Another Li-Alternator question...

[SailingHarmonie]

I know this is a topic beat to death, and this is really just a question to satisfy my curiosity.

I have a 465 A-hr, 24 V house bank of Firefly AGMs. They work great, and hopefully will continue to for years. I have no plans to switch to Li because the existing system satisfies our needs, and we have lots of other things to spend money on.

Solar supplies about half our energy needs, and the balance comes from a WhisperPower DC generator. That's all fine, and it works great...

But I have a question...

We have a 50Amp small frame, internally regulated 24V alternator on our drive engine. It is the one that came with the engine when it was installed 27 years ago, and has 9000 hours on it, so it is (obviously) fully suited to the service it is asked to perform. It's job is to fully supply the power we use when underway motoring (averaging about 12 Amps), with enough left over to give a net positive energy flow to the batteries. It is not designed or used a a primary charging tool.

It is not unusual that we start the engine when the batteries are lower down on the SOC scale. Maybe as low as 65%. When that is true, that little 50 Amp alternator runs at full output for on the order of 3 to 4 hours before it reaches its set point voltage and starts to ramp down the Amps. It generates its hot rated 42 to 45 Amps continuously. Basically, for that time it is no different than if it was connected to a large Li battery bank.

So all that leads to my question: Why do Li battery charging cycles chew up alternators that are not temperature regulated, and our system does not? Surely after 3 hours, the alternator has reached a steady state temperature with a constant power output?

[glenn.225]

I suspect whoever converted your alternator from 12 to 24 volts or the original manufacturer put very high-quality components in it. And that it is only delivering 40 odd amps instead of 70 odd that could possibly be asked to, makes it durable.
Three decades ago, most automotive 24-volt gear was for ambulances or other specialty vehicles, so generally, it was rugged equipment.

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[lmxr]

Originally Posted by SailingHarmonie

It is not unusual that we start the engine when the batteries are lower down on the SOC scale. Maybe as low as 65%. When that is true, that little 50 Amp alternator runs at full output for on the order of 3 to 4 hours before it reaches its set point voltage and starts to ramp down the Amps. It generates its hot rated 42 to 45 Amps continuously. Basically, for that time it is no different than if it was connected to a large Li battery bank.

There's still some difference. The steady-state internal resistance of a large LFP bank tends to be even lower than that of a good lead-acid bank. And because the internal alternator regulators do not measure (nor estimate) the current, alternators can go beyond their hot rating if connected to a sufficiently low voltage, low resistance load. Finally, LFP people tend to run their batteries to low SOC (which is good) but this also means that the batteries' terminal voltage will be only 13.3 V or so for several hours. I expect your lead-acid batteries to climb quite rapidly to around 13.6 V - feel free to correct.

The voltage setpoint and voltage sense also plays a role. Many modern alternators sense the positive battery voltage right at the battery thanks to a dedicated sense wire (running from the alternator to the battery) and try to push this voltage to their high setpoint of 14.3 V or more. Meanwhile, many older alternators are set to only about 14.0 V and they do not compensate for any voltage drop on the positive battery cable.

Originally Posted by SailingHarmonie

So all that leads to my question: Why do Li battery charging cycles chew up alternators that are not temperature regulated, and our system does not? Surely after 3 hours, the alternator has reached a steady state temperature with a constant power output?

Indeed, after 3 hours the engine room and alternator temperatures tend to be in steady state. What I guess is that your alternator has a powerful fan that can keep it cool. Perhaps it's also on a relatively small pulley appropriate for a marine diesel engine and you might also have a well-ventilated engine room. In general, alternator fans have decreased in size over the last decades in order to increase the alternator's efficiency and therefore lower the car's fuel consumption.

Great job on achieving 9000 hours with your engine - you must be taking good care of her.