Battery Charging With Two Alternators

[PaulRainbow]

I don’t really see the difference between having two alternators connected in parallel to one battery, versus having two alternators connected to two separate batteries connected together via a Cyrix relay.

If they are both connected to the same battery each alternator will sense the output of the other alternator, this can cause an issue, depending on the alternators.

If each alternator is connected to a different battery it will only sense the terminal voltage of the battery (again, depends on the alternator, to a degree). When the VSR closes it will parallel the two banks, but they will still have different terminal voltages, depending on the state of charge of each bank. When they both reach the same terminal voltage, or thereabouts, one alternator might reduce its output, as above, but as the batteries will all be mostly charge it isn't likely to matter much, as the batteries aren't likely to be able to accept the full output of one alternator.

To a degree this will depend on the alternators, the batteries and their state of charge.

 

[jakew009]

If they are both connected to the same battery each alternator will sense the output of the other alternator, this can cause an issue, depending on the alternators.

If each alternator is connected to a different battery it will only sense the terminal voltage of the battery (again, depends on the alternator, to a degree). When the VSR closes it will parallel the two banks, but they will still have different terminal voltages, depending on the state of charge of each bank. When they both reach the same terminal voltage, or thereabouts, one alternator might reduce its output, as above, but as the batteries will all be mostly charge it isn't likely to matter much, as the batteries aren't likely to be able to accept the full output of one alternator.

To a degree this will depend on the alternators, the batteries and their state of charge.

Are you sure

 

[jakew009]

Or with a load causing one battery to have a much lower terminal voltage.

I'll wait for one of the batteries to discharge and repeat the test if you want me to...

 

[PaulRainbow]

Or with a load causing one battery to have a much lower terminal voltage.

I'll wait for one of the batteries to discharge and repeat the test if you want me to...

Not sure of your point ?

 

[B27]

I don’t really see the difference between having two alternators connected in parallel to one battery, versus having two alternators connected to two separate batteries connected together via a Cyrix relay.

There may well be no difference at all.
If the alternators interact, that may be influenced by the resistance between each alternator and the battery, and the resistance between the two alternators.. It's not a situation where the behaviour of the alternator is necessarily specified. Different alternators have different regulators. I don't see much merit in a 'suck it and see' approach.

 

[B27]

Using one engine unless in a confined space, which is something I've heard of but had forgotten, suggests that I should stick with one alternator, one battery, but fit a VSR linking the banks. I know Victron do one that works in both directions, I don't know if they all work that way.

I sailed on a cat for a few trips, pottering slowly with no wind, it was economical to use one engine. Also seemed to more than halve the noise.
But that boat had feathering props.
What Neeves said, is partly just going slower uses less fuel?

 

[jakew009]

Not sure of your point ?

You said "When the VSR closes it will parallel the two banks, but they will still have different terminal voltages,"

That is electrical nonsense. They will instantly come into balance (bar the resistance of the cable between them) as shown by the video.

My point is there is literally no difference (from the alternator's perspective) between connecting both alternators to one battery, vs connecting each alternator to it's own battery and then paralleling them with a Cyrix as you suggested.

 

[PaulRainbow]

You said "When the VSR closes it will parallel the two banks, but they will still have different terminal voltages,"

Don't distort what i said. I actually said "When the VSR closes it will parallel the two banks, but they will still have different terminal voltages, depending on the state of charge of each bank"

That is electrical nonsense. They will instantly come into balance (bar the resistance of the cable between them) as shown by the video.

So a large bank of heavily discharged domestic batteries will instantly have the same terminal voltage as a tiny, fully charged engine battery ?

My point is there is literally no difference (from the alternator's perspective) between connecting both alternators to one battery, vs connecting each alternator to it's own battery and then paralleling them with a Cyrix as you suggested.

Your point is nonsense.

 

[Refueler]

Not sure of your point ?

I'm struggling with that post as well ...

I think we can agree that just paralleling two alternators without any form of control is not advised. The matter is to find solution to have both alternators outputting - but able to charge either bank depending on one or two engines in use.

 

[jakew009]

Don't distort what i said. I actually said "When the VSR closes it will parallel the two banks, but they will still have different terminal voltages, depending on the state of charge of each bank"

The state of charge makes no difference, they will still have the same voltage.

So a large bank of heavily discharged domestic batteries will instantly have the same terminal voltage as a tiny, fully charged engine battery ?

of course

 

[B27]

If they are both connected to the same battery each alternator will sense the output of the other alternator, this can cause an issue, depending on the alternators.

If each alternator is connected to a different battery it will only sense the terminal voltage of the battery (again, depends on the alternator, to a degree). When the VSR closes it will parallel the two banks, but they will still have different terminal voltages, depending on the state of charge of each bank. When they both reach the same terminal voltage, or thereabouts, one alternator might reduce its output, as above, but as the batteries will all be mostly charge it isn't likely to matter much, as the batteries aren't likely to be able to accept the full output of one alternator.

To a degree this will depend on the alternators, the batteries and their state of charge.

The OP is speaking of 3 x 100Ah house batteries
The alternators could be as small as 35A, so not much more than a C/10 charge rate.
If the batteries are down to 80% or so, they should take more than that, if the alternator can show them 14V or more, allowing for a bit of wiring drop.
If the OP has any good reason to fit 3 house batteries, i.e. he expects to discharge them down to 60% now and then, and he wants the alternator to be a significant way of charging them, then there appear to be gains from either using both alternators (if that works) or upgrading the 'house' alternator.
Personally I'd be looking at fitting a bigger alternator, 120A is cheap and commonplace these days.
But then, I like to motor for as little as possible and then get the sails up. If you like spending 10 hour days motoring, then clearly a C/10 chare rate is not a problem. If you've got an acre of solar it's not a problem either, or if you are happy for the solar to catch up over the next week.

 

[Refueler]

The state of charge makes no difference, they will still have the same voltage.



of course

At the moment the VSR closes - the batterys will still have their different voltages.

The charge will now be passing via VSR to the two banks and the charge rate going in will be based on the charge level of the respective battery / bank. The voltage now will be usually shown as the charge voltage output reqd for each battery.

Only if the VSR closed and there was no charge source - would the batterys then equalise out as there is no opposing charge ....

This also can be observed on the old 1-2-off switch .. where a meter on each battery can be used to clearly show that when alternator is feeding via the both function - batterys still show different. If you check the amps - you will find the lower voltage battery taking more amps etc.

Its physics me ol' fruit !!

 

[jakew009]

Its physics me ol' fruit !!

George Ohm would be turning in his grave.

 

[PaulRainbow]

The state of charge makes no difference, they will still have the same voltage.

This clearly demonstrates that you don't know what you are talking about.

 

[B27]

George Ohm would be turning in his grave.

He would if he saw you tacking an 'e' onto his first name.

 

[B27]

This clearly demonstrates that you don't know what you are talking about.

Two batteries will have 'the same voltage' if they are paralleled by zero ohm conductors.
Clearly some current will flow, so with real conductors, there will be a potential difference.

What's 'the same voltage' anyway? =/- 10mV? a few microvolts? DC or including AC, RF and noise?

 

[jakew009]

This clearly demonstrates that you don't know what you are talking about.

Rather than just arguing with me on here why don’t you go and try it. You might be surprised what happens.

 

[jakew009]

Two batteries will have 'the same voltage' if they are paralleled by zero ohm conductors.
Clearly some current will flow, so with real conductors, there will be a potential difference.

What's 'the same voltage' anyway? =/- 10mV? a few microvolts? DC or including AC, RF and noise?

Agree. The only difference in voltage between the two battery terminals after they are connected in parallel will be caused by the resistance of the parallel wire itself.

The current that flows between the two batteries will of course also be limited by the internal resistance of the batteries themselves (which will be much greater on a lead acid).

Lithium batteries have a much lower internal resistance, which is why paralleling them requires much more thought.

 

[PaulRainbow]

Rather than just arguing with me on here why don’t you go and try it. You might be surprised what happens.

Or not

 

[PaulRainbow]

Rather than just arguing with me on here why don’t you go and try it. You might be surprised what happens.

You said "The state of charge makes no difference, they will still have the same voltage."

Most people here will know that's nonsense.