Alternators control the output by modulating the magnetic field of the rotor. When more current is needed to maintain bus voltage then more field current is applied and at a given RPM the output current and voltage increases. 

 

When the alternator is spinning slowly, like at idle, and the demand for current is high, like recharging after an extended starting sequence, the alternator regulator may reach the maximum amount of field current that can be applied. In this case the alternator is current limited by the physics of the situation, where the RPM is too low to support the current demands and the bus voltage will be at less than desired until either the RPM is increased or the current demand is decreased: the field is already maxed out. 

 

As the RPM increases the amount of power available increases, this is reflected in more current. The current output will increase until one of two things happen:

1. The regulation voltage is met and the regulator starts to reduce the field current, reducing the output current, to maintain the desired bus voltage.

2. The output is curtailed by the internal resistance of the output windings. With high RPM the output could easily be 60 amps for alternators that are otherwise rated at 40 amps. As the alternator heats up the current capability will gradually reduce from the 50%? overage down to something closer to the rated output.

 

The point to take away from this is that alternators typically do not have a "hard stop" current limit, if the regulation voltage is not met then they may put out substantially more current than you would expect.

 

At the same time, an operator should not worry about running an alternator at 100% rated output continuously. If the alternator can't do that then don't install it.

 

The question a builder should ask with regard to lithium batteries is why would you install a battery that wasn't compatible with my charging system? By compatible, I mean the battery must be able to take all the current that the charging system can output less the minimum expected load from the rest of the airplane. If you want to be conservative ignore the min expected load: the max current of the charging system, including the 50% overage, should be less than the maximum current the battery can take.

 

Alternators with lithium batteries start to work you into an undesirable corner here. The alternator *could* put out more current than you would ever want to go to the battery and you must plan for that. At the same time the alternator should be sized for the max steady state load (think, night, IFR, pitot heat on) to be no more than 80% rated output, where you can't bank on that extra 50%(or whatever it is) power being available. And you may not be able to really know what the upper limit of current is out of an alternator for planning purposes. So the 40 amp alternator is only good for 32 amps steady state but you need to plan for 60 amps charging rate being available. The problem gets worse if you have more than one power source. A few thoughts:

 

1. Get a battery than can charge at a rate at least 50% more than the alternator rating. 40 amp alternator? Get a battery that can take 60 amps. A 60 and a 40 amp alternator? Get a battery than can take 150 amps.

 

2. Get an alternator rated at 2/3rds or less of the max charge rate of the battery you are using.

 

3. If there is any concern about the alternator failing because it is run at 100% output for extended periods of time then get another power source. The last thing you want to be doing is tooling along in the clouds and wondering if you can run the pitot heat because you're concerned about burning up the alternator. 

 

Either way, you shouldn't put something in your airplane that won't be compatible across all foreseeable cases. The case of concern here is having a battery that can be overwhelmed by the charging system.

An alternative: The Monkworkz generator (2.6 lbs, 30 amps) applies a current limit that is stable across operating conditions. The current limit is a "hard stop". The device directly monitors current output hundreds of thousands of times a second and reduces the output voltage until the current limit is respected. It is rated for 30 amps, and will never put out more than 30 amps. In this case, based on the 80% rule you can plan for 24 amps, and select a battery that can cope with 30 amps charge rate and your done.

https://docs.google.com/document/d/1BckaSN06Lv4BYY07AfHk3YRvx8b60yybpX0qHB-Mymc/edit