Charging batteries with the engine

Hard to imagine that an engine rated to run a multi-ton vehicle down the road, using a fraction of its power to run an alternator while wasting the rest just turning itself over, could be more economical than a generator on which the engine is sized to the power needs of the electrical generator attached to it.

On my class C, the alternator seems to charge the house battery a little bit faster than the converter. It has nice heavy (2 gauge or so I think) wire between the house and chassis battery. It really boils down to which charge source can maintain a higher voltage at the house battery, which in turn depends on the converter (multistage vs. single stage), the voltage regulator for the chassis, the alternator's capacity, the voltage drop of the wiring, etc.

I the generator, none the less, rather than starting the main engine. It's not really great for an engine to idle for long periods, and it's sometimes useful to have AC power for other things besides just charging the house battery.

Of course, if you have need of driving somewhere anyhow, it's a no-brainer: don't bother with the generator.

Redram99 wrote:
Hi all.

I have a 2011 triple e senator 25d. Great motor home. E450 v10.

Just curious, what is the output of the alternator vs the standard converter?

It seems to charge the house batteries off the engine alternator very quickly.
Would that be nearly as economical as running a 4000 Onan?

Thanks.

You must be thinking the quick rise in voltage means they are charged up. In fact that is just the beginning of getting them charged up, which can take hours "depending".

You need a battery monitor to tell what is going on while they are being charged. After you stop the charging, and they have no loads on them either for a while, then you can take their voltage and get an idea of their state of charge ("resting voltage")

The alternator amps to the battery will be reduced by the usual thin wire back to the battery and also from the way the voltage regulator for the alternator works wrt the engine battery. MHs are not all the same, but eg, ours gets about 25 amps to the house battery bank once thing settle from the initial start (takes about five minutes to settle) while the converter can do twice that. The converter only works off shore power or generator, so alternator charging while driving is a good bonus feature.

The rated output of the alternator is just one factor, the speed at which the alternator turns makes a huge difference also.

Under SAE J56 standards the ‘rated’ output of the alternator is at 6,000 RPM.

Most automotive alternators are designed to give a pretty solid output at about 2,000 RPM but don’t give much current at idle.

Depends, if it has a high output alternator.

If you have just a standard alternator most likely running the charger off the generator will be faster.

If you have a big 300amp continuous output alternator (most automotive alternators put out the rated amount for a short period to replace the motor starting draw then drop way back), it can probably do it faster.

Of course if you are driving anyway, take the free charging.