# Re: [Flightgear-devel] batteries, alternators, volts, amps - electrical system

```Just some general info that may or may not be useful.

General aviation batteries : Typically lead acid
```
Alternators are far more efficient at *low* RPM than generators but are still
terrible energy converters (unless you really need heat).
The average general aviation alternator takes a little over two watts of
energy (from the engine) to produce one watt of electrical power.

Automotive alternators are usually of the Lundell type but there are newer
types being used now. I should imagine light aircraft also use the Lundell
type.

The power output versus speed (RPM) for an alternator is almost perfectly
linear so that should make things a little easier to model.
What's not so easy to figure out is how much power is delivered at engine idle
as this varies widely from one alternator to the next. I've seen anything
from 25% to 80% on automobiles.

On the battery side of things the discharge and charge rates are not going to
be easy to model. Maybe a simple approximation using a lookup table will do
as an interim measure.

Paul

On Wednesday, 1 June 2005 00:06, Curtis L. Olson wrote:
> I need to do some work beefing up our electrical system model a bit.
> I'd like to add a simple model for a battery where output varies with
> time and a simple alternator model where output varies with rpm.  I'm a
> complete moron when it comes to electrical stuff so I'm not even sure
> I'm asking a sensable question.  Does anyone know of a good online
> reference(s) or even just send me some reasonable info.  In the end I'd
> like to be able to be able to output bus voltage (downstream of the
> battery + alternator) and also model an ammeter.
>
> I already have a way to "back propogate" the total current draw on the
> system, where the individual electrical system outputs can be marked
> with some current draw when they are on, however, I need to work on the
> input side of the equation.
>
> Again, I want to start pretty simple and not get drug down with too
> fancy of an implimentation.
>
> Thanks,
>
> Curt.

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