Charging lifepo4 without charge controller - strange theory?

[Pincones]

I heard a strange charging theory yesterday and I wanted to propose it here and see what others thought.

Someone recently commented that since lifepo4's will take as much amperage as you can give them very quickly, you can feed them power directly from a high amperage 12v alternator (in this case a homemade 12v gas generator with a high-end vehicle alternator producing the power) to the battery. Literally just run your wire from the alternator to the battery posts and zap that battery to full charge in just a few hours (we're talking parameters around 12v 50amps going into a 12v 200ah battery, for instance). This person's theory was that 1. you're not going to overwhelm the thirsty lithium battery since they like getting charged quickly, and 2. the alternator tops out at 14.5v, so it 'can't overcharge' the voltage of the battery, therefore running it through a charge controller of some sort wouldn't be needed.

I'm hesitant on this. Thoughts?

 

[DThames]

1. The battery is a load.....a heavy load that looks like a short circuit, almost.
2. Any power source will have an internal resistance (among other things) that you need to consider. That internal resistance will make heat when you pull amps through it.

So, would you like to short circuit the output of your alternator? Most would say, "No". However, if you can limit the current so that very low resistance of the battery will not short the alternator, then you can put some charge into the battery. For example of you have 14v out of the alternator and you put a 0.5 ohm (very large watt rating) resister across the output of the alternator, you would have a 28amp load on the alternator. If you then disconnected that and connected that resistor in series with the battery, and again connected that circuit to the alternator, you would have a load no more than 28amps. As the battery charged up, the amps would drop. This is not a great way to charge a battery but it could be done.

Amps = voltage (difference)/resistance. A big battery will have almost zero resistance (equivalent circuit), so can create a HUGE load on a power source.

 

[Luthj]

All alternators are inherently currently limited by their basic design. A 100A alternator will only put out ~100A. Most charging wiring is sufficient to limit charge current even to LFP banks of considerable size.

Most alternators designed in the last 20 years also reduce their output voltage (and thus charge current) as they heat up.

 

[HRTKD]

Is the alternator getting any air flow to promote cooling?

 

[Pincones]

I think in this scenario the alternator in question is mounted to a 6 or 8hp gas engine which is outside and may or may not be stored in a wooden housing to reduce noise. That's the best answer I got on airflow.

Is it true though that you 'can't' overcharge a lifepo4 this way? I suppose that's what's throwing me off. Allegedly the alternator will only give as many amps as is called for, and allegedly the battery will call for all the amps it can give until the BMS cuts the power off. That was the proposed theory. But as mentioned here, that might damage the alternator- unless the alternator would stop giving amps if nothing was calling for them?

I'm not savvy on alternators in the slightest. Just puzzling the suggestion out. It seemed far "too simple" to be fool proof when I heard it. The person with the theory hasn't tried this on a lifepo4, but intends to at some point.

Based on my own knowledge (which has its limits for sure) if I were to charge this way, I would run it through a charge controller.

 

[DThames]

Victron test video

 

[Bob B]

The flaw in the original logic is that an individual battery cell can be WAY over voltage if there is a balance or connection problem .... and the overall pack voltage is still fine.

It is also possible to overheat and ruin the alternator.

 

[Pincones]

The flaw in the original logic is that an individual battery cell can be WAY over voltage if there is a balance or connection problem .... and the overall pack voltage is still fine.

It is also possible to overheat and ruin the alternator.

Isn't that balance/connection problem relevant for most or all forms of charging anyway? Or is that problem amplified in this scenario?

I will try to understand the technicalities of this better so I can regurgitate it effectively to the one wanting to do this. They're accustomed to lead acids, so I wonder if they are aware of LFP's influence on the alternator's RPMs and output, as pointed out in the video.

Are there any downsides to running this charge system through a charge controller for LFPs? That might explain why someone would want to circumvent a CC? Aside from the cost of the charger itself of course.

 

[Bob B]

I may have mis-read your original post a little .... I was thinking of charging without a BMS .... not using a charge controller is different..... but the risk of damage to the alternator is still relevant.

 

[Pincones]

If the alternator is mounted on a gas engine that has an idle switch that you can use to raise or lower RPM, would that circumvent the overheating at low RPM's issue? Also, what if you changed the gearing ratio from the engine to the alternator to influence RMPs of the alternator?

Edit: In case I was vague about it, the LFP that might be fated to this experiment is a pre-made LFP complete in a battery case with a built in BMS, but this method would not utilize a charge controller of any kind. The wires would be running from the alternator straight to the battery posts.

 

[Bob B]

I think you will just have to try it out while monitoring the alternator temperature closely to see how it works .... there will be a lot of variables between different types of alternators. There are alternators designed to operate this way, but you would spend more on the alternator than on a charge controller.

 

[DThames]

If the alternator is mounted on a gas engine that has an idle switch that you can use to raise or lower RPM, would that circumvent the overheating at low RPM's issue? Also, what if you changed the gearing ratio from the engine to the alternator to influence RMPs of the alternator?

Years ago, drag racers (street car type) would put an oversized pulley on the alternator to slow it down as high engine RPM was normal (racing). I am sure you could go the other way to speed it up. Some pulleys have built in fans, which might help.

 

[Luthj]

That victron video originally showed a shot of the control laptop, with the alternator running at half of normal idle speed, then it started smoking. They later edited the video with different footage, but still showed the overheated alternator.

First, they are trying to sell you something.
Second, I question if the conditions were real world.
Third, that alternator is a very old model.

I routinely charge my 560AH lithium pack directly from my alternator (2/0 gauge cables!). I get 100A charging at hot idle (200A alternator). Its a Bosch internal fan model, and the output voltage (and thus current) will derate as the regulator warms up. I have never see it more than 120F over ambient at the coils.

Most of the nuked alternator talk comes from marine applications. Most of these yachts have ancient alternator designs, which were marginal when new. Then they are stuck in a hot marine engine compartment with minimal air flow. Then they are run at idle for extended periods. These effects can combine to nuke an alternator. Even then I have never seen it happen on a vehicle or alternator in free air.


Obviously a BMS with disconnect is required, as with any basic voltage source. I would also make sure a 10W or similar resistor is wired across the alternator output, so in the case of a sudden disconnect, the voltage spike is surprised. A suitable Zener diode (say 16V) could also be used.

 

[Pincones]

Interesting responses!

I don't intend to try it without seeing someone else try it first I'll let this person put the theory to the test if they want to on their own LFPs. But I'll ask about what alternator they're using.

So it sounds like;
1. as long as the alternator has protection from heat (be it a fan, regulator, gearing, and/or manually adjusted RPMs) and
2. as long as when the BMS shuts the batteries off, the alternator has some kind of protection(like what, the resistor?)
it might work without a charge controller?

 

[Bob B]

Interesting responses!

I don't intend to try it without seeing someone else try it first I'll let this person put the theory to the test if they want to on their own LFPs. But I'll ask about what alternator they're using.

So it sounds like;
1. as long as the alternator has protection from heat (be it a fan, regulator, gearing, and/or manually adjusted RPMs) and
2. as long as when the BMS shuts the batteries off, the alternator has some kind of protection(like what, the resistor?)
it might work without a charge controller?

A third factor to consider is that the alternator can be damaged by a sudden disconnect if the BMS see's an error condition or reaches full charge.

There is a reason so many people are using the DC-DC chargers.

 

[HRTKD]

That victron video originally showed a shot of the control laptop, with the alternator running at half of normal idle speed, then it started smoking. They later edited the video with different footage, but still showed the overheated alternator.

First, they are trying to sell you something.
Second, I question if the conditions were real world.
Third, that alternator is a very old model.

I agree with this. In my opinion, it was fear mongering on the part of Victron. However, it certainly got my attention.

The general premise that a LiFePO4 battery can draw so many amps such that it overloads and damages the alternator, may or may not be valid. There are likely thousands of Battleborn drop-in LiFePO4 batteries sitting on trailer tongues, connected to tow vehicles through a 7-pin connection. If the unregulated charging of the battery can damage the alternator, I would expect it to also blow fuses on the 7-pin circuit.

I don't think we have a complete understanding of all this and it certainly could vary from vehicle to vehicle. If I was using the alternator to charge my LiFePO4 battery bank, I would play it safe and use a DC-DC charger.

 

[Luthj]

The alternator itself may be fine. In a vehicle (assuming not a second alternator) there is always a battery to buffer if a load is shed (such a BMS). The voltage spike can be pretty high (like 50V transient) A large capacitor can also be used to buffer the voltage spike.

There is a way to avoid a voltage spike. Take the field wire or the B+ supply to the regulator, and disconnect it via the BMS. The easiest way is to use a relay or mosfet to disconnect the B+ or ground supply to the regulator when the bms disconnects(which one depends on the alternators layout). Either one being disconnected will immediately drop the alternators output to zero.

Another option is a 5AH lead acid battery wired in parallel with the LFP pack, which will absorb the voltage spike. One could also use a 16V zener diode in series with a resistor. That would suppress the spike above that voltage.

A final option is the sterling alternator protection device.

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As a note these measures are not needed if you are charging from a primary alternator where an existing lead acid starting battery is always present.

 

[Luthj]

I don't think we have a complete understanding of all this and it certainly could vary from vehicle to vehicle.

All alternators use the same basic design. A regulator supplies between 0V and B+ to the rotor via the slip rings. This excites the field coils which is then rectified to DC. The regulator monitors the B+ and supplies more or less current to the rotor coil by adjusting the rotor voltage. The maximum current the alternator can output is limited by the resistance of the rotor coil and the field coils. There is no way to exceed the output limit of the alternator for more than a few milliseconds.

Now some alternators are marginal in their design. Either due to regulators with no temperature feedback, poor design, out of spec ambient temperatures/environments, etc. Not very common on modern medium duty vehicles which have lots of high power electronics and AC/radiator fans.

Drop in lithium batteries, or DIY with FET BMS have much higher resistance due to the BMS. This is quite effective at limiting charge current when combined with the typical cable lengths.

My previous 500AH lead acid pack can max out my 200A alternator for 40 minutes when deeply discharged. No overheating at all. It may accelerate alternator wear, but its not an expensive part compared to the charging it gives me.

With modern alternators an analog core regulator is tied to a digital monitoring chip (basically the last 20 years of designs). As the regulator heats up, the output of the alternator is reduced. This is done by dropping the output voltage. For example my 200A unit will output 150A at idle when its cold, and 100A when hot. In very hot temps it drops to 90A at idle.

Some vehicles have energy saving alternators which drop to around 12.5V after the starter battery is charged. There are typically ways to correct this, but its vehicle specific.

My final point, is that I often see posters (no one here really) parroting that LFP needs a "proper profile". The typical alternator outputs between 13.5-14.2V, which is perfect for charging LFP. Its well below the stress point of 3.65pc, but still get into the 90% range of SOC. In fact many DC-DC chargers are outputting 14.4V+, which is probably more stressful long term than just using the alternator directly.


What batteries really need DCDC chargers? LEAD ACID! Alternators don't typically reach the voltages which lead acid needs for longevity, especially with even small voltage drops in the wiring.

 

[Bob B]

I watched a Youtube video .... I think it was Mortons On The move .... but don't remember for sure. They were sponsored by Battleborn and a truck camper manufacturer to highlight their products with video during an whole summer trip to Alaska and British Columbia.
They retrofit the camper to remove the onboard generator making room for as many BB batteries as they could fit in that compartment. They had solar .... but a large part of their strategy was to charge from the truck's alternator. The truck was a new F350 diesel with dual alternators .... so they figured hey we will just charge directly and the 2 alternators will share the load.
Turned out that the alternators duty cycle most of the time so at any give time only 1 is being utilized.

Bottom line was ..... the onboard batteries would seriously overheat the alternators. They had to do an emergency install of a manual disconnect switch and when the batteries were at a low SOC, they could only leave them connected for a few minutes at a time. Once the batteries were at a higher SOC, they could leave everything connected.

This was from a "looking Back .... what would we do differently" video after their trip was over.

I guess what I'm saying is that I'm sure there are some alternators which will be fine connected direct .... but there are others that will not ..... better to do it the safe way up front.

Ahh ... found the video...

 

[HRTKD]

All alternators use the same basic design. A regulator supplies between 0V and B+ to the rotor via the slip rings. This excites the field coils which is then rectified to DC. The regulator monitors the B+ and supplies more or less current to the rotor coil by adjusting the rotor voltage. The maximum current the alternator can output is limited by the resistance of the rotor coil and the field coils. There is no way to exceed the output limit of the alternator for more than a few milliseconds.

Now some alternators are marginal in their design. Either due to regulators with no temperature feedback, poor design, out of spec ambient temperatures/environments, etc. Not very common on modern medium duty vehicles which have lots of high power electronics and AC/radiator fans.

Drop in lithium batteries, or DIY with FET BMS have much higher resistance due to the BMS. This is quite effective at limiting charge current when combined with the typical cable lengths.

My previous 500AH lead acid pack can max out my 200A alternator for 40 minutes when deeply discharged. No overheating at all. It may accelerate alternator wear, but its not an expensive part compared to the charging it gives me.

With modern alternators an analog core regulator is tied to a digital monitoring chip (basically the last 20 years of designs). As the regulator heats up, the output of the alternator is reduced. This is done by dropping the output voltage. For example my 200A unit will output 150A at idle when its cold, and 100A when hot. In very hot temps it drops to 90A at idle.

Some vehicles have energy saving alternators which drop to around 12.5V after the starter battery is charged. There are typically ways to correct this, but its vehicle specific.

My final point, is that I often see posters (no one here really) parroting that LFP needs a "proper profile". The typical alternator outputs between 13.5-14.2V, which is perfect for charging LFP. Its well below the stress point of 3.65pc, but still get into the 90% range of SOC. In fact many DC-DC chargers are outputting 14.4V+, which is probably more stressful long term than just using the alternator directly.


What batteries really need DCDC chargers? LEAD ACID! Alternators don't typically reach the voltages which lead acid needs for longevity, especially with even small voltage drops in the wiring.

You have a better understanding of alternators that I do and I couldn't argue most of your points unless I wanted to look stupid.

On your final point, I will agree with you again. Short term, alternator charging of LiFePO4 isn't a big deal, as long as we're not stressing the alternator. And I don't think we are. My concern is the long term charging. I drive for hours at a time. I'm a Point A to Point B kind of tourist. It's not the journey that matters, it's getting to where I'm going so I can relax. A 13 hour drive - with one or two stops for fuel and coffee - is not unusual for me. My truck can maintain a lot of volts/amps over an extended period of time (two alternators). I worry that my LiFePO4 battery bank could see 14+ volts for a long time. Sure, the BMS is going to cut off charging, but the BMS is supposed to be the last line of defense.