Sterling Alternator to Battery device, and alternator output thoughts

[sailingharry]

Over on the Lithium forum, the Sterling Alternator to Battery (A2B) device is often mentioned. Digging into it, it is a clever and simple (but expensive) way to convert a "dumb" internally regulated alternator into a 3-stage temperature protected alternator. But what does it do, and what does it mean?

Here's what I glean from the literature. It is basically an energy pump. It sucks electrons off the alternator output, and pushes them to the battery. It does this fast enough (watching battery volts and alt temps, of course) such that the output voltage drops enough to put the alternator in full field (max output). Pretty clever.

So what is the impact and how does it compare?

First, I am a big proponent of using "amps" as a proxy for power, and like Ah and such. But I am an engineer, and am fully aware that it is an imperfect unit. To sharpen your pencil, you need to use Watts (power). And remember that W is A*V, and that power cannot be created (for this discussion, I am going to assume that efficiencies are 100%, another inaccuracy).

I am also going to assume that alternators, like solar panels, are constant current devices, not constant power devices. A 100A alternator puts out 100A at 12V, and at 15V. This is an exceptionally hard fact to determine (we get lots of info about output vs RPM), but this link has a curve that confirms that: February | 2016 | A Turn of the Nut

So, first case: A smart regulator charging a battery at bulk (say 14.5V), so still at full field. This is putting 100*14.5=1.45kW into to the battery. Sweet.

Second case: A A2B is trying to get to acceptance volts, so going 100%. It has the alternator pulled down in volts to get it to full field. Lets assume that's 13.8V at the alternator. The alternator is putting out 100A at 13.5, or 1.35kW. The A2B isn't 100% efficient, but let's assume it is. So it is putting in 1.38kW, or 9% less power. Not really all that bad -- it's only a cost of about 9A, but still.

Third case -- novel concept that may not work. Instead of a Balmar, and instead of an A2B, what about a new approach. Send battery volts unregulated to the field (obviously, that will need to be addressed in a real project), or full-field (same as with the Balmar regulator). Have the battery down from a long night, so charging at 13.0V (so with max output, that's 1.3kW). Insert a MPPT solar controller in the middle. It will adjust it's "power suck level" to get maximum power (remember, a solar panel has a similar output profile), which is probably around 15V (maybe even higher?). So, at 15V and 100A, you are generating 1.5kW. With the MPPT conversion, you are charging at 115A. Life might get even more interesting if the max voltage of a "12V alternator" was closer to 16 or 18V.

As a simplification, if you already have a smart regulator, you could set it to a max bulk voltage (say 15.5?) and it will full field when needed, and back off on temp and reduced demand.

Now, an MPPT is crazy expensive (a 100A runs around $700). So this is all an academic thought experiment. Kind of like Chotu's post about the extra power buried in the voltage profile of LFP. Educational, but no real-world applications.

[svtickles]

We have an Hitachi OEM alternator rated at 80 Amps. The PO installed a Sterling 80A AtoB device onboard. Since the alternator warms up quickly to its "hot" operating parameters, the output is merely 30-35 amps regardless of rpm. My impression is that the Sterling device is not doing anything. I couldn't find a temperature sensor although the manual implies one was included and I assume it's installed somewhere (otherwise, the stock alternator would have gone up in smoke when I replaced my service bank batteries to LFP).

In retrospect, upgrading the alternator would have been the better choice for the previous owner. However, anything over 50 Amps requires a V belt replacement to serpentine. Balmar offers a kit. I heard Yanmar offers one but I couldn't find it.

I'm very tempted to upgrade to a Balmar XT 170 Amps alternator... but regulator/controller, APM, belt upgrade kit, and relay between motor start and service bank comes in at $2240. Yikes!

[sailingharry]

Originally Posted by svtickles

In retrospect, upgrading the alternator would have been the better choice for the previous owner. However, anything over 50 Amps requires a V belt replacement to serpentine. Balmar offers a kit. I heard Yanmar offers one but I couldn't find it.!

That is conventional wisdom, but not a hard rule.
I'm a little bit with a little 27 horsepower volvo, I installed a 120A externally regulated automotive style alternator. Between a cheap alternator and 220Ah of FLA golf cart batteries, I could rarely get more than 70 or 80A out, but the V belt was fine.
Our current boat has a 130A externally regulated Powerline alternator, frequently puts out over 100A. I have put close to 1,000 hours on that alternator, usually under significant charge load, and only broken one belt. Belt dust has been fairly minor. I wouldn't shy away from a 100 to 130A alternator just because of the V belt.