What happens when you have multiple DC input voltages?

[markopolo-ClassB]

What happens when you have multiple similar DC input voltages all functioning at the same time?

Example:
Genset running 14.4v from converter and engine idling with say 13.9v from the alternator via a diode type isolator

My guess is that loads will see 14.4v at the batteries until the amperage draw is great enough to reduce that voltage. Then there's the next level of support at 13.9v ................. ?

Or is there a blending of the available amperage?

I haven't given it much thought and a Google search hasn't turned up anything yet. I would like to know though.

We can have multiple DC inputs, alternator, generator, solar all with different output voltages or a single input wired in such a way that there's a voltage on one set of wires compared to another set. In my van for example I can parallel coach and chassis batteries via two paths at the same time. One path would have almost no voltage drop and the other path, through the isolator, will have some voltage drop.

Any ideas as to how multiple DC input voltages work together simultaneously?

 

[gerrym51]

on roadtreks site in th e-trek section at one time it implied(or i inferred) that it said all could be used simultaneously amd their system would contol the 3. don't know if it's true.

 

[booster]

Voltage is just like water pressure, it flows from high to low. The highest voltage will set the system voltage as long as it can supply enough current to maintain that voltage. Once the voltage starts to drop from high current drain on the highest voltage, the other sources will start to pick up load as their voltage gets to be the same as the now reduced system voltage. I think all the sources would have blocking diodes in them, except for the relay type separators, so they don't really backfeed. If you have a source that can backfeed, that is lower than the system voltage, it magically turns to a load instead of a source.

 

[booster]

on roadtreks site in th e-trek section at one time it implied(or i inferred) that it said all could be used simultaneously amd their system would contol the 3. don't know if it's true.

This was what one of the original questions about eteks. They were adding all the charging sources together in saying how much charging they had, but if they aren't at all the same voltage and contributing you wouldn't get that. If they are all putting out max, and are pulled down off of setpoint voltage, then it would be possible. No one ever got answers about it and how the sources were getting controlled, proprietary, yeh know!

 

[markopolo-ClassB]

I get it.

You need to know what is in your system.

If your batteries are low and you have an old Magnatek at 13.8v and a diode type isolator with say a .4v drop netting 14v and you run the Onan and the van engine to charge quickly then when the voltage reaches 13.9v you should shut off the Onan keep idling the van engine a bit more and save some fuel.

If your batteries are low and you have an newer Tripp-Lite at 14.4v and a diode type isolator with say a .4v drop netting 14v and you run the Onan and the van engine to charge quickly then when the voltage reaches 14.1v you should shut off the van engine and keep the Onan running and save some fuel.

In my case with parallel cabling from the alternator (marine switch) and the isolator means that I need to size the wire on the marine switch side to carry the full load. Really both sides should be sized to carry the full load. With the old alternator there's some sharing of the load because the voltage drops but it is quite possible that with the new alternator, if it maintains high enough voltage, the entire load will be carried on the marine switch (preferred) side when running the microwave oven for example.

Swapping out the isolator (with voltage drop) with a relay type separator (minimal voltage drop) will allow load sharing on the two parallel feeds because the voltage will quickly be the same under load.

 

[booster]

Sounds like you have it figured very well. One other advantage of swapping out the diode isolator for a relay type is that, if your inverter is like ours, the current it needs to run the microwave drops very quickly with increased voltage so you could wind up at lower amps, and lower heat, to the inverter, than just the E=IR would predict.

 

[gerrym51]

Mark- my eyes glazed over 1/2 way thru your post.

 

[markopolo-ClassB]

It doesn't really apply to your new Zion. The second alternator aka "engine generator" will be the overriding source of both voltage and amperage in your new Zion.

It could be of interest to anyone adding new parts, retrofitting, or older rigs if trying to speed up bulk charging.

I have an engine battery and a house battery separated by about one foot inside my van. That's where I'll put the separator at least for now. Nice short cable runs. Looks like at least 4 gauge wire from the under the hood common power junction to that second engine battery. Largest load would be the 600w cooking/900w power consumption microwave oven. 900w/14 volts = 64 amps + say 10% losses = 70 amps or so. 4 gauge will be ok if I can keep the voltage up around 14 volts.

 

[booster]

Mark- my eyes glazed over 1/2 way thru your post.

And it actually gets worse when you consider you can have at least 3 different voltages coming out of you shore charger, 3 more out of your solar controller, and the alternator can vary continuously over about a 2.0 volt range. The big deal is too make sure you are using the power source that is the most appropriate for the task. In Marko's example, the alternator through the bypass for the microwave, but it it would probably be best to not use that source for battery charging because it is not as well controlled as the shore charger is. The power sources can also not play well together and mess up the nice automatic features they have.

I think this is a timely discussion, especially when compared to the transit discussion mlts22 has going.

I don't think it will be long before most of the stuff will be able to work well together. The Magnum inverter/charger that mlts22 is going to use does its charging control off a shunt to batteries, measuring actual current to them, which is perfect and a step above the current measuring we have with our Blue Sea charger which uses the charger current and gets messed up by coach loads. The Blue Sky solar controller he is going to use also uses a shunt in the line to the batteries so the same applies. That way they won't interfere with each other at all, and coach loads are of no consequence to the charging. Both of these systems will give him the very best charging and battery life possible, I think.

The missing link in the current technology is the charging control from the engine alternator or engine generator. Sterling makes some pretty neat battery to battery chargers, up to 180 amp capacity, but I am unsure how well they will do on the quality of the charge profile and control. The appear to not run off a shunt to the coach batteries, and claim an internal calculation to determine charge times. This would probably mean that they would also be fooled by coach loads in use, and not be nearly as good at never over or under charging batteries. Same would be true for the "smart regulators" added to the alternators. Both would certainly be better than nothing, though..

I hope that someone like Magnum addresses the engine charging issue and builds it into their systems. If they did solar also, all 3 systems could be in one control and run off the same shunt. At that point, you would have true "turn it on and let it run with no input" capability, with near perfect charging at all times. If that doesn't happen, and Sterling or someone else, comes up with a shunt controlled 12v to 12v charger, you would also be able to have the same quality with 3 separate systems.

Of course, if lithium systems get the prices down, and cold weather questions ironed out, all this may be mute, as they have their own management systems for ending and starting charge, so all the sources of power could just change to single voltage controlled with no other controls needed.