Safe to Power a DC to DC charger *Directly* via Alternator?

[OTRwSolar]

Can a dc to dc charger be safely powered by an alternator *directly*?

Describing a scenario where the only connection on the alternator's pos (+) output terminal is the dc to dc charger. There's no other load, device, or battery connected to the alternator output except the dc to dc charger. Thanks.

 

[Supervstech]

I wouldnt think so.
Alternator needs a stable energy source.

 

[OTRwSolar]

My assumption is it would be safe. In a typical dc to dc charger vehicle install, the battery is paralleled with the alternator output. Not sure if the battery provides any protection prior to the alternator reaching full voltage upon startup. Maybe a 5-10 sec delay before actively connecting the dc to dc charger would be appropriate, I don't know. In any case, once the alternator reaches operating voltage/RPM, I would assume there'd be no concern.

 

[mikefitz]

No it won't work. The alternator needs a power source to start up.

 

[OTRwSolar]

Agree an alternator needs a energy/power source to operate. When I mentioned no other load or device was connected, only the dc to dc charger, I was only addressing the connections on the alternator's positive output terminal.

Have a 2nd alternator on a vehicle that is already operational; however, the output is not connected to the vehicle's electrical system. It operates independently of the vehicle's primary alternator.

If the 2nd alternator's output is connected directly to the input of the dc to dc charger are there any operational issues to consider?

I addressed some of my concerns in my previous post.

 

[John Frum]

The starter battery will tend to somewhat smooth out alternator ripple.
In the absence of this buffering effect, the dc2dc charger will have to contend with more ripple.

Alternator Ripple Voltage Testing

Before computers and networks in vehicles, an alternator that was producing AC voltage or ripple through the electrical system would cause flickering lights and maybe an ignition problem. Today, excessive ripple voltage can cause communication issues with networks and communication DTCs in...

www.underhoodservice.com

 

[Sabre36]

Read about this: https://sterling-power.com/products/alternator-linearisation-device-17-5v-70f-ald-17500

 

[boondox]

Yep, the field coil has to be energized for the alt to put out and the battery does indeed help smooth the output. It could be a very small battery.

 

[OTRwSolar]

The starter battery will tend to somewhat smooth out alternator ripple.
In the absence of this buffering effect the dc2dc charger will have to contend with more ripple.

Alternator Ripple Voltage Testing

Before computers and networks in vehicles, an alternator that was producing AC voltage or ripple through the electrical system would cause flickering lights and maybe an ignition problem. Today, excessive ripple voltage can cause communication issues with networks and communication DTCs in...

www.underhoodservice.com

Thanks for the link. Help put things into perspective.

I get the impression that as long as the AC ripple is less than 50mv, it shouldn't pose a significant problem. I guess any given dc to dc charger's tolerance for ripple will ultimately how long/well it works connected directly to the alternator.

The dc to dc chargers I'm using are relatively inexpensive. Once I verify my alternator's ripple is minimal, I may just hook 'em up and give it a try.

Thanks again.

 

[OTRwSolar]

Read about this: https://sterling-power.com/products/alternator-linearisation-device-17-5v-70f-ald-17500

Thanks for the link. Looks like the perfect protective solution for what I'm trying to do, but it sure is pricey. Just might have to use a battery.

 

[OTRwSolar]

Yep, the field coil has to be energized for the alt to put out . . . .

Had a rep from Nations alternator mention this. I'm a total alternator newb. Is the field coil energized via one of the smaller, secondary alternator terminals, or via the main, positive output terminal?

 

[boondox]

Had a rep from Nations alternator mention this. I'm a total alternator newb. Is the field coil energized via one of the smaller, secondary alternator terminals, or via the main, positive output terminal?

Well, it depends on the alternator. I think if the alternator has residual magnetism it will start producing without a battery. In which case a cap would be fine to smooth out the power. If you have a brand new alt, spin it and see if it puts out.

 

[FilterGuy]

My understanding is that if the alternator is run without being hooked to the battery, the voltage will go way high and blow out the rectifier circuit.

This is why marine battery disconnects sometimes come with an extra circuit switch that will disconnect the field coil if the battery is disconnected.


This article does a good job of describing it: https://www.bluesea.com/resources/91

The switches that don't have this often come with a label that says to shut down the engine first.