More alternator paralleling

[Timleech]

There have been various discussions recently about what happens when you put two alternators in parallel, do they 'fight' each other and so on (they don't).

 

I've put a short video up on utoob to illustrate what does happen.

This 24 Volt system has two alternators, one connected to the start batteries and the other to the domestics. The two batteries are connected together by a Voltage Sensitive relay, this is 'on' at the time of the video so the two alternators can be regarded as being in parallel feeding one battery bank. Each alternator has its own ammeter, which reads alternator current and nothing else.

 

The left-hand ammeter is for the start battery alternator.

 

 

The battery monitor is reading the domestics voltage, though of course all should be the same.

Note that the domestic alternator gives a useful current at a lower engine rpm (actually about 20A at tickover) than the other, but the start alternator starts to work as soon as the revs rise slightly.

The domestic alternator is regulated to 28 Volts, so as soon as the volts rise above this it cuts off and the start alternator does all the work. The start alternator is regulated to 29 Volts.

The regulator on the domestic alternator is adjustable, so I'll be tweaking it to be nearer to 29 Volts in due course and then they will share the work better.

 

Edit - you really need to have the sound on, to get an indication of engine revs.

 

The main purpose in having two substantial alternators is to provide power when under way, for microwave and electric kettle etc. (no room for gas).

Edited February 13, 2013 by Timleech

[Timleech]

A fresh observation on the above -

 

The 'starter' alternator (CAV AC7 with 440 regulator) provides the signal for an electronic tachometer.

 

I was tweaking the two regulator voltages yesterday, and found that if the 'domestic' regulator is set even fractionally higher than the start regulator, the AC7 shuts down so completely that the tacho stops working. Also the warning light flashes on at odd moments.

I've now arrived at settings, with domestic reg set slightly below start reg, which mean that when on a heavy load both alternators are properly doing their bit, and the tacho is working properly.

 

Tim

[Sir Nibble]

Serious alternator, nice to see it given some work to do.

[MikeV]

I've now arrived at settings, with domestic reg set slightly below start reg, which mean that when on a heavy load both alternators are properly doing their bit, and the tacho is working properly.

Mine are the other way round. The domestic alternator seems to be at 14.4V and the start alternator 14.1V. I rarely parallel them as, unless the domestic batteries are heavily discharged, the domestic alternator does all the work anyway. If I want to get the batteries charged a bit faster I put on the TravelPower and the mains charger which seems to have a similar 14.4V output at normal temperatures and adds about 10 amps during the early part of the charging cycle. Edited April 7, 2013 by MikeV

[dmr]

Not sure I fully understand what you are trying to do here, though I confess I have not watched your video as broadband speed is limited right now.

My understanding is that Alternator Sharing only works when charging in Bulk mode, which means the voltage is lower than the regulator voltage ("held down" by the batteries), and so both alternators are going flat out and so the regulators are just fully turned on. Hence the regulator voltage does not matter.

In absorption mode the regulators ARE controlling the voltage but as the current is low it can usually be supplied by just one alternator. It would be good practice to un-parallel the alternators at this stage.

Now just at the transition things do become uncertain, especially if you have a strong alternator and a weak alternator and are changing engine speed.

I think I have seen cases were the weaker alternator shuts down the stronger one which is not ideal, though I don't fully understand how this happens.

This transition should not usually last long so is probably not worth worrying about.

I think trying to adjust alternator voltages so that they share the load during absorption is a hiding to nowhere!

 

.............Dave

[Timleech]

Not sure I fully understand what you are trying to do here, though I confess I have not watched your video as broadband speed is limited right now.

My understanding is that Alternator Sharing only works when charging in Bulk mode, which means the voltage is lower than the regulator voltage ("held down" by the batteries), and so both alternators are going flat out and so the regulators are just fully turned on. Hence the regulator voltage does not matter.

In absorption mode the regulators ARE controlling the voltage but as the current is low it can usually be supplied by just one alternator. It would be good practice to un-parallel the alternators at this stage.

Now just at the transition things do become uncertain, especially if you have a strong alternator and a weak alternator and are changing engine speed.

I think I have seen cases were the weaker alternator shuts down the stronger one which is not ideal, though I don't fully understand how this happens.

This transition should not usually last long so is probably not worth worrying about.

I think trying to adjust alternator voltages so that they share the load during absorption is a hiding to nowhere!

 

.............Dave

 

It's much more to do with sharing an 'active' load - such as inverter - than battery charging. My adjustments while under load tell me that the regulator voltage settings do indeed affect the relative contributions.

Not sure why it should be 'good practice' to separate the alternators when the charge current reduces?

 

Tim

[dmr]

It's much more to do with sharing an 'active' load - such as inverter - than battery charging. My adjustments while under load tell me that the regulator voltage settings do indeed affect the relative contributions.

Not sure why it should be 'good practice' to separate the alternators when the charge current reduces?

 

Tim

 

This is all interesting (to me at least). I would expect the alternator with the highest voltage would deliver all the current, but as soon as the load increases enough to pull the alternator voltage down just a fraction then the second alternator would start to help.

I don't know enough about the circuit inside a regulator but most controllers do let (require) the voltage fall just a little as the load increases so this would certainly allow sharing. So how does the current balance of your alternators vary as you change the load?????

You could maybe even control the split by slightly changing the difference is regulator voltage?

Why do you actually want to share the load? Even with a "dominant" alternator it should still all be ok as the higher voltage one will provide as much current as it can, and the other will step in to help when the first one runs out of go?

 

Its good practice to split the banks if you do not have the same type of batteries for domestic and start, so that each can have its optimum voltage

during absorption. Even with identical batteries its probably still wise, especially if you are liveaboard. For example I charge our domestics (Trojans) at between 14.8 and 15v to get maximum charge but the starter battery (which has a much easier life) is happy with 14.4 to 14.6 so why give it more stress than it needs!

 

It seems that fitting mega alternators (175amp and greater) is becoming the norm but I feel sharing a couple of smaller alternators is the nicer way to go.

 

.........Dave

[Timleech]

This is all interesting (to me at least). I would expect the alternator with the highest voltage would deliver all the current, but as soon as the load increases enough to pull the alternator voltage down just a fraction then the second alternator would start to help.

I don't know enough about the circuit inside a regulator but most controllers do let (require) the voltage fall just a little as the load increases so this would certainly allow sharing. So how does the current balance of your alternators vary as you change the load?????

You could maybe even control the split by slightly changing the difference is regulator voltage?

Why do you actually want to share the load? Even with a "dominant" alternator it should still all be ok as the higher voltage one will provide as much current as it can, and the other will step in to help when the first one runs out of go?

 

Its good practice to split the banks if you do not have the same type of batteries for domestic and start, so that each can have its optimum voltage

during absorption. Even with identical batteries its probably still wise, especially if you are liveaboard. For example I charge our domestics (Trojans) at between 14.8 and 15v to get maximum charge but the starter battery (which has a much easier life) is happy with 14.4 to 14.6 so why give it more stress than it needs!

 

It seems that fitting mega alternators (175amp and greater) is becoming the norm but I feel sharing a couple of smaller alternators is the nicer way to go.

 

.........Dave

 

 

I've not really tried changing the load a lot, but within the limits I've tried (Henry vacuum cleaner on low & high power) the balance remains pretty good.

 

My original posting was really just to show to any remaining doubters that the world does not come to an end if you put two alternators in parallel. The follow up post was to draw attention to an incidental snag which I hadn't really expected. My system has been made up to make what I see as best use of what was available to me, to suit my perhaps slightly unusual requirements. Small essentially very basic boat but with occasional heavy electrical load. It seemed a logical thing to do to try to get both alternators to be contributing when there is a large load, as the adjustment possibility was there. Of course if the volts drop enough, that will happen but I hope to minimise any heavy loading on the batteries.

 

Tim