LiFePO4 direct to alternator

[sunshine_eggo]

Long wires and voltage drop can be a great way to limit current. Would never recommend wire that can't handle the ampacity, but a 5% voltage drop would cause the alternator to "see" peak voltage while the battery is 0.7V lower. To maintain this state, the current decreases thus effectively regulating current continuously as the battery voltage rises to meet alternator.

Many have connected LFP direct to alternators in RVs and had no issues due to the marginal wiring and excessive voltage drop so common in RV 12V systems.

 

[ehahn]

Long wires and voltage drop can be a great way to limit current. Would never recommend wire that can't handle the ampacity, but a 5% voltage drop would cause the alternator to "see" peak voltage while the battery is 0.7V lower. To maintain this state, the current decreases thus effectively regulating current continuously as the battery voltage rises to meet alternator.

Many have connected LFP direct to alternators in RVs and had no issues due to the marginal wiring and excessive voltage drop so common in RV 12V systems.

So instead of adding any kind of resistor, I just add smaller wire and maybe a smoke alarm? lol people are really going to love me in this thread if I do that too ?

 

[sunshine_eggo]

So instead of adding any kind of resistor, I just add smaller wire and maybe a smoke alarm? lol

Wires are low Ω resistors.

As I indicated, I would not recommend wire of insufficient ampacity. If you use properly rated wire, there is no concern for fire.

Smaller gauge wire with higher rated insulation would work. Marine (105°C) and silicon (200°C) wire can handle higher current for a given gauge, but they will run hotter and have a higher resistance than the same gauge wire run at lower current due to lower rated insulation.

Example:

2awg 60°C wire is rated for 95A
4awg 90°C wire is rated for 95A

Voltage drop of 4awg would be about 60% higher than 2awg.

 

[Captain Hook]

Assumptions:
I understand LiFePO4 has a low internal resistance, so when you hook it up to an internally regulated alternator, the voltage differential from the battery and alternator divided by the battery’s low internal resistance causes a massive spike in current. The alternator quickly reaches max output and the alternator voltage drops such that ohms law is satisfied for the charging circuit. With a large enough battery bank, the input current isn’t a big deal for the LiFePO4. However, running the alternator at max output can cause the alternator to overheat, therefore we need a way to drop its output voltage and therefore output power to keep it from overheating (a classical duty of an external regulator). As long as you can keep it from overheating, you can continue to charge albeit at a limp output, but so far all seems well. The alternator will produce all it can until the battery pack is charged enough to allow the alternator to hit its max regulated voltage, at which point the voltage differential between the alternator and the battery will begin to decrease and therefore the circuit current will begin to decrease as the LiFePO4 continues to complete its constant voltage charge stage. As long as the BMS has overcharge protection, the battery will accept charge until full.

Question:
What am I missing? It seems with all the parameters correctly configured, the only thing I need to charge LiFePO4 off of an alternator is a way to regulate alternator temperature?

Another question. Is it possible that the BMS gets confused about when to deny charge when the battery is charging at the same time it’s under load? How does the BMS prevent overcharging? Does it disconnect the circuit completely, or maybe insert a diode into the circuit such that the battery can only discharge until the circuit voltage drops below the battery open circuit voltage?

Why ask?
Because I discovered a company claiming to make alternators with internal regulators that also are smart enough to regulate temperature

270 Amp XP High Output Alternator (Ford Expedition)

Our XP series high-output alternators are the ultimate upgrade when you need the absolute biggest and most sophisticated high output alternator on the planet. All of our XP series alternators are designed for one purpose, brute force idle output. When you need the most extreme idle output with...

www.dcpowerinc.com

And I’m wondering if I can cut a lot of complexity and cost out of converting an alternator to external regulation, wiring up an external regulator, current shunts, SOC measurement, temperature sensors, meh - I hope I can just drop this thing in.

I charge my 2 100ah lifepo4s at the same time as my lead. Been doing it for a year and I have had zero issues. My alternator is 70amps. Now someone please tell me why I should not do this. The technical talk sounds good but I needed to charge them and I gambled and it works for me.

 

[12vDC]

STRAIGHT ALTERNATOR & DCDC CHARGER

I have a 3x100Ah battery lithium battery bank in my van.
Curious, would there be an issue to wire in a relay activated by a switch (for direct lithium alternator charging) and also run a DCDC charger simultaneously?

 

[HRTKD]

STRAIGHT ALTERNATOR & DCDC CHARGER

I have a 3x100Ah battery lithium battery bank in my van.
Curious, would there be an issue to wire in a relay activated by a switch (for direct lithium alternator charging) and also run a DCDC charger simultaneously?

What is the source of power for the DC-DC charger?

 

[12vDC]

What is the source of power for the DC-DC charger?

An OEM SLA feeding an isolated Orion30.

The alternator is a Nations280.

280 Amp Sprinter Van Alternator Overview​

• Rated Voltage- 14V
• Rated Output- 280 Amps
• Idle Output- 200 Amps
• Stator- Ultra High Efficiency six phase high output
• Rotor- Precision balanced OEM high output
• Rectifier- 12, 75 amp diodes special high efficiency aluminum heat sinks
• Voltage Regulator- OEM manufactured

 

[time2roll]

STRAIGHT ALTERNATOR & DCDC CHARGER

I have a 3x100Ah battery lithium battery bank in my van.
Curious, would there be an issue to wire in a relay activated by a switch (for direct lithium alternator charging) and also run a DCDC charger simultaneously?

What is the point?

 

[12vDC]

Didnt feel like ignoring the unhelpful response….

With a larger alternator presumably it is designed to handle more heat than a “standard” alternator. If I wished to provide a larger source current at IDLE, I would engage the switch and perhaps triple my available charge capacity (for a low SOC). In that case, I would like to run another set of wires and take advantage of the capability.

As my breaker(s) are buried and not so easy to access, if it were no hazard to have both sources of charging simultaneously I would rather do that.

Hence my query.

 

[HRTKD]

Can your LiFePO4 battery bank handle that many amps? I have a 560 Ah (2x280Ah) but my BMS will limit charging to 240 amps. The alternator system in my truck could theoretically produce more than 240 amps.

 

[time2roll]

In essence if both are connected the DC-DC would be back feeding itself and could be an issue. Maybe the DC-DC would self limit amps but would effectively accomplish nothing. Check with the manufacturer but I think it would be in peril. Consider a battery switch that would select one or the other.

Before installing the extra connection I would jump it and see if there was any real benefit. The LFP runs about a volt higher than lead-acid at 50% charge so the alternator may not get the voltage right on the battery through a long wire compared to a DC-DC charger mounted right close to the battery getting a charge. So even with more amps available the voltage spread is small enough that current may not move well.

 

[12vDC]

Can your LiFePO4 battery bank handle that many amps? I have a 560 Ah (2x280Ah) but my BMS will limit charging to 240 amps. The alternator system in my truck could theoretically produce more than 240 amps.

I would only really want the extra juice under less than ideal circumstances such as in a snow storm or other non-PV charging scenarios, to replace some stored energy from an idle.

Mfr specs state max charge/discharge 100A but recommends 0.2C charge for my selection of battery.

Theoretically, if my bank was almost dead it could ask for 300Ah from the alternator if it was able to provide it. But say just as much even at around 20% SOC. But in actuality, not so much current available at idle. Nation specs idle current at 200A for my model.

 

[12vDC]

In essence if both are connected the DC-DC would be back feeding itself and could be an issue. Maybe the DC-DC would self limit amps but would effectively accomplish nothing. Check with the manufacturer but I think it would be in peril. Consider a battery switch that would select one or the other.

Before installing the extra connection I would jump it and see if there was any real benefit. The LFP runs about a volt higher than lead-acid at 50% charge so the alternator may not get the voltage right on the battery through a long wire compared to a DC-DC charger mounted right close to the battery getting a charge. So even with more amps available the voltage spread is small enough that current may not move well.

I dont actually know but I do not agree that the DCDC would backfeed - that assertion is opposite to how the module works with input output. The current and voltage regulation only goes in one direction.

If the DCDC would limit current “and accomplish nothing” that is actually a plus as far as running the two lines in parallel. The direct line would feed as many amps the bank would ask for given the alternators ability to provide at idle vs competing with the DCDC for power draw.

What do you mean “jump it” before I add the extra connection? You mean test it out with jumper cables?

 

[time2roll]

I dont actually know but I do not agree that the DCDC would backfeed - that assertion is opposite to how the module works with input output. The current and voltage regulation only goes in one direction.

If the DCDC would limit current “and accomplish nothing” that is actually a plus as far as running the two lines in parallel. The direct line would feed as many amps the bank would ask for given the alternators ability to provide at idle vs competing with the DCDC for power draw.

What do you mean “jump it” before I add the extra connection? You mean test it out with jumper cables?

Think of two batteries in parallel. Then put a DC-DC charger between them while leaving the direct parallel connection. This would accomplish nothing and possibly damage the DC-DC charger.

Not jumper cables but the wire intended for use. Just saying make a test to verify if it actually does what is expected before pulling the wire through, behind, and over to get it all neatly in place. Not sure the size or distance but I believe it may not provide as much current as expected.

 

[12vDC]

Think of two batteries in parallel. Then put a DC-DC charger between them while leaving the direct parallel connection. This would accomplish nothing and possibly damage the DC-DC charger.

Not jumper cables but the wire intended for use. Just saying make a test to verify if it actually does what is expected before pulling the wire through, behind, and over to get it all neatly in place. Not sure the size or distance but I believe it may not provide as much current as expected.

This vid describes a safer way to DIRECT and DCDC charge in parallel. But apparently it does more than accomplish nothing.