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Old 29-09-2021, 14:27   #1
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Will a DC-to-DC charger protect the alternator from lithium BMS cutoff?

I have an outboard with a 60 amp alternator connected to a 12V AGM starter battery. I also have two 12v lead acid house batteries connected in series. The alternator regulator is currently of the dumb variety with no temperature sensor. It would be painful to try and swap out the regulator for something smarter.

My starter battery is showing signs of weakness. I'd like to replace all 3 batteries with lithium. However, lithium BMS shutoff will likely roast the alternator's regulator. I could use an APD but most of them are sacrificial and this is in all likelihood a common occurrence.

If I place a DC-to-DC charger between the alternator and the battery will the charger always present enough load to the alternator to prevent regulator burn out even when the BMS shuts off?

The additional problem is that the lithium charging load at low RPM may overheat the alternator. However, I believe I can reduce the chance of this by limiting the size of the DC-to-DC charger. I am assuming capping the chargers draw to half of the alternator output, 30 amps, will prevent the low speed overheating issue.
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Old 29-09-2021, 15:12   #2
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Re: Will a DC-to-DC charger protect the alternator from lithium BMS cutoff?

The power coming out of a alternator is spikey at best. It needs to go to the capacitance of a battery to smooth it out. A DC to DC charger will let the magic smoke without the capacitance of a starter battery.
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Old 29-09-2021, 16:35   #3
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Re: Will a DC-to-DC charger protect the alternator from lithium BMS cutoff?

Alt to new agm engine battery. Dc to dc from engine battery to lithuim house bank.

Why do you want a lithuim engine bank? Cranking amps is very low.
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Old 29-09-2021, 16:49   #4
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Re: Will a DC-to-DC charger protect the alternator from lithium BMS cutoff?

These days solar is enough so it doesn't make sense to use an alternator.
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Old 29-09-2021, 18:26   #5
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Re: Will a DC-to-DC charger protect the alternator from lithium BMS cutoff?

Quote:
Originally Posted by NPCampbell View Post
I have an outboard with a 60 amp alternator connected to a 12V AGM starter battery. I also have two 12v lead acid house batteries connected in series. The alternator regulator is currently of the dumb variety with no temperature sensor. It would be painful to try and swap out the regulator for something smarter.

My starter battery is showing signs of weakness. I'd like to replace all 3 batteries with lithium. However, lithium BMS shutoff will likely roast the alternator's regulator. I could use an APD but most of them are sacrificial and this is in all likelihood a common occurrence.

If I place a DC-to-DC charger between the alternator and the battery will the charger always present enough load to the alternator to prevent regulator burn out even when the BMS shuts off?

The additional problem is that the lithium charging load at low RPM may overheat the alternator. However, I believe I can reduce the chance of this by limiting the size of the DC-to-DC charger. I am assuming capping the chargers draw to half of the alternator output, 30 amps, will prevent the low speed overheating issue.
Small FLA start battery and b2b charger to LFP house bank.
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Old 29-09-2021, 18:27   #6
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Re: Will a DC-to-DC charger protect the alternator from lithium BMS cutoff?

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Originally Posted by seandepagnier View Post
These days solar is enough so it doesn't make sense to use an alternator.
I still have a 75 amp alternator to charge my LFP house bank setup just as I stated in my previous post recommendation
Solar isn't always enough . Especially in winter
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Old 30-09-2021, 16:44   #7
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Re: Will a DC-to-DC charger protect the alternator from lithium BMS cutoff?

We use a Victron 30amp, 12v to 12v charger between our start battery (AGM) and our house bank (Li). The alternator goes to the start battery. Any excess flows to the house bank.
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Old 01-10-2021, 05:01   #8
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Re: Will a DC-to-DC charger protect the alternator from lithium BMS cutoff?

We charge from externally regulated alternator directly to the Lithium house bank with the APD in the circuit. Then B2B to (in our case) LA windlass battery. We start off the house bank with the second battery available only for backup.

So both, I guess. I wouldn't trust the B2B to absorb much current.

We max the absorption voltage at 14, and haven't yet seen the bank get anywhere near cutoff.
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Old 01-10-2021, 18:41   #9
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Re: Will a DC-to-DC charger protect the alternator from lithium BMS cutoff?

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
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Old 01-10-2021, 19:16   #10
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Re: Will a DC-to-DC charger protect the alternator from lithium BMS cutoff?

Quote:
Originally Posted by NPCampbell View Post

My starter battery is showing signs of weakness. I'd like to replace all 3 batteries with lithium. However, lithium BMS shutoff will likely roast the alternator's regulator. I could use an APD but most of them are sacrificial and this is in all likelihood a common occurrence.

If I place a DC-to-DC charger between the alternator and the battery will the charger always present enough load to the alternator to prevent regulator burn out even when the BMS shuts off?

The additional problem is that the lithium charging load at low RPM may overheat the alternator. However, I believe I can reduce the chance of this by limiting the size of the DC-to-DC charger. I am assuming capping the chargers draw to half of the alternator output, 30 amps, will prevent the low speed overheating issue.
A lithium start battery makes no sense for several reasons. Price for one and lithium's are not built to start an engine.

Use a lead/acid battery for starting.

A DC-DC charger wires between the start battery and the lithium bank. The alternator's output is wired to the start battery. Victron has the best in my opinion. https://www.victronenergy.com/dc-dc-...orion-tr-smart

Here is a link to several videos showing Orion installations.
https://www.youtube.com/results?sear...=victron+orion

The Orion will protect your alternator as it will always be connected to the lead/acid start battery - Agm or flooded as you prefer.
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Old 01-10-2021, 19:35   #11
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Re: Will a DC-to-DC charger protect the alternator from lithium BMS cutoff?

Quote:
Originally Posted by rgleason View Post
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
Actually the first reply answered the question abjectly in an in unconventional manor
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Old 01-10-2021, 19:43   #12
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Re: Will a DC-to-DC charger protect the alternator from lithium BMS cutoff?

Quote:
Originally Posted by rgleason View Post
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
It will not. A alternator to battery charger may. Download the manual for one and read it.

https://sterling-power.com/collectio...ttery-chargers
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Old 02-10-2021, 00:02   #13
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Will a DC-to-DC charger protect the alternator from lithium BMS cutoff?

Actually the answer is it might.

Firstly most modern alternators have clamping to ISO-16750-2. Peak load dump clamped to 35v ,

Secondly a small load is all that’s required to quench the alternator load dump issue. The risk with dc dc converters is they could be effectively “ off” when the load dump occurs.

Protecting the alternator can be as simple as switching in a resistor for about 300mS . The idea of leaving the starter battery inline is a typical “workaround” but in my view it’s not an optimum solution the dc dc solution is better , as in my view batteries at different SOCs should not be charged together. To me the battery needing most charging should be directly connected to the charge sources.

I charge my starter only from a Victron dc dc converter , all charge source go directly to the domestics. ( currently all FLA )

If investing in lithium my view is sort out the alternator disconnect issue and allow the alternator to charge the lithium’s and dc dc charge the starter
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Old 02-10-2021, 04:19   #14
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Re: Will a DC-to-DC charger protect the alternator from lithium BMS cutoff?

@Newhaul. What does "..let the majic smoke" mean? I need a translation to english please.
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Old 02-10-2021, 05:23   #15
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Re: Will a DC-to-DC charger protect the alternator from lithium BMS cutoff?

A couple of comments / questions.

A response indicated that lithiums are not meant to start engines. I strongly disagree! You can buy off the shelf lithium engine start batteries, but they really aren't any different. And engine cranking load is far less than typical inverter loads, and lithiums are better at high loads than lead acid. A lithium battery makes a perfectly acceptable start battery.

I have a question or concern about charging the AGM and B2B to the lithium. In OP's case, the alternator is fairly weak, and so this might work. But it is often touted as the answer, and on larger installations, mine for instance, with a 200 amp or more alternator source it seems a waste to put a 50 or 100 amp B2B in the line and limit charging to those rates.

With OP's internal regulator, he has a challenge. But with external regulators, it seems to me that it would be far superior to interrupt the field wire then to interrupt the alternator output. You switch 10 amps versus 200, and there is no chance of frying the alternator. Of course, with drop-in solutions, there's almost no opportunity to do this.

To the OP's original concern about protecting the alternator from a disconnect, I wonder if a better solution would be a load that gets switched into the circuit when the engine is running. A simple relay on the ignition switch, that would turn on a load like a light bulb. If you can find an old 100 watt incandescent household bulb, that would draw very little current at 12 volts, but would probably protect the alternator just fine. It would suck down one amp of the alternator output, and disconnect when the engine was off.

But even that answer gets me thinking. When the engine is running, you most assuredly have other things on the boat running. Chart plotter, VHF, stereo, maybe autopilot or running lights, etc. Are these loads not enough to protect the alternator from spiking? Is the concern more theoretical or special case than as widespread as it is thought?
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