Hybrid Car – More Fun with Less Gas

safe use of car alternator?

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Posted by Robert Morein on August 4, 2004, 5:43 am
Because my Chinese generator is late in arriving, I want the capability of
recharging my battery based home backup system from a car alternator.

The battery bank will not be connected directly to the car alternator. The
setup is thus: a 1.5KW/4KW surge AIM inverter is connected to the car DC
terminals. The AC output of the AIM connects via 100' power cord to a 24V
25A switching power supply. The power supply connects to an Outback MX-60
charge controller, which charges a 660 ah 12V bank, at up to 50a, limited by
the rating of the switching power supply and the (as yet) unknown safe
operating parameters of the car alternator.

The MX-60 allows me to set charging current limits.  Thus, I can tightly
control the amps drawn by the AIM inverter from the car DC system.

How many amps can be drawn, at engine idle, from a typical car alternator --
in this case, a 70 amp Bosch -- without damaging the unit?

The car motor is a 5 cyl diesel, and thus relatively immune to overload.

Posted by dave on August 4, 2004, 2:42 pm
At idle ? A 70 amp alternator ?  With the stock pulley ? With the
stock regulator?

You can do this but WHY? You probably will be lucky to get 10 amps.
After 30 minutes the alternator will probably give you less.

To do this right you need to size the pulley diameter smaller, use a
hot rated alternator of 150Amps or so, use a 3 step regulator. A
double belt pulley will probably be needed.

Consider buying a GENERATOR

Posted by Robert Morein on August 4, 2004, 8:42 pm

Please re-read my post. I bought a generator.

Posted by DJ on August 5, 2004, 3:59 pm

 Considering the potential damage you may cause to that car, loss of a
nice alternator, loss of an automotive battery, and add to that the
expense of buying a REALLY HEAVY DUTY extension cord, and putting gas
in the car and risk of overheating it by running it without moving it,
yeah, I'd suggest going to buy a cheap little portable generator, too.

Fact is, automotive alternators are designed to charge a single
automotive battery over a long period, at a "float" charge. I'd be
surprised to see it put out more than 10amps consistently as it sits
in the car.


Posted by Robert Morein on August 5, 2004, 4:21 pm

I have a RAMCO diesel generator coming. It's delayed by the importer due to
missing parts. The question relates to emergency use.

The engine cannot overheat at idle. It's a 5 cyl Mercedes diesel. These
engines run so cool there's an accessory water pump to make sure the
passenger compartment gets enough heat. Diesels idle with 1/2 the fuel
consumption of an Otto engine, and are not stressed the way Otto engines are
at idle.

Your statement that the alternator is designed to merely float the battery
does not conform with the realities of a modern automobile. The older
Mercedes are famous for conservative design. In this car, the Bosch
alternator must be able to supply, at idle:

headlights: 110 watts
front radiator fan: 100 watts
interior AC ventilation fan: 100 watts
Total: 310 watts  = 26 amps.

To give an idea of the quality of the electrical components, every
electrical connector in this car uses silver plated, soldered, screw-machine

My question is related to emergency use only.

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