DC to DC regulator circuit to handle lots of amps

MisterBill2

Joined Jan 23, 2018
18,179
Certainly A high power chopper switching at a high frequency can produce a lot of harmonic energy, no question about that. This application would be switching at a much lower rate, probably less than once per second. Also, the current being switched is only the field current of the alternator. Given that the residual magnetism in the field does not instantly change, the 140 amps will not instantly change, and so the harmonic energy in the output is far less. So the two applications are a bit different. BUT thanks for mentioning the fact that precautions must be taken.
 

shortbus

Joined Sep 30, 2009
10,045
. I guess for $200 it's not horrible but would have loved to have something less expensive
What is the alternator being put on? You seem to be thinking that by modifying the new alternators regulator it will still keep the rest of the vehicle running and satisfied. So the expense of another alternator and it's wiring is just part of the project. Once you change the regulator it will only work for your Lipo, and not even really do that correctly. Most new vehicle engines don't even have enough room to add another alternator. Then you get to the lipo being charged, don't lipos need a BMS? To handle charge equaling? But then again this may be a stand alone engine not in a vehicle.
 

Thread Starter

SailCS33

Joined Mar 18, 2021
53
What is the alternator being put on? You seem to be thinking that by modifying the new alternators regulator it will still keep the rest of the vehicle running and satisfied. So the expense of another alternator and it's wiring is just part of the project. Once you change the regulator it will only work for your Lipo, and not even really do that correctly. Most new vehicle engines don't even have enough room to add another alternator. Then you get to the lipo being charged, don't lipos need a BMS? To handle charge equaling? But then again this may be a stand alone engine not in a vehicle.
The engine now has Hitachi 60amp which charges an AGM. It works just fine for that. LIFEPO4 acceptance rate is very high and so will try to make the alternator run at it's max which will heat it up and either reduce output to a useless numbers or burn it and kill the alternator.
The idea was to put a larger alternator but limited its output to only a fraction of the rated capacity to keep it producing the needed 40A but for longer periods of time.

P.s. 40 is plenty... even 30 would be fine. The important thing is how long can it do it.
 

MisterBill2

Joined Jan 23, 2018
18,179
I did once "use up" a battery that was not intended for vehicle operation because the charging system did deliver current suitable for a wet standard battery instead of an AGM battery. So it seems that really what the TS is wanting is a scheme to reduce the maximum charge current because the battery is not the type that the regulator is designed for. That does make sense. There are adjustable regulators available, but for this application it may even be reasonable to simply insert a resistance into the alternator field circuit to reduce the alternator maximum output. That resistance will need to dispose of quite a bit of heat and so it will not be small. It would be best to have it in the cooling fan's air stream.
Of course, a replacement regulator could find a bit of room under the hood of most cars, but not much room. And it will need to be built to survive the under hood environment.
 

crutschow

Joined Mar 14, 2008
34,285
The engine now has Hitachi 60amp which charges an AGM.
The idea was to put a larger alternator but limited its output to only a fraction of the rated capacity to keep it producing the needed 40A but for longer periods of time.
It would seem that the Hitachi 60A alternator would readily generate 30-40A continually without a problem, so why not just limit its current?

A resistor as MB2 suggested may be all you need.
You would probably need a 10W resistor of about 6 ohms or so (TBD).
You could buy a few 5W power resistors of lower value, and then experimentally put them in series and/or parallel until you get the desired maximum current.
 
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shortbus

Joined Sep 30, 2009
10,045
The engine now has Hitachi 60amp which charges an AGM. It works just fine for that. LIFEPO4 acceptance rate is very high and so will try to make the alternator run at it's max which will heat it up and either reduce output to a useless numbers or burn it and kill the alternator.
The idea was to put a larger alternator but limited its output to only a fraction of the rated capacity to keep it producing the needed 40A but for longer periods of time.

P.s. 40 is plenty... even 30 would be fine. The important thing is how long can it do it.
So the idea is to use the lipo as the vehicles battery, replacing the AGM? You still need a BMS for that. Lipos don't charge like a AGM or flooded lead acid battery.
 

Thread Starter

SailCS33

Joined Mar 18, 2021
53
So the idea is to use the lipo as the vehicles battery, replacing the AGM? You still need a BMS for that. Lipos don't charge like a AGM or flooded lead acid battery.
No, the starting/main battery is not changing. I'm just changing the house battery. It's going from AGM 100ah to 200ah lifepo4. I already have a BMS for that (happens to be Daly Smart BMS.). No additional charge profile management is needed as there's really no bulk, absorption or float. I just need to give it 30-40A at 14ish volts and it will be a happy camper until 3.65v each cell (x 4).
 

Thread Starter

SailCS33

Joined Mar 18, 2021
53
It would seem that the Hitachi 60A alternator would readily generate 30-40A continually without a problem, so why not just limit its current?

A resistor as MB2 suggested may be all you need.
You would probably need a 10W resistor of about 6 ohms or so (TBD).
You could buy a few power resistors of lower value, and then experimentally put them in series and/or parallel until you get the desired maximum current.
I will certainly try that but everything I'm reading about these internally regulated alternators is that they get hot quickly and to mitigate that the regulator drops the voltage.

About the resistor. I have to apologize for my lack of knowledge, if i add a resistor in series, would it only reduce the voltage? Ideally I'd like to keep the voltage at around 14v but reduce/limit the current output to keep the heat at manageable levels.
 

crutschow

Joined Mar 14, 2008
34,285
About the resistor. I have to apologize for my lack of knowledge, if i add a resistor in series, would it only reduce the voltage?
I'm taking about a resistor in the rotor excitation current line.
A resistor would reduce the maximum current to the rotor, and thus reduce the maximum alternator current output.
But that won't work if the alternator has an internal regulator.
 

Thread Starter

SailCS33

Joined Mar 18, 2021
53
I'm taking about a resistor in the rotor excitation current line.
A resistor would reduce the maximum current to the rotor, and thus reduce the maximum alternator current output.
But that won't work if the alternator has an internal regulator.
Thank you, I really appreciate the patience with answering all the questions!
 

GetDeviceInfo

Joined Jun 7, 2009
2,192
No, the starting/main battery is not changing. I'm just changing the house battery. It's going from AGM 100ah to 200ah lifepo4. I already have a BMS for that (happens to be Daly Smart BMS.). No additional charge profile management is needed as there's really no bulk, absorption or float. I just need to give it 30-40A at 14ish volts and it will be a happy camper until 3.65v each cell (x 4).
If this being your load, why do you need to alter the source?
 

GetDeviceInfo

Joined Jun 7, 2009
2,192
Because of the lifepo4 acceptance rate it will max out almost any alternator (or any other source). It does need at least 13.6v to change though when it gets to 80%+ SOC.
BMS not programmable to a lower capacity? I don’t think switching it’s source is going to work in your favour. Voltage will fall off as you enter current limit
 
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MisterBill2

Joined Jan 23, 2018
18,179
Reducing the field's excitation reduces the voltage, which is how the current gets reduced. The fact is, that is the only way to control current in a load that is not adjustable, such as a battery. The current is driven by the voltage, and so reducing the voltage to set the current is the only way that will happen. Doing it with a reistor does the same thing, but it is less efficient.
 

GetDeviceInfo

Joined Jun 7, 2009
2,192
Yes, but you are not driving a battery, rather a bms, which in turn is charging the battery. Would it provide a meaningful charge profile if it was reset from low voltage every time it entered a higher current mode. As goofy as it sounded, the previous suggestion of a charger ic, provides the mosfets you’ve been speaking about, along with current profiling. The alternator is simply a current magnifier. Of course you could program charger functions into the mcu and run it as you’ve suggested, in place of the bms.
 
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MisterBill2

Joined Jan 23, 2018
18,179
Yes, but you are not driving a battery, rather a bms, which in turn is charging the battery. Would it provide a meaningful charge profile if it was reset from low voltage every time it entered a higher current mode. As goofy as it sounded, the previous suggestion of a charger ic, provides the mosfets you’ve been speaking about, along with current profiling. The alternator is simply a current magnifier. Of course you could program charger functions into the mcu and run it as you’ve suggested, in place of the bms.
INCORRECT! This is the battery maintaining system! That is the sole reason for the question that started the thread. Nothing between the alternator and the battery, except possibly a diode, and I am jut guessing about that part. The alternative, originally considered, and rejected, was a series current limiting system of some kind. Instead, in the same general manner as has been used for at least 50 years, an alternator with field control to regulate the charging current is being discussed. And the field current does control the charging current by varying the voltage, since voltage is what pushes the current through.
 

GetDeviceInfo

Joined Jun 7, 2009
2,192
INCORRECT! This is the battery maintaining system! That is the sole reason for the question that started the thread. Nothing between the alternator and the battery, except possibly a diode, and I am jut guessing about that part. The alternative, originally considered, and rejected, was a series current limiting system of some kind. Instead, in the same general manner as has been used for at least 50 years, an alternator with field control to regulate the charging current is being discussed. And the field current does control the charging current by varying the voltage, since voltage is what pushes the current through.
I may have mis interpreted post #27
 

MisterBill2

Joined Jan 23, 2018
18,179
I may have mis interpreted post #27
It does seem a bit different, but the system appears to be a motor home with multiple power systems, with this part being the lighting and utility battery while the starting and engine battery is separate.
So now the consideration of how to charge two different kinds of batteries is a bit confusing.
Quite probably some additional explanation from the TS is in order.
It might even be that a simple solution is not possible, only a more complex solution, if both batteries are to be charged at once.
 

Thread Starter

SailCS33

Joined Mar 18, 2021
53
My apologies if any of the information was not initially shared. I had no intentions of hiding anything. Just didnt think it has any relevance.
Surely hope this will not complicate things. Here is what I have. In a sailboat, I have a small 3 cylinder diesel engine with the previously mentioned Hitachi 60A. Not sure if this is of any importance but the engine to alternator has 1:2 pulley ratio. Engine idles at 1000 rpm, full speed is 3000rpm. The alternator turns at double the speed respectively.
Up until now I had two 100Ah AGM batteries. 1 batt is dedicated to starting the engine and 1 is "house" (accessories, electronics, etc.)
I am replacing the house battery with the different chemistry, LiFePo4 and in the process adding more amp/hours (200ah).
As I already said, the lifepo battery has a BMS which protects the battery from over charger/discharge, temp and other things.
The main/starting battery is maintained by a small 50watt solar panel as well as DC to DC charger. Source is the 'house' bank. The house battery is charged by the alternator.

This system works just fine with exception of the alternator not being designed to output 30-40Amps @ 14+v for 3 hours at a time without overheating. Hence, the original question. Note that I am purposely not looking to output 140A or more from any alternator no matter how big or expensive. This is a dangerous territory and the risk outweigh the benefit.

Back to restating the original question. I would like to make (or buy if I have to) a device that will limit the alternator output to 40A. My rational is that if I do install 140A alternator but only pull 40A max, it will be able to produce the 40A without overheating.

Note that lifepo battery does not require any particular charge curve. It will accept almost any amperage at around 14v. As the battery charges its acceptance will reduce starting at about 80% SOC and will continue to taper off. once the battery reaches 14ish volts it is considered 100% charged and the charger is disconnected. No float.

From everything I've read so far it sounds to me that my main problem is the voltage regulator inside the alternator. To address this I am learning now how to either remove the one thats built in or buy one without one built in.
 
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