Retrofitting a LiFeYPo4 battery charger

Hello forum,

My name is Hermann, I am 66 years old, and I live with my wife on our catamaran in Greece.

I hope to find the right answers to my questions in this forum.

I am in the planning phase for replacing the lead-acid batteries in my boat with LiFeYPo4 batteries.
To ensure proper charging of the LiFeYPO4 cells, they are monitored by a battery management system (BMS), and the charging process is interrupted when a precisely defined voltage is reached.

And now, here's my problem:
I would like to charge the batteries, among other things, using a powerful generator. If the "charging voltage" has been reached, I cannot simply disconnect the alternator from the vehicle's electrical system, as this would likely damage it.
The BMS outputs a signal when the voltage reaches a certain level, which I can use to control a relay.
Can I simply use the relay to disconnect the excitation voltage of the DF unit and thereby interrupt the charging process?
Is there any way to access the DF line and route it out of the Lima?

Greetings from Greece.
Hermann.

Hello Herrmann,
"If the load is suddenly disconnected from the Lima, a voltage spike occurs. (Google keyword: Load dump)." Older power supplies can damage their diodes, while newer ones use Zener diodes, which reduces the likelihood of this happening.
However, the BMS is not an Ohmic load; it likely draws power from your supply with a pulsed waveform. That certainly won't do the "nakten" Lima any good.
I assume the Lima is connected to a diesel engine that sometimes runs, but mostly doesn't. Because the starter motor of the diesel engine draws high currents for a short period, I would keep the entire electrical system of the diesel engine completely original and separate the "consumer network" with the LiPos from it. This ensures that the lead-acid starter battery remains as a voltage stabilizer.

When the engine is running, you can charge the house battery via the on-board electrical system; otherwise, you might be able to charge it using solar panels and wind power.
You can also find some information about this issue on microcharge.de (no relation or affiliation).

Best regards, gnash.

Hello gnasch,

Thank you for your explanation.

I probably should have elaborated more on my motivation.

Two older diesel engines, each with 55 horsepower, are installed. Each engine has its own onboard electrical system, which is currently used to charge the service batteries. An a motor, an additional alternator (Lima) is attached, which is only intended for the new service batteries (LiFeYPo4), while the old service batteries will be removed.

What I didn't understand is your sentence:
"However, the BMS is not an Ohmic load; it's likely drawing power from your supply with a pulsed waveform." "That certainly won't do the 'nakten' Lima any good."

The BMS is galvanically isolated and is powered by the LiFeYPo4 battery. It controls an external relay (200-500A) when a certain value is reached. This relay can, for example, disconnect all charging devices from the mains. Alternatively, I could also disconnect the alternator's excitation voltage, causing it to simply run without generating power. Or am I making a mistake in my thinking?

Best regards, Hermann.

Hello Herrmann,
You haven't specified the voltage level and charging strategy used for your new batteries. If your charger with BMS only acts as a resistive load, I would be less concerned. However, even a generator without external excitation can self-excite at slightly higher voltages due to residual magnetism. In that case, it will simply generate its idle voltage. I would thoroughly measure and analyze the processes, including any transient behavior, using the KO (likely referring to a specific instrument or method).

best regards,
gnash

Hello gnasch,

I would like to limit the voltage range of my LiFeYPo4 battery bank to a maximum of 13.8 volts and a minimum of 12.5 to 12.0 volts. In this area, the batteries have the longest lifespan.

When the battery reaches its lower voltage range, I want to charge it with the highest possible current.

I have an acquaintance who has been using this system on his boat for quite some time. He solves the shutdown problem by connecting an additional battery to the alternator via a blocking diode. However, that would not be the ideal solution.

Since I'm still in the planning phase, I haven't purchased any components for a new battery bank yet. That's why I also don't know what these processes look like when the alternator is switched off.

Would a circuit diagram be helpful to you?

Best regards, Hermann.

I think you'll have to use an externally adjustable LiMa regulator that can be influenced from the outside. Then you can have it automatically limit the voltage to 13.8V. Then the BMS doesn't need to shut down anymore.

Alternatively, I could install the existing external unit and allow the BMS to interrupt the current flow to the excitation winding before the battery itself is disconnected.

Greetings.
Guste.

Hello Guste,

That's exactly what I intended to do. I wanted to disconnect the excitation winding from the BMS to interrupt the charging process.

Gnasch was concerned that the LIMA might still self-excite at slightly higher voltages due to residual magnetism. Would that be problematic? Will this create a voltage spike, and thus a risk for the diodes?


Regards,
Hermann.

Damn, mistake.

"even at slightly higher voltages, it can self-excite."
should be
"even at slightly higher speeds, it generates its own excitement."

The reason is that the iron content in the rotor leaves behind a residual magnetism, which can then induce a small voltage in the stator winding, amplifying the magnetic field and energizing the generator.
Sorry for the error that distorted the meaning...

Best regards, Gnasch.

Couldn't we prevent that somehow? I mean, if the Lima (alternator) starts generating voltage again, but the DF (field winding) is still disconnected, then shouldn't there be no significant current flowing that could damage the diodes, right?

Best regards,
Hermann.

You don't have to prevent that. If you follow Guste100's suggestion and use a regulator set to 13.8V, then there will be no fluctuations in the current flow.
Best regards, Gnasch.

Hello,

Yes, I understand now.

I'll take a look online to see if I can find a suitable regulator.

Thank you for your help.
Hermann.

In my opinion, there's a difference between interrupting the DF (Direction Finder) and interrupting the excitation winding.

If the DC power supply is interrupted and the generator goes into self-excitation, the regulator injects a current into the excitation winding, allowing it to generate its full power.

Conversely, a break in the excitation winding allows only a small amount of self-excitation, but not the full power output.

That's my subjective assessment, at least.

Regards,
Guste.

Controller

Best regards, Gnasch.

Hello Guste,

Yes, I agree. I don't know much about lighting systems. Is it possible to access the lead wire connected to the excitation winding?

Hello gnasch,

Thank you for the link.
That would be the right regulator. If I'm correctly informed, the lithium battery still needs to be disconnected from the battery, because, unlike a lead-acid battery, a maintenance charge is not beneficial.

I'll double-check that again.
Best regards,
Hermann.

The normal charging voltage is 3.6V per cell. With your 13.8V, you're at 3.45V/cell for a 4S configuration, which is on the safe side. At that voltage, no current flows through the regulator. I assume you sail your catamaran more than you motor it. When the engine is not running, you can disconnect the alternator using a battery switch.
Is there still a shipment of solar panels?
Best regards, gnasch.

Hello gnasch,

Yes, I think that's a good approach.

I'm not the only one who's interested in LiFeYPo4 cells. However, it's quite unusual to see them used in the boating sector. The charging options are a bit more problematic. I'll be researching this for a while longer before I buy the cells. A friend of mine has been using these batteries on his boat for 5 years now, and he's very happy with them. Therefore, there is no reason to believe that using this technique is wrong.

I still have 900 watts peak of solar panels, and I will definitely find a use for them. Let's see how I can best utilize them with the LiFePO4 batteries.

Thank you again for your support.

Best regards from Greece, which is now quite warm.
Hermann.