since it includes screenshots the data will be for installs documented before Sept 15, 2022. | Since it includes screenshots the data will be for installs documented before Sept 15, 2022. | ||
Line 55: | Line 59: | ||
{{https:// | {{https:// | ||
- | The average max current drawn from the alternator is **0.33C**, or 1/3rd of what the warnings predict. | + | The average |
+ | |||
+ | - higher voltage of LiFePO4 compared to lead; and (V in the forumula) | ||
+ | - the resistance introduced by chassis grounding (R in the formula) | ||
+ | |||
+ | 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 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:// | ||
- | There is an outlier that pulls 0.67C. | ||
- | Remember The listed Max current is as a worst-case scenario that is typically only seen when charging first kicks in. With direct-charging current falls off linearly((or close enough)) as the house bank voltage rises. | ||
==== current drawn expressed as percentage of alternator rating ==== | ==== current drawn expressed as percentage of alternator rating ==== | ||
Line 69: | Line 83: | ||
- | On average, direct-charging 100Ah of LFP consumed ~22% of the alternator' | + | On average, direct-charging |
Line 78: | Line 92: | ||
==== failures ==== | ==== failures ==== | ||
- | There are two failures, and neither of them come from the van-relevant section. | + | There are three failures |
Line 85: | Line 99: | ||
**500Ah** of LFP direct charged with a **225A** alternator. | **500Ah** of LFP direct charged with a **225A** alternator. | ||
- | Mortons on the Move were able to overheat (not damage) the 225A primary alternator with 500Ah pulling 180A between the tow vehicle and travel trailer. | + | Mortons on the Move were able to overheat (not damage) the 225A primary alternator with 500Ah pulling 180A on 1/0 cables |
+ | |||
+ | They ended up installing a DC-DC charger ([[https:// | ||
Line 93: | Line 109: | ||
**300Ah** of LFP (no BMS) direct charged with a **90A** alternator. | **300Ah** of LFP (no BMS) direct charged with a **90A** alternator. | ||
- | Victron damaged a 90A car alternator with 300A of lithium on a bench (no airflow as when driving). | + | [[electrical: |
+ | === 1977 RV === | ||
- | === lessons === | + | The 60A alternator in RJS's 1977 RV died after approximately 1 year of direct-charging 200Ah of Lithium at rates up to 41A through a manual switch. |
- | Don't direct-charge banks that have 2-3x the capacity of your alternator' | ||
- | Do not idle to charge. | + | |
+ | === lessons === | ||
+ | |||
+ | - Think twice about direct-charging banks that have 2-3x the capacity of your alternator' | ||
+ | - do not idle to charge. | ||
===== tweaking current with resistance | ===== tweaking current with resistance | ||
+ | We don't have a lot of control over alternator voltage (diode-based isolators nothwithstanding) but we can affect resistance. | ||
+ | |||
+ | |||
+ | >> [with lead chemistries] the battery resistance [is] the large and controlling factor, **in the case of LFP, its the wiring** and the battery resistance is inconsequential. -- MechEngrSGH((https:// | ||
+ | |||
+ | ==== NEG return through chassis ==== | ||
+ | |||
+ | >> using a copper cable ground between the starter and the auxiliary battery is actually part of the overcurrent problem. Using a frame ground tends to add a lot of resistance to the network... -- MechEngrSGH((https:// | ||
+ | |||
+ | |||
+ | Direct-charging setups typically only run the POS wire from the battery((or alternator)) to the relay; | ||
+ | |||
+ | Steel has much more resistance than copper: | ||
+ | |||
+ | >> copper (considered 100%), Aluminum is 71%, Brass is 25%, **steel is 12-14%** (depending on the alloy), lead (solder) is around 12% and 304 stainless steel (what stainless fasteners are made from) is 2.5%((YT comment by WorkingOnExploring)) | ||
+ | |||
+ | ...so a chassis NEG return will increase resistance on that leg by something like 8x. The mass of the chassis material is so great no heating will be observed. | ||
+ | |||
+ | Corollary: | ||
+ | |||
+ | |||
+ | |||
+ | ==== case study ==== | ||
+ | |||
The owner of the van in the higher-current outlier mentioned above had very low resistance in his wiring. | The owner of the van in the higher-current outlier mentioned above had very low resistance in his wiring. | ||
Line 125: | Line 169: | ||
- | Let's assume battery resting voltage is 13.0 and alternator voltage is 14.0 and circuit resistance is 20m Ohm. | + | 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 ===== | ||
- | [[electrical: | ||
- | It would also reduce | + | Anything that reduces the voltage difference (" |
+ | ==== diode-based isolators ==== | ||
- | ==== application ==== | + | [[electrical: |
+ | It would also reduce final charging voltages, so this might allow vehicles with high chassis voltages like to safely charge LFP. Example: | ||
- | Let's assume battery resting voltage is 13.0 and alternator voltage is 14.0 and circuit resistance is 20m Ohm. Diode isolator cause 0.4v loss. We'll do it with a relay first: | ||
- | | + | **Example**: |
+ | |||
+ | | ||
| | ||
Let's add the diode isolator((assumes the diode isolator has the same internal resistance as the relay)) to reduce apparent alternator voltage: | Let's add the diode isolator((assumes the diode isolator has the same internal resistance as the relay)) to reduce apparent alternator voltage: | ||
- | * I=V/R = (**13.6v**-13v)/0.020m Ohm = 0.6v/0.005m Ohm = **30A** | + | * I=V/R = (**13.3v**-13v)/0.020mR = 0.3v/0.020mR = **15A** |
- | + | ||
+ | |||
+ | ==== state of charge ==== | ||
+ | |||
+ | When the bank State of Charge (and therefore voltage) is higher the delta and current will be reduced. | ||
+ | |||
+ | **Example**: | ||
+ | |||
+ | * I=V/R = (14v-**12.5v**)/ | ||
+ | * I=V/R = (14v-**13.5v**)/ | ||
+ | |||
+ | |||
+ | ==== concurrent charging sources ==== | ||
+ | |||
+ | Even if SoC has not increased meaningfully yet other charging sources can drive up apparent bank voltage, thereby reducing the delta and current. This voltage " | ||
+ | |||
+ | In practice the most common [[electrical: | ||
+ | |||
+ | |||
+ | **Example**: | ||
+ | |||
+ | * I=V/R = (14v-**13.0v**)/ | ||
+ | * I=V/R = (14v-**13.3v**)/ | ||
+ | |||
+ | Note: this occurs because alternators are voltage regulated while solar charge controllers can output whatever voltage they want internally as long as voltage setpoints at the terminals are exceeded. | ||
===== current demands of DC-DC chargers vs isolators ===== | ===== current demands of DC-DC chargers vs isolators ===== | ||
- | [For this section | + | For these examples |
- | Charging | + | ==== combiner charging current acceptance patterns ==== |
+ | {{ https:// | ||
+ | Lead batteries have roughly-linear voltage curves. | ||
- | {{https:// | + | * at very low states of charge (<10% state of charge) voltage will be very low, around 12.0v. This means current demanded will be maximal((I=V/R)) briefly, then quickly level off as voltage comes off the bottom knee. ~120A. |
+ | * in the middle 80% voltage will be stable, in the 13.2v - 13.6v range. Charging current will be moderate and stable. | ||
+ | * at high states of charge (>90% state of charge) voltage will climb above 14.0v. | ||
- | This crude and fictional graph illustrates | + | Word to the wise: if your SoC is ≤20% you may want to allow solar or other charging to bring up voltage a bit before activating alternator charging. |
+ | ==== DC-DC charging current acceptance | ||
+ | |||
+ | For this section we will assume the DC-DC is sized for the moderate currents seen in the middle 80% above (~50A) | ||
+ | |||
+ | * at very low states | ||
+ | * in the middle 80% voltage will roughly be the DC-DC' | ||
+ | * at high states of charge DC-DC will hold rated output slightly longer then taper faster at the end, 50A, then falling to 0A. | ||
+ | |||
+ | |||
+ | ==== comparison ==== | ||
+ | |||
+ | {{ : | ||
+ | |||
+ | As long as we stay away from the bottom knee (lowest 10% of SOC) our // | ||
+ | |||
+ | **Current levels with DC-DC will remain the same regardless of bank size**.((other than very small banks)) | ||
+ | |||
+ | **Current levels with combiners will vary with bank size**, about 0.33C and tapering (see above). | ||
+ | |||
+ | |||
+ | ---- | ||
- | The isolator starts out high at 85A and current starts falling off immediately. | ||
- | The DC-DC holds a steady 60A until the 14.4v setpoint is reached around 50 minutes. | ||
==== breakeven ==== | ==== breakeven ==== | ||
- | Note: the crossover point between green and purple at ~20mins marks the time DC-DC charging passes more current than the isolator. | + | Breakeven points (the amount of time it takes for each to replace |
+ | But we can try to estimate how much driving it would take to replace 10Ah in a 200Ah bank (going from 50% to 55% SoC, for example: | ||
+ | * 60A DC-DC - 10 minutes | ||
+ | * 50A DC-DC - 12 minutes | ||
+ | * combiner - ~12 minutes | ||
+ | * 40A DC-DC - 15 minutes | ||
+ | * 30A DC-DC- 20 minutes | ||
+ | * 20A DC-DC - 30 minutes | ||
- | ===== assessing your own setup for direct | + | If the SoC was very low (near cutoff) the combiner would slightly faster, ~7.5 minutes. |
+ | ===== 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, | + | * you **want predictable charging |
- | * 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 | ||
+ | * [[electrical: | ||
+ | * **if isolator wiring is already present** from a previous bank, test it while using a monitor or BMS info to watch actual current. | ||
+ | * **if starting from scratch** there is less to go on. | ||
+ | * The most conservative approach would be to size the wiring for 1C (non-critical). | ||
+ | * 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. | ||
+ | |||
+ | |||
+ | |||
+ | ===== 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. | ||
- | - if the isolator is already in use with lead-chemistry banks before the upgrade, | + | |
- | - Read and understand your Li battery manufacturer' | + | |
- | - Read and understand your alternator' | + | |
- | - Read and understand | + | - **voltage when the bank is at 100% State of Charge** (likely close to chassis voltage) - this will be the highest voltage the battery sees |
- | | + | - Read and understand your **Li battery manufacturer' |
- | - make a first approximation | + | - Read and understand your **alternator' |
- | - install [[electrical: | + | |
- | - ensure the fuse between your chassis and battery bank is sized so //you cannot draw more than the alternator and wiring can handle// | + | - Read and understand your **isolator' |
- | - make the first test run a brief one and with the Li fairly well charged.((higher states of charge will typically lessen current demands to some degree))((if you have paralleled batteries you might want to do this step with just one in place to get a feel for the draw.)) | + | - now make a **sanity check** |
+ | - ensure the **fuse** between your chassis and battery bank is sized so //you cannot draw more than the alternator and wiring can handle// | ||
+ | - make the first test run a brief one and with the Li fairly well charged.((higher states of charge will typically lessen current demands to some degree))((if you have paralleled batteries you might want to do this step with just one in place to get a feel for the draw.)) | ||
- 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 | + | - 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 | + | - [[electrical: |
Line 216: | Line 337: | ||
+ | ===== objections ===== | ||
+ | |||
+ | No one is insisting you must to direct-charge your LFP or claiming it is a good fit for all use cases. | ||
+ | |||
+ | Let's address some other common objections. | ||
+ | |||
+ | ==== the alternator cannot fully charge LFP ==== | ||
+ | |||
+ | Yes, it can. LFP will charge to ~100% SoC quite smartly at ≥13.8v. | ||
+ | |||
+ | Which brings us to the next point: | ||
+ | |||
+ | ==== BMS disconnect will damage the alternator ==== | ||
+ | |||
+ | - 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. | ||
+ | |||
+ | ==== 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: | ||
+ | |||
+ | This info has been moved to the Other Reports tab on [[https:// |