Will a DC-to-DC charger protect the alternator from lithium BMS cutoff?

[mitiempo]

Originally Posted by NPCampbell

2) Amp-hours. The massive trolling motor is an option on this boat, so the added 24V "house" batteries don't actually drive anything but the trolling motor. This means the starter battery is starting the engine AND driving the power trim, 2 charts, a network, LiveScope, sink pump, live well, bilge, stereo, etc. I'd feel way more comfortable with a starter battery that could actually provide it's listed amp hours.

Correct me if I am wrong - you have a 12 volt start battery and a 24 volt "house bank" that is only used for the trolling motor. And all house loads are powered by the start battery.

If the above is true I would install a lead acid start battery. Charge the 24 volt bank with a DC to DC charger.
I would run all house loads - plotter, stereo, bilge pump, etc - from the 24 volt bank with a DC to DC converter. They are not expensive. I used 2 on a 24 volt boat recently.

The weight saving by switching the start battery from lead acid to lithium isn't that great, maybe 30 lbs. But going to a lithium start battery will add complication and expense besides the battery.

[NPCampbell]

Originally Posted by lmxr

@OP: your outboard, does it have one or two positive cables? Is it one positive cables that carries both starter motor and alternator current? Or does it have two positive cables, one for the starter motor (and engine electronics) and one for the alternator? I assume the latter?

In case of a single cable: How about a small, cheap, sealed lead-acid starter battery connected in parallel to the LFP starter? Whenever the LFP battery cuts out, the LA battery can act as a sink. Though do make sure to fully charge both batteries before putting them in parallel.

In case of a double cable: Alternator output goes to a small LA battery. Then charge the LFP from the small LA using a B2B. The LA is just there to smooth the alternator output and to act as a sink. Would that be an option?

Here's the description of the alternator, the regulator is internal (I have the 3.0L). It doesn't use the magnet on the flywheel approach:

Found on all Mercury Marine 3.0L V-6 and 2.5L OptiMax V-6 models, this is the most powerful outboard charging system in the industry. Fully regulated, this system provides 35 amps (493.5 watts) on conventional 3.0L V-6s and 40 amps (564 watts) on OptiMax models at only 1000 RPM, while most competitors are providing that much only at W.O.T. (Wide Open Throttle). This is a marinized version of an automotive-style alternator and belt-driven off of the crankshaft. This field wound configuration, meaning there are no magnets passing by the stator, allows the system to put out current only as required by the battery. This eliminates the heat build-up problems associated with high-output stator-driven designs and maximizes horsepower by supplying current only on demand.

That bolded part is kinda interesting. A lot of automotive designs have precautions against load dumps.

[NPCampbell]

Originally Posted by mitiempo

Correct me if I am wrong - you have a 12 volt start battery and a 24 volt "house bank" that is only used for the trolling motor. And all house loads are powered by the start battery.

Correct.

Originally Posted by mitiempo

If the above is true I would install a lead acid start battery. Charge the 24 volt bank with a DC to DC charger.
I would run all house loads - plotter, stereo, bilge pump, etc - from the 24 volt bank with a DC to DC converter. They are not expensive. I used 2 on a 24 volt boat recently.

The weight saving by switching the start battery from lead acid to lithium isn't that great, maybe 30 lbs. But going to a lithium start battery will add complication and expense besides the battery.

The weight savings would actually be over 100 pounds but I could achieve 2/3 of that just by swapping out the 24v batteries. However, your suggestion would solve some of my issues. I don't want to re-wire the rat's nest under the console and converting down from 24v to 12v would solve that. The only remaining issue I would have is keeping that starter battery topped up between weekend uses. A 10w solar tender can probably do that between weekend uses. Especially so if the starter battery is removed from it's obligation of powering everything but the starting duties.

[mitiempo]

Originally Posted by NPCampbell

The only remaining issue I would have is keeping that starter battery topped up between weekend uses. A 10w solar tender can probably do that between weekend uses. Especially so if the starter battery is removed from it's obligation of powering everything but the starting duties.

If the start battery is only used for starting it will not need charging other than from the engine if used frequently.

[goboatingnow]

Originally Posted by newhaul

You really need to keep track of what you post



Post#22

It’s a myth you need a battery with an alternator per se



Leading to my question

"Originally Posted by*newhaul*



And how do you propose to accomplish a charging demand without having a battery in the loop"



Please answer I would really like to know . It could save money for my customers.

You really need to keep track of the context in which I made that comment. I was referring at the time to the issue of alternator disconnect under lithium systems. The context is about load dumping in that situation

Clearly there is no battery when the battery is disconnected

I presume even your “customers “ don’t want to charge a battery that is full and has tripped a HVC. Alternately it could be a death wish or an insurance scam , doesn’t sound like a customer anyone would want.

[newhaul]

Originally Posted by goboatingnow

You really need to keep track of the context in which I made that comment. I was referring at the time to the issue of alternator disconnect under lithium systems. The context is about load dumping in that situation

Clearly there is no battery when the battery is disconnected

You still have not answered the question
Originally Posted by*newhaul*



And how do you propose to accomplish a charging demand without having a battery in the loop"

[goboatingnow]

In my opinion and is all charge sources should directly connect to the battery bank requiring the most charging , most usually the domestic bank. The starter battery is then charged via a dc dc converter.

This is the most efficient mechanism and avoids any issues with overcharging the starter battery

The only issue that must be handled is unexpected alternator disconnect under HVC for lithium. The issue doesn’t arise for FLA

I currently charge my domestic FLA in this manner

The alternator issue can be dealt with in a number of ways including alternator “ protectors “ , load switching or field disconnect.

[goboatingnow]

Originally Posted by newhaul

You still have not answered the question

Originally Posted by*newhaul*







And how do you propose to accomplish a charging demand without having a battery in the loop"

Sorry I’m an electronics engineer , what’s a “charging demand “ I’ve designed alternator controllers, never heard the term before

An alternator needs a load it doesn’t have to be a battery

But you understand the context of my original comment , which was to disconnect the alternator under HVC, clearly if a battery needs recharging it will be connected to whatever charging sources exist.

[newhaul]

Originally Posted by goboatingnow

Sorry I’m an electronics engineer , what’s a “charging demand “ I’ve designed alternator controllers, never heard the term before

An alternator needs a load it doesn’t have to be a battery

But you understand the context of my original comment , which was to disconnect the alternator under HVC, clearly if a battery needs recharging it will be connected to whatever charging sources exist.

You are the first one I have ever heard use that term " charging demand" .
Hence my question about it .

[goboatingnow]

Originally Posted by newhaul

You are the first one I have ever heard use that term " charging demand" .

Hence my question about it .

I don’t think you read my post , here’s where you quoted me and asked the question i don’t believe I ever
used the phrase “ charging demand “ I believe you actually used the term.

Again you are deliberately avoiding the context of my posts.

[newhaul]

Originally Posted by goboatingnow

I don’t think you read my post , here’s where you quoted me and asked the question i don’t believe I ever
used the phrase “ charging demand “ I believe you actually used the term.

Again you are deliberately avoiding the context of my posts.

Attachment 246514

My bad I must have not been awake when I read your post #22 I read charging you wrote changing.
But fact remains how do you propose protecting an alternator from a sudden hvd situation where it is putting out significant amperage and then suddenly nothing ?
If I understand you recommend running some type of load to absorb the spike that will happen when ( in the unlikely event ) there is an HVD situation .
Please as I have requested elaborate on this .

Common knowledge says you do need a battery to smooth out the choppy and spikey voltage coming from an alternator before any loads are applied.

[goboatingnow]

Originally Posted by newhaul

My bad I must have not been awake when I read your post #22 I read charging you wrote changing.

But fact remains how do you propose protecting an alternator from a sudden hvd situation where it is putting out significant amperage and then suddenly nothing ?

If I understand you recommend running some type of load to absorb the spike that will happen when ( in the unlikely event ) there is an HVD situation .

Please as I have requested elaborate on this .



Common knowledge says you do need a battery to smooth out the choppy and spikey voltage coming from an alternator before any loads are applied.

That’s fine anyone can make a mistake


You might take a look at the output waveform from a modern alternator it’s not bad as you might think

There are several ways to deal with the alternator during a HVC ( That’s the common nomenclature ) event.

The first is to control the alternator output , via the field feed. This is the best and requires a alternator regulator with such an ability.

The next option is to add a protection device which can clamp the output. Most alternators today are rated to clamp to IEC16750-2 test B which requires suppression in the alternator to 35v for 400ms. In fact everything on your boat should actually survive this test in the first place. ( sadly some cheap marine stuff doesn’t )

So a protection device suitably rated to handle either Test A or the easier Test B , can do the job , all it really does is provide an alternative load ( usually a big TVS diode or diodes )

The third choice is to provide an alternative load that’s switched in before the HVC event. Again in any alternator system you have to watch di/dt values. Preferably Also the load needs to switch when the alternator is not at max capacity again to minimise di/dt

All this is standard stuff but an alternator doesn’t “need “ a battery per se ( there are of course many reasons why one is present in the circuit.

It’s instructive to put a scope on automotive systems and actually monitor voltage and current waveforms. It’s a very dirty system irrespective of the alternator.


So if at disconnect of the battery , the battery is consuming 10A and the rest of the boat is consuming nothing , then switching in a 2-2A load over 100mS will actually prevent any significant spike as the alternator regulator will handle the change . This can easily be seen on a test rig. Clearly that’s not worse case but even in worse case an alternative load can be switched in to allow the battery disconnect. you could even temporarily switch in an alternative battery until the HVC ended etc.

Any number of ways of doing it.

The OPs question concerned a DCDC battery charger. The issue is if the load via the dcdc is small at the point of disconnect the di/dt remains very large and the spike issue remains. This is why I said “ it depends”


I used to be involved in design in truck DC systems for a while

[goboatingnow]

Common knowledge says you do need a battery to smooth out the choppy and spikey voltage coming from an alternator before any loads are applied

An alternator is a 3 phase sine wave power supply. This is then rectified. A modern IC based regulator then controls the voltage as the current demand changes. The battery acts as a capacitor rather like the dc power supply capacities you’d have in a mains power supply unit.

The output isn’t spiky or choppy per se. It’s quite clean actually. In the past voltage regulation was electro mechanical and that system was poor and generated lots of spikes

You can ( if you fool the electronics ) run your car without its battery if you know how. Modern cars use the alternator as a dc power source , very little battery charging goes on at all in fact the alternator regulator isn’t actually designed as a battery charger at all. Which is why in boats the stock alternators underperform as boats use their alternators as chargers rather then power sources

[newhaul]

Originally Posted by goboatingnow

That’s fine anyone can make a mistake




There are several ways to deal with the alternator during a HVC ( That’s the common nomenclature ) event.

Ok so tell us all that deal with this stuff every day .
What is HVC?

A BMS lists its disconnect on high voltage as a HVD situation .
Aka High voltage disconnect.

[CatNewBee]

Originally Posted by rgleason

Nobody has answered the question asked by OP in a direct manner!


Will a DC-to-DC charger protect the alternator from lithium BMS cutoff?

The question stands unanswered

No. It will not.

The question should be why would the battery disconnect in the first place if the alternator is working properly?