Briggs and gm charger

Ignoring failure due to temperature/current for the moment... Alternators are pretty well know for outputting "rated current" when you first start the car, and then ramping down very quickly as things warm up...

You might find a specification... Here is Balmar's ratings of RPM vs Temperature vs Output current (very good products in general):

https://balmar.net/balmar-technology/alternator-output-curves/

And they drop output current by 10-20% from "cold" to "hot". (I think "car alternators" can drop by 50% or even more as they heat up... But could not find a quick link for that).

Next, you need to look at the charging voltage for the battery... A FLA battery (deep cycle lead acid, flooded cell) will take around 14.75 Volts as Absorb set point... Below 80% State of charge, the battery will take (FLA: efficiently) >20% rate of charge (>20 amps into a 100 AH battery bank). From ~80-90% state of charge, the FLA will reduce accepted charging current... And around 90-100% state of charge, the charging current will drop to around 1% or less rate of charge.

EQ charging voltage is higher (15-16 volts or so), and typically around 5% (down to 2.5%) rate of charge.

Of course, with a standard car alternator (at least the older "simpler" cars), charge around 14.0 to 14.2 volts (and around 13.8 to 14.4 is probably "within specs.").

Which brings us to the next issue... What is the charging voltage you want for your battery bank--Charging at 14.0 volts is going to be much slower than charging at 14.75 volts (short/heavy cables to keep wiring voltage drop to ~0.05 to 0.10 volts max suggested).

And how many integrated alternators have adjustable voltage regulators?

You could, rewire the alternator to supply 0-4 amp (typical) field current to the rotating field pieces. Manual pick a number, or build a manual/automatic regulator... Or get a marine (typical) alternator with external/adjustable output voltage:

https://balmar.net/balmar-technology/multi-stage-regulation/

So, we now are in the range of alternator fuel efficiency vs costs of parts/components for the charging system...

http://www.delcoremy.com/documents/high-efficiency-white-paper.aspx

Assuming you are OK with charging at 14.x volts, or have a fixed/manual charging system (cheap version) with a B&S motor. B&S motors tend to be less fuel efficient running at 3,600 RPM... And running at 3,600 RPM with less than full motor load--My first estimate is that Gasoline motors consume 100% rated fuel flow at 100% rated HP, linearly fuel flow drops as motor reaches 50% load (and 50% fuel flow). Below ~50% rated load, fuel flow (for gasoline/propane engines) tend to stay at 50% fuel flow as loads drop.

Getting a "modern" inverter generator where they throttle back as the electrical loads drop, tend to stay relatively fuel efficient down to 25% of rated load...

If you have an old flat head B&S 3hp engine, and a 63 Amp @ 12 volt generator (~765 Watts)... More or less, that would draw 2hp (belt and alternator losses) from the engine. Not sure (cannot find any "real numbers") but I would suspect an old flat head engine is not very fuel efficient. The old number was ~1/2 lb of fuel (gasoline) per HP hour...

• 2 HP * 1/2 lb per HP hour * 1 hour = 1 lb of gasoline running a 1 HP alternator
• 1 lb fuel per hour * 1/6 bls per gallon = 1/6 gallon per hour = 0.17 GPH

http://www.delcoremy.com/documents/high-efficiency-white-paper.aspx

So, lets look at some small gensets and possible fuel usage... 

https://www.amazon.com/All-Power-America-APG3014G-Generator/dp/B083LCZ7YZ

Lets assume that the numbers are somewhat accurate

• 2000 Watt Peak Power, 1400 Watt Rated Power
• 3 HP 98cc 4-Stroke OHV Engine; Runs 9 hrs @ 1/2 load, Operating Noise 65 dB
• 1. 3 Gallon Fuel Capacity, Low Oil Level Shut Down 
• assume at <50% load (less than 1,000 Watt load and 50% fuel flow
• 765 DC Charging Watts / 0.80 AC charger eff = 945 Watts AC load from charger
• 765 Watts charging / 14.5 volts charging = ~53 Amps charging current
• 1.3 Gallons per tank / 9 hours per tank = 0.144 GPH

Take a smaller Honda eu2200i running down to 25% rated power (there are lots of small inverter-generator brands out there that seem to have similar fuel usage numbers):
https://powerequipment.honda.com/generators/models/EU2200i#Specs

o Run Time per Tankful 3.2hr @ rated load 8.1 hrs @ 1/4 load
o 1,800 Watt * 0.25 (1/4 load) = 450 Watts
o 0.95 gallons per tank / 8.1 hours per tank (1/4 load) = 0.117 GPH (at 450 Watts)
o  0.117 GPH * 1/450 Watts * 945 Watt AC charger load = 0.245 GPH (or a bit less @ 945 Watts)

So--If you are running less than ~0.25 gallons per hour with your home made generator, and you can get the charging voltage you want--It may be a cost effective solution.

Older (and possible even current) B&S engines seem to have a reputation for being loud and not very fuel efficient.

Just to make up some numbers for comparison. Say 100 Hours per year (5 hours per day for 20 days of usage):

o 0.245 gph * 100 hours * $3.00 per gallon (I live in expensive California) = $73.5 per 100 hours (year?)

It does not seem like you would want to spend a lot of money on an expensive "fuel efficient" genset+AC battery charger... But that does leave you with the question of how you will get the "optimum" charging voltage/current for your battery bank...

The Honda numbers are pretty accurate (and probably conservative)... Just trying some different calculations and see what comes out.

-Bill "just playing with numbers" B.