Battery Charging With Two Alternators

[Boathook]

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



of course

I've had 2 batteries connected together and the voltage has been the same with both. When disconnected one dropped like mad as it was knackered and the other rose up.
Both batteries the same age and always connected together.

I wondered why the battery capacity seemed limited.

All sorted by 2 new batteries ........

 

[jakew009]

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

Most people here will know that's nonsense.

If you have one AGM battery at 90%
charge at say 12.7V

And another AGM battery at 70% charge (12.3V)

And you connect them together with a cable, the voltage measured at both battery terminals will instantly change to approx 12.5V volts.

If you immediately remove the cable connecting them together, they will return to their original voltages again.

I’m not sure how else I can explain it to you, I even provided a video demonstrating it.

 

[PaulRainbow]

If you have one AGM battery at 90%
charge at say 12.7V

And another AGM battery at 70% charge (12.3V)

And you connect them together with a cable, the voltage measured at both battery terminals will instantly change to approx 12.5V volts.

If you immediately remove the cable connecting them together, they will return to their original voltages again.

I’m not sure how else I can explain it to you, I even provided a video demonstrating it.

What are those batteries in your videos ?

 

[jakew009]

What are those batteries in your videos ?

Victron AGM. Just what I had to hand

 

[Boathook]

If you have one AGM battery at 90%
charge at say 12.7V

And another AGM battery at 70% charge (12.3V)

And you connect them together with a cable, the voltage measured at both battery terminals will instantly change to approx 12.5V volts.

If you immediately remove the cable connecting them together, they will return to their original voltages again.

I’m not sure how else I can explain it to you, I even provided a video demonstrating it.

Still have a problem with a knackered battery; and both connected together means both are knackered. Thats what I have found out over the years.

 

[Neeves]

Running on one engine can save some fuel, but Neeves calculations are off. Running on one engine does not cut the fuel bill in half. If you shut one engine down you either lose speed, or have to increase RPM of the running engine.

Correct - but if you run 2 engines flat chat you do not double speed, compared to one engine flat chat - but you double fuel consumption. If you turn off one engine you lose some speed but half fuel consumption. I don't recall drop in speed.

The speed will vary depending on props, vessel and how clean the bottom.

The data will be different for different vessels and is something worth doing by an owner. Stemar's engines for his cat are the same size as our engines, both 20hp - theoretically and crudely he should be able to motor faster than we did - in practice that may be incorrect.

Jonathan

 

[Refueler]

George Ohm would be turning in his grave.

Yes - particularly on your post !

 

[Refueler]

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.

Your original reply is in error ... because at the moment of paralleling - the batterys will still have their different voltge levels. As another says - the equalisation will commence once connected - but would need zero Ohm connection to be equal once equalisation has completed - which in actual fact is infinite time .... YES infinite ...

Its infinite because as the voltages approach each other - the net inbalance reduces and internal resistance of the batterys causes extension of time to balance. Only if you allow a practical range to be taken as equal - can it be finite.

Second as already explained - if a charge source is applied during the parallel connection - the charge source if sufficient will suppress any balancing of the two batterys and each will charge up .. as they charge up - their resistance to charge will cause them to approach each other in voltage etc.

Both scenarios are easily checked and confirmed if you have meters connected to each battery .......

 

[Refueler]

I've had 2 batteries connected together and the voltage has been the same with both. When disconnected one dropped like mad as it was knackered and the other rose up.
Both batteries the same age and always connected together.

I wondered why the battery capacity seemed limited.

All sorted by 2 new batteries ........

Correct ... but static batterys paralleled is not what the other guy is maintaining ...

I too have similar situations ...

 

[Refueler]

If you have one AGM battery at 90%
charge at say 12.7V

And another AGM battery at 70% charge (12.3V)

And you connect them together with a cable, the voltage measured at both battery terminals will instantly change to approx 12.5V volts.

If you immediately remove the cable connecting them together, they will return to their original voltages again.

I’m not sure how else I can explain it to you, I even provided a video demonstrating it.

Now you are changing your position from each battery voltage - to the net average ...

 

[Refueler]

Correct - but if you run 2 engines flat chat you do not double speed, compared to one engine flat chat - but you double fuel consumption. If you turn off one engine you lose some speed but half fuel consumption. I don't recall drop in speed.

The speed will vary depending on props, vessel and how clean the bottom.

The data will be different for different vessels and is something worth doing by an owner. Stemar's engines for his cat are the same size as our engines, both 20hp - theoretically and crudely he should be able to motor faster than we did - in practice that may be incorrect.

Jonathan

Trouble is how do you rate consumption ?

Fuel per hour regardless of speed / distance covered ?
or
Fuel per distance covered ?

This makes a big difference .... as the two engine scenario is far more efficient at cruise throttles than single engine ..... when considering Fuel per distance covered.

 

[jakew009]

Your original reply is in error ... because at the moment of paralleling - the batterys will still have their different voltge levels. As another says - the equalisation will commence once connected - but would need zero Ohm connection to be equal once equalisation has completed - which in actual fact is infinite time .... YES infinite ...

Its infinite because as the voltages approach each other - the net inbalance reduces and internal resistance of the batterys causes extension of time to balance. Only if you allow a practical range to be taken as equal - can it be finite.

Watch my video. That is not what will happen. Before you connect the batteries the batteries will have different voltages. After you connect them the voltages will be identical. After you disconnect them again the batteries will return to their original voltages (plus / minus whatever charge managed to transfer in the time the batteries were connected).

Second as already explained - if a charge source is applied during the parallel connection - the charge source if sufficient will suppress any balancing of the two batterys and each will charge up .. as they charge up - their resistance to charge will cause them to approach each other in voltage etc.

This is even more crazy. You cannot have two different voltages at the ends of the same bit of wire if no current is flowing down it!

V = I x R. If I is 0, V is also going to be 0. Therefore the voltage drop would be zero over the cable, and the voltage would be identical at either end.

In reality the current from the alternator will also mostly flow over the parallel link and into the less charge battery until both batteries are in balance.

So both alternators will be effectively connected to the same battery, which was the entire point of my original post.

Both scenarios are easily checked and confirmed if you have meters connected to each battery .......

I have already posted a video showing exactly what happens. You seem to keep posting crazy theories about charge sources suppressing battery balancing over a parallel cable.

 

[Refueler]

Watch my video. That is not what will happen. Before you connect the batteries the batteries will have different voltages. After you connect them the voltages will be identical. After you disconnect them again the batteries will return to their original voltages (plus / minus whatever charge managed to transfer in the time the batteries were connected).



This is even more crazy. You cannot have two different voltages at the ends of the same bit of wire if no current is flowing down it!

V = I x R. If I is 0, V is also going to be 0. Therefore the voltage drop would be zero over the cable, and the voltage would be identical at either end.

In reality the current from the alternator will also mostly flow over the parallel link and into the less charge battery until both batteries are in balance.

So both alternators will be effectively connected to the same battery, which was the entire point of my original post.




I have already posted a video showing exactly what happens. You seem to keep posting crazy theories about charge sources suppressing battery balancing over a parallel cable.

You still haven't opened your view to the fact that you are on a different road and fail to understand why ..

My post and explanation bears directly on the subject of the thread - while yours veers off ...

You still ignore that fact that a charge load is being applied when the batterys parallel ... this in effect then creates a 'valve situation' where one battery lower state than the other will receive higher % of charge ... the common cables will show averaged figures while the batterys themselves will only become equal to each other after time ... the charge rate will also determine how much of the higher level battery flows to the lower - based on internal resistance and cable capabilities ....... BUT if there was no charge on the system - then the flow of equalisation between the batterys will flow based on the internal resistance of the batterys ...

I can show a video that says the world is flat ... doesn't mean it is !!

 

[jakew009]

You still haven't opened your view to the fact that you are on a different road and fail to understand why ..

My post and explanation bears directly on the subject of the thread - while yours veers off ...

You still ignore that fact that a charge load is being applied when the batterys parallel ... this in effect then creates a 'valve situation' where one battery lower state than the other will receive higher % of charge ... the common cables will show averaged figures while the batterys themselves will only become equal to each other after time ... the charge rate will also determine how much of the higher level battery flows to the lower - based on internal resistance and cable capabilities ....... BUT if there was no charge on the system - then the flow of equalisation between the batterys will flow based on the internal resistance of the batterys ...

I can show a video that says the world is flat ... doesn't mean it is !!

The original question was along the lines of “can I connect two alternators to one battery”

Someone posted something about the two alternators would interfere with each other as they would sense the voltage of the other.

Someone else then posted “you could connect each alternator to it’s own battery and then connect the batteries with a voltage sensitive relay”

I pointed out the ridiculousness of the above statement in my first post (20) as illustrated by the below diagram. Assuming the voltage is greater than 13V (which it will be if the alternator is running) you have wired them in exactly the same way.




We have then continued to have a slightly surreal conversation where you have continued to argue that the voltage measured on the battery terminals will be different even though they are connected in parallel with a bit of wire as shown below. But I think you have now slightly changed your position in your most recent post (if we ignore the bit about the 'valve' situation caused by the alternator which is nonsense).



For what it's worth, I don't actually know exactly what would happen if you connect two alternators in parallel. My gut feeling is that if they were similar models with similar regulators, they would actually share the load just fine and both contribute to charging the battery,

If they had different voltage set points, you would find that one would switch off before the other, but whilst the voltage was below the threshold, they would both contribute to charging (assuming the battery could accept all the charge current).

 

[PaulRainbow]

Correct - but if you run 2 engines flat chat you do not double speed, compared to one engine flat chat - but you double fuel consumption. If you turn off one engine you lose some speed but half fuel consumption. I don't recall drop in speed.

Was there a drop in speed or not ? Of course, there has to be.

When you turn one engine off you lose some speed, but the engine that is still running will increase it's power, as a result of the governors opening up. The extra fuel that the single engine is now using means that you are not using half the fuel per hour and the drop of speed means that you are not using half the fuel per distance travelled.

The speed will vary depending on props, vessel and how clean the bottom.

The data will be different for different vessels and is something worth doing by an owner. Stemar's engines for his cat are the same size as our engines, both 20hp - theoretically and crudely he should be able to motor faster than we did - in practice that may be incorrect.

Jonathan

 

[Refueler]

The original question was along the lines of “can I connect two alternators to one battery”

Someone posted something about the two alternators would interfere with each other as they would sense the voltage of the other.

Someone else then posted “you could connect each alternator to it’s own battery and then connect the batteries with a voltage sensitive relay”

I pointed out the ridiculousness of the above statement in my first post (20) as illustrated by the below diagram. Assuming the voltage is greater than 13V (which it will be if the alternator is running) you have wired them in exactly the same way.

View attachment 176990


We have then continued to have a slightly surreal conversation where you have continued to argue that the voltage measured on the battery terminals will be different even though they are connected in parallel with a bit of wire as shown below. But I think you have now slightly changed your position in your most recent post (if we ignore the bit about the 'valve' situation caused by the alternator which is nonsense).

View attachment 176991

For what it's worth, I don't actually know exactly what would happen if you connect two alternators in parallel. My gut feeling is that if they were similar models with similar regulators, they would actually share the load just fine and both contribute to charging the battery,

If they had different voltage set points, you would find that one would switch off before the other, but whilst the voltage was below the threshold, they would both contribute to charging (assuming the battery could accept all the charge current).

Rubbish - my position has not changed in any way at all ... YOU on the other hand have ...

Bye Bye ... glad I am not having you near my electrics. You completely ignore and twist to suit ...

 

[B27]

The point is, an alternator is not a 'dumb' DC voltage source like a battery, it is a machine which is attempting to control its output to achieve a set voltage. That's where it gets complicated.
The regulator in the alternator controls the field current in response to the voltage it sees.
There's a delay implicit in that.
So if the voltage is changing due to external causes (e.g. another alternator), the regulator may do the wrong thing.

 

[rogerthebodger]

The point is, an alternator is not a 'dumb' DC voltage source like a battery, it is a machine which is attempting to control its output to achieve a set voltage. That's where it gets complicated.
The regulator in the alternator controls the field current in response to the voltage it sees.
There's a delay implicit in that.
So if the voltage is changing due to external causes (e.g. another alternator), the regulator may do the wrong thing.

That is the point the regulator on each alternator will see the voltage of the single battery and adjust the field current on each alternator to control the changing voltage of each alternator.

The field current on one alternator could be shut off but will not affect the second alternator

 

[Neeves]

Was there a drop in speed or not ? Of course, there has to be.

When you turn one engine off you lose some speed, but the engine that is still running will increase it's power, as a result of the governors opening up. The extra fuel that the single engine is now using means that you are not using half the fuel per hour and the drop of speed means that you are not using half the fuel per distance travelled.

I'm sure you might be right Paul but if you are not in a great hurry it uses less fuel, for distance travelled, to run one engine at cruising revs than 2 engines (at cruising revs). If you have no timetable and are a parsimonious Scot it is better to arrive late but with a less depleted wallet (especially if fuel is not easily accessible). But in answer to the unasked question, one liter of fuel on one engine takes you, significantly, further than 1 liter of fuel for 2 engines. Once you get upto, or near, hull speed - it does not matter how much fuel you use - you will not travel faster - in a displacement hull. Now if you are motoring around slowly with no specific destination, a day at leisure, motoring on 2 engines , slowly, may be completely different to 2 engines (in such circumstances we would not have the engines on at all -but simply sail).

Now this was our experience, which may be different to any results Stemar finds - and he, having been prompted by this thread, might think to validate or not. I am very comfortable in accepting that an experience with two drives close together (without a large drive separation) and in the absence, or not, of a planing hull - might be different. It may also be that designs of props may have a big impact, we had 3 bladed folding Volvos.

I can only report our experience - over 25 years - on one yacht.

Our experience was that one engine was, significantly, more cost effective, for distance travelled than 2. We never worried about arrival time (we are not scared of the dark) but if you have a 1000nm passage, a finite fuel tank, no opportunity to replenish and a return journey to make then......you are not profligate with unnecessary diesel usage and need to know, with a degree of confidence what is your resource. Rather than motor we would simply find the nearest anchorage and wait till the forecast wind arrived, it, inarguably , saves fuel.

Stemar has a totally different yacht to ours - he can easily check my rantings - hopefully my rantings might prompt him to check the characteristics of his yacht and engines. He can then make his own decisions. I don't know Stemar's yacht but think it is of a significantly different design to our 38' x 22'6" x 7t cat - but we both, interestingly, had/have 2x20hp engines. I wonder what his conclusions might be - in the fullness of time. I suspect he can get to hull speed more easily than us - and he may find 2 engines are, even, less efficient than we found.


How to check fuel usage - we had a tall, rectangular, fuel tank, maybe 1m tall (located directly and just forward, underneath the mast), it had a simple sight tube down the side, from top to bottom, with calibrated marks every 20l (it was a 200l tank). Crude -but over 25 years -sufficiently statistically robust. For long periods off grid we would supplement with 20l 'jerry cans' - are we allowed to call them 'jerry cans' - ? If we were motoring we would run at 3,000 revs so measure in terms of hours, or parts of (but usually multiples of hours) at 3,000 revs. If it was a simple day out - we would not measure - not relevant.

Jonathan

 

[rogerthebodger]

The power that is required to drive a boat through the water is a function of the hull shape and a function of speed through the water.

The power developed by an engine to obtain the speed is mainly a function of the amount of fuel consumed and this can/ would generally be the same if supplied by a single engine or multi engines it the power that is consumer.

There is an efficiency factor which will affect the total fuel consumption.

You have the basic efficiency of the engine / prop / drive chain.

If you use a single engine/prop/drive chain, there could be a difference in efficiency when using twin engines