Lithium batteries and smart alternators

[TB]

Hi all. I have some very specific questions I'm hoping somebody (@drivesafe?) can answer to help me with my aux battery system design.

1. Does one of the LR modules monitor the alternator's total current output and lower the voltage when needed to keep the alternator from burning out?

2. Imagine a VSR-like system connecting a high capacity lithium aux battery to the Defender. The aux battery has been drained to 11.5V. You start the engine and the smart alternator ramps to 14.6V with the intent of rapidly topping up the AGM stop/start crank battery. The relay connects the aux battery. If the lithium battery is rated to sustain a charge current of 200A, what gives? Does the alternator voltage drop to keep its total output within safe levels? Does the battery demand kill the alternator? Does the alternator kill the lithium battery by heroically attempting to hold 14.6V and shoving way more than 200A into the battery?

3. What is the actual output current capacity of the D240 Defender alternator?

[JessicaTam]

I am thinking the answers to your questions would also depend on the auxiliary battery controller.

I.e. is it suited to lithium second battery installations.

[TB]

I am thinking the answers to your questions would also depend on the auxiliary battery controller.

I.e. is it suited to lithium second battery installations.

Well, no, the question includes the premise that I'm using a "VSR-like system". That means some kind of switch that connects the lithium battery to the vehicle circuit with no ability to influence voltage or current. I'm asking explicitly about the interaction of a lithium battery with the vehicle alternator in the absence of any more sophisticated controller.

[biggin]

I was under the impression that you’d need a dc/dc charger for the lithium battery. I’m sure Drivesafe will be along shortly.

[drivesafe]

Hi TB.

To your first question, there is no need to monitor the current output of any alternator as a protection measure.

If you attempt to overload an alternator, it automatically reduces the voltage to the point where the current load no longer exceeds the alternators maximum output.

NOTE, when any vehicle is driven in bumper to bumper traffic, because the motor is running at idle, the maximum output of any alternator will be low and easily exceeded thousands of times over its lifespan, but there’s no harm done.

BTW, you may have seen the YouTube videos, where they cooked alternators.

These videos are a con job, intended to suck unsuspecting potential customers into buying something they never needed.

You can not imitate those tests in a real life situation.

Again, you can not over load an alternator and burn it out.

Next, and unfortunately, regardless of what make of vehicle you have, Land Rover or otherwise, you can not use a VSR and lithium batteries in any vehicle with a SMART alternator.

You MUST use a DC/DC device to charge the lithium AUXILIARY battery and you can NOT use a lithium CRANKING battery.

Nether battery can be charged using a SMART alternator.

[TB]

Thanks Drivesafe. Your answers are along the lines of what I was hoping for, though it's unclear to me how a short-circuited alternator self-limits to avoid damage from excessive current. I do understand that the rotor field strength and hence alternator voltage is externally controlled, and these days that's done by a computer based on things like the min/max safe voltages in the circuit and the charge state of the cranking battery. I'm still puzzled by what would happen when you attach a hugely capable lithium battery like the DCS 200Ah slimline (which is rated for a constant 140A charge and 200A peak charge rates) in a state of low charge. How similar is that to a short circuit? If it's at a lower voltage than the cranking battery, will the car be pushing the alternator harder and harder trying to charge the crank battery, unaware of the 200A+ virtual black hole it's dumping power into? Will the other circuits in the car still work or will they suffer a brownout?

Be assured that I know a smart alternator in its usual configuration won't work well with a direct-connected lithium aux battery, because after the cranking battery is sufficiently charged (by the computer's measure) it'll drop voltage to a level that the aux will be running the vehicle and not charging to anywhere near its capacity. I do think there are setups where a lithium crank battery can work just fine, provided everything's calibrated to the right voltage thresholds.

For various reasons I'm keen on the CTEK D250SE (20A DCDC) and Smartpass 120S (fancy MOSFET VSR thing) combination but unsure as to whether I can integrate them successfully with the Defender's oddball 12V behaviour. If the alternator is safely current-limited then I think I can do it. If not, I have to pursue a very different setup.

[drivesafe]

Hi again TB.

As I posted, an alternator can not be over loaded because it drops the voltage if the current load is greater than the maximum current output of the alternator.

So if you connect a very low charged lithium battery to the alternator and say the lithium battery tries to pull more current than the alternator can produce, the alternator VOLTAGE will drop.

Lower the charge VOLTAGE on any battery, not just a lithium battery, and the battery ITSELF, will draw less current.

The system is self protecting.


As to maximum loads, I have many D3 and D4 customers, who tow caravans with as many as 3 large lead acid batteries.

After staying in one campsite for a few days, they may drive for 3, 4, or 5 hours straight, to their next stopover and their alternators can be running full on for the entire drive time and no problems occur.

Also again, you can NOT use any form of conventional VSR system to charge lithium batteries in a vehicle equipped with a SMART alternator.

The alternator is not the issue, it is the vehicle’s BMS that is the problem.

With a lead acid battery, the fully charged voltage is 12.7-8v, and any voltage below 12.4v will have the alternator working to charge the battery.

A lithium battery has a fully charged voltage of 13.2-3v, so when ever the BMS “sees” this 13.2v, it considers the battery to be fully charged and as such, drops the alternator voltage.

With a lead acid battery, as the alternator voltage drops, so does the lead acid battery voltage.

With a lithium battery, the voltage remains at 13.2v till the battery is over 80% discharged.

Then and only then, will the BMS “think” the battery needs charging.

You can literally leave home with fully charged lithium batteries and arrive at your camp site with two lithium batteries in a near flat state.

This is because the BMS “sees” the batteries as being fully charged and runs the alternator at a lower voltage.

Any alternator voltage below 13.2v means your vehicle is running on the lithium batteries and not the alternator. And this is why it flattens the lithium batteries.

[TB]

Yep, many of the things you just wrote mean the same things as what I wrote. We are in agreement on the core problem with lithium aux batteries via VSR in a standard vehicle with smart alternator. I put it to you though that the vehicle's BMS is not measuring cranking battery voltage so much as it is monitoring the current in and out of the cranking battery and correlating that with the circuit voltage. A calibration cycle runs the alternator at high voltage constantly until the battery accepts no further charge, and after that the BMS counts the coulombs (1/3600 of an Ah) that leave the battery so as to make room for energy capture during engine braking but not allow it to be depleted so far that it would fail to start the engine. Controlling the voltage output of the alternator is how it controls whether the net flow of current goes into or out of the cranking battery at any point in time.

So yes, a vehicle with smart alternator tries to create a steady state where the cranking battery is only partially charged, which on my Defender right now (parked) leaves it sitting at 12.57V. And yes, that voltage is very much downhill from a lithium cell's perspective so all the charge in the lithium would drain out to service the vehicle's needs and it would never get charged beyond those brief moments when the alternator is assisting with engine braking or replenishing the cranking battery after an engine start.

Anyway... the CTEK combo puts a form of VSR with very high current capacity (120A sustained) in parallel with a 20A DCDC that has a lithium profile. During those times when the alternator's output voltage is higher than what the DCDC is putting to the lithium, the Smartpass connects the aux battery to the vehicle and permits a much higher rate of charge. So, engine braking becomes a big help. As soon as the current tries to flow the other way, because the alternator has dropped its output, the Smartpass breaks the connection and we rely on the DCDC alone for charging. However while the alternator is on, the Smartpass disconnects its attached consumers (fridges etc) from the battery and instead powers them from the alternator so we get the full 20A DCDC charge rate for the lithium.

While monitoring my vehicle I see an occasional peak around 14.8V which worries me a bit but its sustained alternator output when in charging mode sits on a steady 14.6V. That's gonna push a very substantial current through the Smartpass into the lithium for at least a short time and it's those currents that I'm worried about – both for the alternator and the battery. The Smartpass is self-protecting and will disconnect after 30 seconds of 350A but I worry that something else will have been damaged by that point.

BTW, there could be a variant of your 160 here, where it acts as a sort of diode to only allow current flow when the vehicle circuit voltage is higher than the aux battery circuit voltage. I had a good look at your website trying to figure out my options.

[drivesafe]

TB, it does not matter if your lithium battery tried to draw 500 amps direct off the alternator, as I have posted and nothing has changed in the way an alternator works, if the current applied to an alternator is greater than the maximum output current of the alternator, the alternator’s VOLTAGE drops.

You can not get around that and this VOLTAGE drop is independent of the BMS.

This is the way ALL alternators work.

Next, there is no such thing as “Energy capture during braking”

The operation of your brake peddle has nothing to do with the way the voltage is controlled by the BMS.

It is the position of your ACCELERATOR peddle that controls the voltage of your alternator.

This is easily demonstrated while driving.

As you drive up a hill, with your foot on the accelerator, with the voltage low, apply a gentle touch to the brake peddle and nothing changes.

When you are coming down the other side, while coasting the voltage will be high, but apply a gentle touch to the accelerator, and the voltage will immediately drop.

Even if you have your foot on the brake peddle, when you touch the accelerator peddle, the voltage drops.

This garbage about the alternator loading up to assist with braking is pure fiction.

If the battery was at 80% or higher, it would only be drawing 5 to 10 amps at most at the maximum voltage of 14.7v.

This would mean instead of taking 1,000 meters to stop, without the alternator’s assistance, you would only need 999.9 meters to stop with the mythical assistance of the alternator.

When a battery has a charged state of 80% or higher, you could have a 1,000 amp alternator and the battery would still only draw 5 to 10 amps max.

[TB]

TB, it does not matter if your lithium battery tried to draw 500 amps direct off the alternator, as I have posted and nothing has changed in the way an alternator works, if the current applied to an alternator is greater than the maximum output current of the alternator, the alternator’s VOLTAGE drops.

You can not get around that and this VOLTAGE drop is independent of the BMS.

This is the way ALL alternators work.

Well, that would be great and I hope that's how it works. Thanks

Next, there is no such thing as “Energy capture during braking”

The operation of your brake peddle has nothing to do with the way the voltage is controlled by the BMS.

It is the position of your ACCELERATOR peddle that controls the voltage of your alternator.

This is easily demonstrated while driving.

As you drive up a hill, with your foot on the accelerator, with the voltage low, apply a gentle touch to the brake peddle and nothing changes.

When you are coming down the other side, while coasting the voltage will be high, but apply a gentle touch to the accelerator, and the voltage will immediately drop.

Even if you have your foot on the brake peddle, when you touch the accelerator peddle, the voltage drops.

This garbage about the alternator loading up to assist with braking is pure fiction.

If the battery was at 80% or higher, it would only be drawing 5 to 10 amps at most at the maximum voltage of 14.7v.

This would mean instead of taking 1,000 meters to stop, without the alternator’s assistance, you would only need 999.9 meters to stop with the mythical assistance of the alternator.

When a battery has a charged state of 80% or higher, you could have a 1,000 amp alternator and the battery would still only draw 5 to 10 amps max.

Um... apologies for saying so, but you have something new to learn here. The entire point of "smart" alternators, the reason they were invented, is to improve fuel economy by using the alternator to capture unwanted kinetic energy in electrical form during **engine braking**. No, it's not a lot of energy. No, it doesn't replace your brakes. But JLR ditched proper centre differentials for inferior clutch-based front driveline disconnect just to get a 1% fuel economy improvement in the 6-cylinder diesel Defenders – so it isn't always the most pragmatic design that we get stuck with.

While rotational speed is a factor in determining an alternator's *power* output capacity, we don't have output voltage correlated to throttle position or engine revs. Output voltage is controlled by adjusting the current in the rotor coil. Older vehicles had analog regulator circuits to do that job. These days a computer calculates what the output voltage needs to be at any point in time and directly manipulate the rotor coil current to achieve that.

With good auto transmissions like the ZF that can lock the torque converter in any gear at any speed we get actual engine braking assist when we ease off the throttle or apply the brake pedal. The computers sense this and cause the alternator to increase the output voltage, so that current will flow into the deliberately-not-fully-charged starter battery. That means we're converting unwanted kinetic energy (which came from fuel) into electrical energy that we can use later. When engine braking has stopped the alternator output voltage will be reduced again by the computer, so that some charge will flow out of the battery and make room for the next capture.

So there's a fraction of kinetic energy extracted from fuel that brakes would have discarded as heat, but instead has been captured via the alternator and battery system then used to supply electrical loads in the vehicle. If you're doubting this explanation, consider that vehicle manufacturers were mandated to implement smart alternators under EURO5 and subsequent emissions reduction standards.